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					Application of Ground-Water Flow and Solute-Transport Models
to Simulate Selected Ground-Water Management Scenarios
in Coastal Georgia and Adjacent Parts of
South Carolina and Florida, 2000 –2100




Scientific Investigations Report 2006-5077

Prepared in cooperation with the
Georgia Department of Natural Resources
  Environmental Protection Division



U.S. Department of the Interior
U.S. Geological Survey
Cover photograph: Arthur Hills Golf Course, Palmetto Dunes,
   Hilton Head Island, South Carolina
Photograph by Alan M. Cressler, U.S. Geological Survey, 2006
Application of Ground-Water Flow and Solute-Transport Models
to Simulate Selected Ground-Water Management Scenarios
in Coastal Georgia and Adjacent Parts of
South Carolina and Florida, 2000–2100

By Dorothy F. Payne, Alden M. Provost, Jaime A. Painter,
Malek Abu Rumman, and Gregory S. Cherry




Prepared in cooperation with the
    Georgia Department of Natural Resources
       Environmental Protection Division




Scientific Investigations Report 2006-5077



U.S. Department of the Interior
U.S. Geological Survey
U.S. Department of the Interior
Gale A. Norton, Secretary

U.S. Geological Survey
P. Patrick Leahy, Acting Director


U.S. Geological Survey, Reston, Virginia: 2006




This report is a Web-only publication: pubs.usgs.gov/sir/2006/5077/
For more information about the USGS and its products:
World Wide Web: http://www.usgs.gov
Telephone: 1-888-ASK-USGS




Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the
U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to
reproduce any copyrighted materials contained within this report.



Suggested citation:
Payne, D.F., Provost, A.M., Painter, J.A., Abu Rumman, Malek, Cherry, G.S., 2006, Application of Ground-Water
Flow and Solute-Transport Models to Simulate Selected Ground-Water Management Scenarios in Coastal
Georgia and Adjacent Parts of South Carolina and Florida, 2000–2100: U.S. Geological Survey Scientific
Investigations Report 2006-5077, 86 p.
                                                                                                                                                                        iii


Contents
Abstract............................................................................................................................................................1
Introduction.....................................................................................................................................................2
     Purpose.and.Scope...............................................................................................................................2
     Description.of.Study.Area....................................................................................................................2
     Previous.Coastal.Sound.Science.Initiative.Studies.........................................................................4
     Method.of.Study.....................................................................................................................................4
     Acknowledgments.................................................................................................................................4
Ground-Water.Flow.and.Solute-Transport.Models...................................................................................5
     Model.Framework..................................................................................................................................8
           Hydrologic-Unit.Layering.............................................................................................................8
           Hydraulic.Properties....................................................................................................................9
           Spatial.Discretization...................................................................................................................9
           Boundary.Conditions....................................................................................................................9
           Pumpage.Distribution...................................................................................................................9
     Model.Calibration................................................................................................................................12
     Base.Case.Simulations:.Representation.of.the.Flow.System.during.2000................................ 20
Simulation.of.Ground-Water.Management.Scenarios...........................................................................26
     Scenario.A:.Representation.of.the.Flow.System.during.1997......................................................27
           Fit.of.Simulated.Heads.to.Observed.Heads.Using.the.MODFLOW.Model........................28
           Ground-Water.Flow....................................................................................................................30
           Solute.Transport..........................................................................................................................35
     Scenario.B:.Effects.of.an.Industrial.Well.Field.Shutdown............................................................35
     Scenarios.C1.and.C2:.Relative.Effects.of.Pumping.in.Chatham.County,.Georgia,.and..
                Southern.Beaufort.County,.South.Carolina........................................................................40
           Scenario.C1:.Elimination.of.Pumping.in.Chatham.County,.Georgia....................................43
                    Ground-Water.Flow...........................................................................................................43
                    Solute.Transport.................................................................................................................43
           Scenario.C2:.Elimination.of.Pumping.in.Southern.Beaufort.County,.South.Carolina......48
                    Ground-Water.Flow...........................................................................................................48
                    Solute.Transport.................................................................................................................52
           Comparison.of.Head.Gradients.in.the.Savannah–Hilton.Head.Island.Area,..
                         Scenarios.C1.and.C2.....................................................................................................53
     Scenarios.D1.and.D2:.Projected.Pumping.to.2035.........................................................................54
           Scenario.D1:.Projection.Based.on.Regional.Economic.Models,.Inc.................................55
                    Ground-Water.Flow...........................................................................................................55
                    Solute.Transport.................................................................................................................61
           Scenario.D2:.County.Comprehensive.Water-Supply.Plan.Projection................................61
                    Ground-Water.Flow...........................................................................................................61
                    Solute.Transport.................................................................................................................71
Model.Limitations.........................................................................................................................................71
Summary........................................................................................................................................................76
Selected.References....................................................................................................................................77
iv


Figures
. 1–2.. Maps.Showing—
.         1..
       . 1. Location.of.model.area,.major.structural.features,.and.24-county.coastal..
          . Georgia.area............................................................................................................................3
.      . 2.. Pumpage.distribution.by.county.for.2000...........................................................................5
. 3–4..Schematic.Diagrams.Showing—
.      . 3.. Model.aquifers.and.confining.units.and.boundary.conditions.for.MODFLOW..
          . and.SUTRA.models.................................................................................................................8
.      . 4.. Simulated.hydraulic-property.zones.by.model.unit.for.the.MODFLOW.model..........10
. 5–9.. Maps.Showing—
.      . 5.. Finite-difference.grid.from.MODFLOW.model.and.finite-element.mesh.from.
          . SUTRA.model........................................................................................................................11
.      . 6.. Distribution.of.ground-water.pumpage.by.model.unit.for.the.MODFLOW..
          . model,.2000,.for.the.Brunswick.aquifer.system.(unit.3),.Upper.Floridan..
          . aquifer.(unit.5),.and.Lower.Floridan.aquifer.(unit.7)........................................................12
.      . 7.. Distribution.of.ground-water.pumpage.by.model.unit.for.the.SUTRA.model,..
          . 2000,.for.the.Brunswick.aquifer.system.(unit.3);.Upper.Floridan.aquifer.(unit.5),..
          . and.Lower.Floridan.aquifer.(unit.7)....................................................................................16
.      . 8.. Simulated.potentiometric.surfaces.generated.by.the.MODFLOW.model,.2000,..
          . for.the.Brunswick.aquifer.system.(unit.3),.Upper.Floridan.aquifer.(unit.5),.and..
          . Lower.Floridan.aquifer.(unit.7)...........................................................................................21
.      . 9.. Potentiometric.surface.of.the.Upper.Floridan.aquifer.generated.by.the..
          . SUTRA.model,.2000..............................................................................................................22
. 10.. Schematic.diagram.showing.simulated.flow-budget.components.and.values..
          for.the.Base.Case.(2000).............................................................................................................23
. 11–17.. Maps.Showing—
.      . 11.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper..
          . . Floridan.aquifer.in.the.Hilton.Head.Island,.South.Carolina,.area.for.the..
          . . Base.Case.(2000)................................................................................................................23
.      . 12.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the..
          . . Upper.Floridan.aquifer.in.the.Hilton.Head.Island,.South.Carolina,.area.for..
          . . 2000.pumpage.during.2010,.2020,.2035,.and.2100..........................................................24
.      . 13.. Change.in.ground-water.pumpage.from.Scenario.A.(1997).to.the.Base.Case..
          . . (2000).by.county..................................................................................................................27
.      . 14.. Change.in.ground-water.pumpage.from.Scenario.A.(1997).to.the.Base.Case.
          . . .(2000).for.the.Upper.Floridan.aquifer.............................................................................28
.      . 15.. Difference.between.simulated.and.observed.water.levels.(residuals).by..
          . . model.layer.for.Scenario.A.(1997)...................................................................................31
.      . 16.. Simulated.potentiometric.surfaces.for.Scenario.A.(1997).for.the.Brunswick..
          . . aquifer.system.(unit.3),.Upper.Floridan.aquifer.(unit.5),.and.Lower.Floridan..
          . . aquifer.(unit.7).....................................................................................................................32
.      . 17.. Change.in.simulated.head.between.the.Base.Case.(2000).and.Scenario.A..
          . . (1997).for.the.Brunswick.aquifer.system.(unit.3),.Upper.Floridan.aquifer.(unit.5),..
          . . and.Lower.Floridan.aquifer.(unit.7).................................................................................33
                                                                                                                                                          v

. 18–19.. Graphs.Showing—
.      . 18.. Simulated.flow.budget.indicating.changes.from.Scenario.A.(1997).conditions..
          . . to.Base.Case.(2000).conditions........................................................................................34
.      . 19.. Comparison.of.simulated.net.landward.flow.for.the.Base.Case.(2000).and..
          . . Scenario.A.(1997),.Scenario.B.(Durango.Paper.Company.pumping.eliminated),..
          . . Scenario.C1.(Chatham.County,.Georgia,.pumping.eliminated),.Scenario.C2..
          . . (southern.Beaufort.County,.South.Carolina,.pumping.eliminated),.Scenario.D1..
          . . (projection.based.on.Regional.Economic.Models,.Inc.),.and.Scenario.D2..
          . . (County.Comprehensive.Water-Supply.Plans.projection)...........................................35
. 20–23.. Maps.Showing—
.      . 20.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the..
          . . Upper.Floridan.aquifer.in.the.Hilton.Head.Island,.South.Carolina,.area.for..
          . . Scenario.A.(1997)...............................................................................................................35
.      . 21.. Distribution.of.ground-water.pumpage.for.the.Upper.Floridan.aquifer.(unit.5).
          . . in.the.southern.part.of.the.model.area.for.Scenario.B.(Durango.Paper..
          . . Company.pumping.eliminated).........................................................................................36
.      . 22.. Simulated.potentiometric.surfaces.for.Scenario.B.(Durango.Paper.Company..
          . . pumping.eliminated).for.the.Brunswick.aquifer.system.(unit.3),.Upper..
          . . Floridan.aquifer.(unit.5),.and.Lower.Floridan.aquifer.(unit.7)......................................37
.      . 23.. Change.in.simulated.head.between.Scenario.B.(Durango.Paper.Company..
          . . pumping.eliminated).and.the.Base.Case.(2000).for.the.Brunswick.aquifer..
          . . system.(unit.3),.Upper.Floridan.aquifer.(unit.5),.and.Lower.Floridan.aquifer..
          . . (unit.7)...................................................................................................................................38
. 24.. Hydrographs.for.St..Marys.well.cluster.(33D071.and.33D073).and.nearby..
          National.Park.Service.well.(33D069),.Camden.County,.Georgia,.2000–2003......................40
. 25.. Map.showing.observed.water-level.change.from.September.2001.through..
          May.2003.in.wells.completed.in.the.Upper.Floridan.aquifer.and.simulated..
          change.in.potentiometric.surface.for.Scenario.B.(Durango.Paper.Company..
          pumping.eliminated).in.Camden.County,.Georgia,.and.Nassau.County,.Florida...............41
. 26.. Graph.showing.simulated.flow.budget.for.Scenario.B.(Durango.Paper.Company..
          pumping.eliminated),.relative.to.the.Base.Case.(2000).........................................................42
. 27–29.. Maps.Showing—
.      . 27.. Simulated.potentiometric.surfaces.for.Scenario.C1.(Chatham.County,..
          . . Georgia,.pumping.eliminated).for.the.Brunswick.aquifer.system.(unit.3),..
          . . Upper.Floridan.aquifer.(unit.5),.and.Lower.Floridan.aquifer.(unit.7)..........................44
.      . 28.. Results.using.solute-transport.model.for.simulated.potentiometric.surface..
          . . for.Scenario.C1.(Chatham.County,.Georgia,.pumping.eliminated).and..
          . . change.in.simulated.head.between.Scenario.C1.and.the.Base.Case.(2000)..
          . . for.the.Upper.Floridan.aquifer.in.Chatham.County,.Georgia,.and.southern..
          . . Beaufort.County,.South.Carolina.....................................................................................45
.      . 29.. Change.in.simulated.head.between.Scenario.C1.(Chatham.County,.Georgia,..
          . . pumping.eliminated).and.the.Base.Case.(2000).for.the.Brunswick.aquifer..
          . . system.(unit.3),.Upper.Floridan.aquifer.(unit.5),.and.Lower.Floridan.aquifer..
          . . (unit.7)...................................................................................................................................46
. 30.. Graph.showing.simulated.flow.budget.for.Scenario.C1.(Chatham.County,.Georgia,.
          pumping.eliminated),.relative.to.the.Base.Case.(2000).........................................................47
vi

. 31–34.. Maps.Showing—
.      . 31.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the..
          . . Upper.Floridan.aquifer.in.the.Hilton.Head.Island,.South.Carolina,.area.for..
          . . Scenario.C1.(Chatham.County,.Georgia,.pumping.eliminated)...................................48
.      . 32.. Simulated.potentiometric.surfaces.for.Scenario.C2.(southern.Beaufort.County,..
          . . South.Carolina,.pumping.eliminated).for.the.Brunswick.aquifer.system.(unit.3),..
          . . Upper.Floridan.aquifer.(unit.5),.and.Lower.Floridan.aquifer.(unit.7)..........................49
.      . 33.. Results.using.solute-transport.model.for.simulated.potentiometric.surface..
          . . for.Scenario.C2.(southern..Beaufort.County,.South.Carolina,.pumping...
          . . eliminated).and.change.in.simulated.head.between.Scenario.C2.and.the..
          . . Base.Case.(2000).for.the.Upper.Floridan.aquifer.in.Chatham.County,..
          . . Georgia,.and.southern.Beaufort.County.........................................................................50
.      . 34.. Change.in.simulated.head.between.Scenario.C2.(southern.Beaufort.County,..
          . . South.Carolina,.pumping.eliminated).and.the.Base.Case.(2000).for.the..
          . . Brunswick.aquifer.system.(unit.3),.Upper.Floridan.aquifer.(unit.5),.and..
          . . Lower.Floridan.aquifer.(unit.7).........................................................................................51
. 35.. Graph.showing.simulated.flow.budget.for.Scenario.C2.(southern.Beaufort.County,.
          South.Carolina,.pumping.eliminated),.relative.to.the.Base.Case.(2000).............................52
. 36.. Map.showing.simulated.chloride.concentration.in.ground.water.in.the.middle.of..
          the.Upper.Floridan.aquifer.in.the.Hilton.Head.Island,.South.Carolina,.area.for..
          Scenario.C2.(southern.Beaufort.County,.South.Carolina,.pumping.eliminated)...............53
. 37.. Cross.section.of.potentiometric.surfaces.for.the.Upper.Floridan.aquifer.for..
          predevelopment,.Base.Case.(2000),.Scenario.C1.(Chatham.County,.Georgia,..
          pumping.eliminated),.and.Scenario.C2.(southern.Beaufort.County,.South.Carolina,.
          pumping.eliminated)....................................................................................................................54
. 38.. Graph.showing.projected.ground-water.use.in.the.24-county.coastal.area.of..
          Georgia,.2000–2050,.by.category...............................................................................................55
. 39–41.. Maps.Showing—
.      . 39.. Distribution.of.the.difference.between.ground-water.pumpage.for..
          . . Scenario.D1.(projection.based.on.Regional.Economic.Models,.Inc.).during..
          . . 2035.and.the.Base.Case.(2000).in.the.Upper.Floridan.aquifer....................................56
.      . 40.. Simulated.potentiometric.surfaces.for.Scenario.D1.(projection.based.on..
          . . Regional.Economic.Models,.Inc.).during.2035.for.the.Brunswick.aquifer..
          . . system.(unit.3),.Upper.Floridan.aquifer.(unit.5),.and.Lower.Floridan..
          . . aquifer.(unit.7).....................................................................................................................58
.      . 41.. Change.in.simulated.head.between.Scenario.D1.(projection.based.on..
          . . Regional.Economic.Models,.Inc.).during.2035.and.the.Base.Case.(2000)...
          . . for.the.Brunswick.aquifer.system.(unit.3),.Upper.Floridan.aquifer.(unit.5),..
          . . and.Lower.Floridan.aquifer.(unit.7).................................................................................59
. 42.. Graph.showing.simulated.flow.budget.for.Scenario.D1.(projection.based.on..
          Regional.Economic.Models,.Inc.).during.2035,.relative.to.the.Base.Case.(2000).............60
. 43–46.. Maps.Showing—
.      . 43.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the..
          . . Upper.Floridan.aquifer.in.the.Hilton.Head.Island,.South.Carolina,.area.for..
          . . Scenario.D1.(projection.based.on.Regional.Economic.Models,.Inc.).during..
          . . 2010,.2020,.2035,.and.2100.................................................................................................62
	      . .44....Distribution.of.the.difference.between.ground-water.pumpage.for..
          . . Scenario.D2.(County.Comprehensive.Water-Supply.Plans.projection).during..
          . . 2035.and.the.Base.Case.(2000).for.the.Upper.Floridan.aquifer..................................64   .
.      . 45.....Simulated.potentiometric.surfaces.for.Scenario.D2.(County.Comprehensive..
          . . Water-Supply.Plans.projection).during.2035.of.the.Brunswick.aquifer.system..
          . . (unit.3),.Upper.Floridan.aquifer.(unit.5),.and.Lower.Floridan.aquifer.(unit.7)...........66                               .
                                                                                                                                                   vii

.      . 46.. Change.in.simulated.head.between.Scenario.D2.(County.Comprehensive..
          . . Water-Supply.Plans.projection).during.2035.and.the.Base.Case.(2000).for.the..
          . . Brunswick.aquifer.system.(unit.3),.Upper.Floridan.aquifer.(unit.5),.and..
          . . Lower.Floridan.aquifer.(unit.7).........................................................................................67
	 47.. Graph.showing.simulated.flow.budget.for.Scenario.D2.(County.Comprehensive..
          Water-Supply.Plans.projection).during.2035,.relative.to.the.Base.Case.(2000)................69
. 48–50.. Maps.Showing—
.      . 48.. Simulated.recharge.or.discharge.(flow.from.or.to.general-head.boundary)...
          . . for.Base.Case.(2000);.Scenario.D1.(projection.based.on.Regional.Economic..
          . . Models,.Inc.),.2035;.and.Scenario.D2.(County.Comprehensive.Water-.
          . . Supply.Plans.projection),.2035.........................................................................................70
.      . 49.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the..
          . . Upper.Floridan.aquifer.in.the.Hilton.Head.Island,.South.Carolina,.area.for..
          . . Scenario.D2.(County.Comprehensive.Water-Supply.Plans.projection)..
          . . during.2010,.2020,.2035,.and.2100.....................................................................................72
.      . 50.. Simulated.head.below.the.top.the.Upper.Floridan.aquifer.for.Base.Case.(2000),..
          . . Scenarios.D1.(projection.based.on.Regional.Economic.Models,.Inc.).and.D2..
          . . (County.Comprehensive.Water-Supply.Plans.projection).during.2035,.and..
          . . boundary-condition.head.above.and.below.the.Upper.Floridan.aquifer..................75



Tables
.      1.. Simulated.pumpage.by.scenario.and.difference.from.Base.Case.(estimated..
           2000.pumpage).by.county.............................................................................................................6
.      2.. Flow-budget.components.for.the.Base.Case,.2000...............................................................22
.      3.. Description.of.conditions.for.pumping.scenarios..................................................................26
.      4.. Model-fit.statistics.for.simulated.heads.for.Scenario.A,.1997.pumping.conditions,..
                                      .
           for.the.MODFLOW.model...........................................................................................................30
.      5.. Flow-budget.components.for.Scenario.A...............................................................................34
.      6.. Flow-budget.components.for.Scenario.B...............................................................................42
.      7.. Flow-budget.components.for.Scenario.C1..............................................................................47
.      8.. Flow-budget.components.for.Scenario.C2..............................................................................52
.      9.. Flow-budget.components.for.Scenario.D1,.2035...................................................................60
.     10.. Flow-budget.components.for.Scenario.D2,.2035...................................................................68
viii


       Conversion	Factors,	Datum,	and	Acronyms
        Multiply                                           By             To	obtain
                                                         Length
        inch (in.)                                         2.54           centimeter (cm)
        foot (ft)                                          0.3048         meter (m)
        mile (mi)                                          1.609          kilometer (km)
                                                          Area
        square mile (mi )
                        2
                                                           2.590          square kilometer (km2)
                                                        Volume
        million gallons (Mgal)                         3,785              cubic meter (m3)
                                                  Flow/Transport.rate
        million gallons per day (Mgal/d)                  0.04381         cubic meter per second (m3/s)
        inch per year (in/yr)                            25.4             millimeter per year (mm/yr)
                                                 Hydraulic.conductivity
        feet per day (ft/d)                                0.3048         meter per day (m/d)


       Temperature.in.degrees.Celsius.(°C).may.be.converted.to.degrees.Fahrenheit.(°F).as.follows:
                                                  °F.=.(1.8.x.°C).+.32
       Temperature.in.degrees.Fahrenheit.(°F).may.be.converted.to.degrees.Celsius.(°C).as.follows:
                                                   °C.=.(°F.–32)./.1.8
       Vertical.coordinate.information.is.referenced.to.the.North.American.Vertical.Datum.of.1988.
       (NAVD.88)..Historical.data.collected.and.stored.as.National.Geodetic.Vertical.Datum.of.1929.
       have.been.converted.to.NAVD.88.for.this.publication.
       Horizontal.coordinate.information.is.referenced.to.the.North.American.Datum.of.1983.(NAD.83)..
       Historical.data.collected.and.stored.as.North.American.Datum.1927.have.been.converted.to.
       NAD.83.for.this.publication.
       Altitude,.as.used.in.this.report,.refers.to.distance.above.the.vertical.datum.
       Acronyms.used.in.this.report:
           CSSI.              Coastal.Sound.Science.Initiative
           CWSP.              County Comprehensive.Water-Supply.Plans
           GaEPD.             Georgia.Environmental.Protection.Division
           GIS.               Geographic.Information.System
           MODFLOW. Modular.Ground-Water.Flow.Simulator
           REMI.              Regional.Economic.Models,.Inc.
           SCDHEC.            South.Carolina.Department.of.Health.and.Environmental.Control
           SUTRA.             Saturated-Unsaturated.Transport.Simulator
           USGS.              U.S..Geological.Survey
Application	of	Ground-Water	Flow	and	Solute-Transport	Models	
to	Simulate	Selected	Ground-Water	Management	Scenarios	
in	Coastal	Georgia	and	Adjacent	Parts	of		
South	Carolina	and	Florida,	2000–2100

By.Dorothy.F..Payne,.Alden.M..Provost,.Jaime.A..Painter,.Malek.Abu.Rumman,.and.Gregory.S..Cherry



Abstract
                                                                   County, South Carolina, on ground-water levels and saltwater
      Regional ground-water flow and solute-transport models       distribution and movement in that area. Results indicate that
for the coastal area of Georgia, and adjacent parts of Florida     pumping in southern Beaufort County has a smaller effect
and South Carolina were used to evaluate the effects of current    on saltwater-plume development than pumping in Chatham
and hypothetical ground-water withdrawal on ground-water           County for plumes west of Hilton Head Island, South Caro-
flow and saltwater transport. The models were designed to          lina. Results also indicate that the effect of pumping on the
simulate the flow system at different scales while being as        plume at the northern end of Hilton Head Island in either
consistent as possible in framework, hydraulic properties,         Chatham County or southern Beaufort County is small,
pumpage distribution, and boundary conditions. Simulation          although pumping in southern Beaufort County may have a
results for future pumpage scenarios were compared with            slightly greater effect on plume growth than pumping in
those during 2000 (the Base Case), or during 2100 for 2000         Chatham County. Furthermore, model results indicate that
pumpage applied after 2000. The regional MODFLOW model             eliminating pumping in Chatham County would result in a
assumes steady-state ground-water flow, and is calibrated to       greater water-level increase at the southern end of Hilton
1980 and 2000 pumping conditions. The SUTRA model of the           Head Island than elimination of pumping in southern Beaufort
Savannah, Georgia–Hilton Head Island, South Carolina, area         County, and that eliminating pumping in southern Beaufort
is run as a transient simulation from a predevelopment (1885)      County would result in a greater water-level increase at the
steady-state flow field to 2004, and calibrated to water levels    northern end of Hilton Head Island than elimination of
in September 1998 and estimated chloride values in 2000,           pumping in Chatham County.
2002, 2003, and 2004.                                                    Scenarios D1 and D2 simulate the effect of projected
      Scenario A illustrates the effects of implementing an        pumpage during 2000–2035, based on two estimates of
interim strategy for managing saltwater intrusion in the Upper     future ground-water needs. Results from both scenarios show
Floridan aquifer of southeastern Georgia. Results show a           substantial water-level declines from 2000 to 2035 and an
combination of rises and declines in head from 1997 to 2000        increase of inflow at the source-sink boundaries. For the sol-
in response to changes in the pumping patterns and only            ute-transport simulations, pumpage at 2035 was held constant
minor changes in the chloride distribution in the Hilton Head      until 2100. For both scenarios, chloride plumes expand during
Island, South Carolina, area. Generally, water levels rose in      2000–2100, but show limited expansion relative to plumes
the Savannah–Hilton Head Island area, and declined in the          that develop for 2000 pumpage applied until 2100. Although
area north of the Gulf Trough.                                     the total pumpage difference between Scenarios D1 and D2
      Scenario B simulates the effect of a 36-million-gallon-      during 2035 is 477 million gallons per day, and pumpage in
per-day reduction of pumpage at a major pumping center             both scenarios is substantially larger than that during 2000,
in Camden County, Georgia. Results show that the largest           distance and hydraulic features, such as the Gulf Trough, and
recovery is limited in extent to the area surrounding St. Marys,   high hydraulic conductivity in the southwestern part of the
Georgia, but a smaller water-level rise of 1–2 feet extends as     model area, limit the effects of these differences on the extent
far north as southern Chatham County, Georgia, and inland          of resultant plumes.
toward the Gulf Trough. Nearest the area where the wells were            Results obtained using the ground-water flow and
turned off, the model predicts a smaller recovery than indi-       solute-transport models used in this study are subject to
cated by observed water levels.                                    the limitations of the models and scenario conditions. For
      Scenarios C1 and C2 illustrate the relative effects of       example, pumpage for Scenario B may not accurately repre-
pumping in Chatham County, Georgia, and southern Beaufort          sent conditions during 2002 when the industrial wells were
2	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

turned off, and projected pumpage for 2010, 2020, and 2035                The USGS developed digital models, as part of the CSSI,
is substantially different in Scenarios D1 and D2. Generally,       which must satisfy multiple objectives at varying scales.
model results are less reliable for scenario conditions that        Objectives include simulation of (1) the regional flow system,
are farther from calibration conditions. Conditions that dif-       including the Brunswick aquifer system and the Lower Flori-
fer substantially from calibration conditions may induce an         dan aquifer, in addition to the Upper Floridan aquifer (Payne
unrealistic response from the model if the model assumptions        and others, 2005); (2) subregional flow and localized seawa-
are violated; for example, if influx from model boundaries is       ter intrusion in the Savannah, Ga.–Hilton Head Island, S.C.,
excessive. Excessive inflow from the model boundaries, par-         area (Provost and others, 2006); and (3) localized saltwater
ticularly from the onshore area, may result in underestimated       intrusion at Brunswick, Ga. To satisfy these objectives, the
drawdown and inflow of saltwater. Model results are most            USGS developed a set of ground-water flow and solute-trans-
reliable in areas where calibration data exist, and for the range   port models with consistent framework, hydraulic properties,
of pumpage conditions for which the models are calibrated.          pumpage, and boundary conditions; these models update and
                                                                    expand on earlier digital models of the area. This suite of mod-
                                                                    els represents an approach to developing consistent, integrated
Introduction                                                        modeling tools that simulate different aspects of a coastal
      Since the 1980s, population growth, increased tourism,        ground-water flow system at varying scales for the purpose of
and sustained industrial activity in the coastal area of Georgia,   addressing water-resource management issues. The hypotheti-
South Carolina, and Florida have resulted in increased ground-      cal scenarios that were evaluated using the two models provide
water pumpage. Recent periods of drought (for example,              insight into the influence of stresses on ground-water flow
1998–2002) also have increased stresses on the coastal              and saltwater intrusion in Georgia and may provide insight to
ground-water system. The principal source of water in the           similar occurrences elsewhere in the Atlantic Coastal Plain.
coastal area is the Upper Floridan aquifer, an extremely per-
meable, high-yielding aquifer that was first developed during       Purpose	and	Scope
the late 1800s and has been used extensively in the area ever
since. Pumping from the Upper Floridan aquifer has resulted               This report describes results from a variety of simulations
in substantial water-level decline near Savannah, Georgia           using the regional flow model of Payne and others (2005) and
(Ga.), and saltwater intrusion at the northern end of Hilton        the Savannah–Hilton Head Island solute-transport model of
Head Island, South Carolina (S.C.), and at Brunswick, Ga.           Provost and others (2006). These simulations were designed to
This saltwater contamination has constrained further develop-       evaluate the effects of current and hypothetical ground-water
ment of the Upper Floridan aquifer in the coastal area, which       withdrawal, and the relative effects of pumping in specific
has created competing demands for the limited water sup-            areas on ground-water flow and saltwater transport. For each
ply. Projected increase in coastal population during the next       scenario simulated, this report describes the purpose, pump-
several decades is expected to result in increased competition      age distribution, simulated head by aquifer unit, differences
for ground water.                                                   in simulated head relative to 2000 conditions, and simulated
      The Georgia Environmental Protection Division (GaEPD),        flow-budget components, including boundary fluxes. For most
as part of an interim water management strategy, capped             scenarios, the simulated chloride distribution in the Savannah–
permitted withdrawal from the Upper Floridan aquifer at 1997        Hilton Head Island area also is presented. Finally, this report
withdrawal rates in parts of the coastal area during 1997–2005      describes the limitations of the applications of these models
to limit further saltwater intrusion (Georgia Environmental         and simulation results.
Protection Division, 1997). To develop a strategy to address              Payne and others (2005) and Provost and others (2006)
these problems and manage projected future coastal water-           describe in detail the models used in this study. The regional
resource needs, the GaEPD has implemented the Georgia               ground-water flow model was constructed using the USGS
Coastal Sound Science Initiative (CSSI), a series of scientific     finite-difference, ground-water flow simulator MODFLOW-
and feasibility investigations designed to assess ground-water      2000 (Harbaugh and others, 2000), and the Savannah–Hilton
resources in the coastal area and address issues of saltwater       Head Island solute-transport model was constructed using the
intrusion and resource sustainability. As part of this initia-      USGS finite-element, variable-density solute-transport simula-
tive, the GaEPD, Skidaway Institute of Oceanography, South          tor SUTRA (Voss and Provost, 2003). Only a brief description
Carolina Department of Health and Environmental Control             of the models is included in this report.
(SCDHEC), U.S. Army Corps of Engineers, and the U.S.
Geological Survey (USGS), as well as private consulting firms,
collected and analyzed hydrogeologic data to refine the concep-     Description	of	Study	Area
tual models of ground-water flow and saltwater transport. The
USGS then synthesized this information into digital models               The GaEPD defines the coastal area of Georgia to include
that describe the ground-water flow system and movement of          the 6 coastal counties and 18 adjacent counties, an area of
saltwater. The GaEPD will use these digital models to help          about 12,240 square miles (mi2) (fig. 1). To account for natural
design a ground-water permitting strategy for the coastal area.     hydrologic boundaries used for model simulation, the regional
                                                                                                                                                                                                                         Introduction	 	 

              84°                                       83°                                                     82°                                                  81°                                      80°
       GWINNETT                                                                                                                                                             RICHLAND      SUMTER




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                                  OCONEE        OGLETHORPE                                                                                   LEXINGTON
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                                            WILKINSON                                                                                                                                                     CHARLESTON
                         BIBB                                                                                  JENKINS
       CRAWFORD                                                          JOHNSON                                              SCREVEN               Savannah–Hilton Head
                                  TWIGGS                                                                                                               Island study area
      TAY




                                                                                            EMANUEL
                     PEACH                                                                                                                                       JASPER
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                       CRISP     WILCOX
                                                           TELFAIR
        LEE                                                                                                                        LIBERTY
                                                                         JEFF DAVIS
                             TURNER             BEN HILL                                    APPLING                      LONG

                                                                                                                Jesup
                                         IRWIN
                    WORTH                                                           BACON                         WAYNE
                                                                COFFEE                                                                MCINTOSH

                                 TIFT
                                                                                                     PIERCE
        MITCHELL




                                              BERRIEN       ATKINSON                                                               GLYNN




                                                                                                                                                                                                    an
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                                                     LANIER                             WARE                                                    Brunswick
31°




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                                                                         CLINCH                                            CAMDEN
                                            LOWNDES                                                   CHARLTON



                                                                                                                                                                                            Atlan
               THOMAS        BROOKS                                                                                       St Marys
                                 GEORGIA                                 ECHOLS                                                                 Fernandina
                                                                                                                         NASSAU
                                 FLORIDA                                                                                                        Beach
                                                      HAMILTON
                                                                                                                                 DUVAL
             JEFFERSON          MADISON

                                                                                                   BAKER                 Jacksonville

                                                    SUWANNEE               COLUMBIA


                             TAYLOR                                                         UNION                       CLAY
30°
                                             LAFAYETTE
                                                                                                      BRADFORD                         ST JOHNS

                                                   DIXIE        GILCHRIST                   ALACHUA                       PUTNAM


                                                                                                                                                                     0                 25                50              75 MILES
                      GEORGIA         SOUTH CAROLINA                        24-county coastal Georgia
                                                                              area as identified by the                                                              0         25           50         75 KILOMETERS
                                                                              Georgia Environmental
                                                                              Protection Division                                                                    Base from U.S. Geological Survey 1:100,000-scale
                                                                                                                                                                     digital data. Structural features modified from
                                                                                                 Northern
                                                              Map area




                                                                                                                                                                     Applied Coastal Research Laboratory, 2002;
                                                                                                                                                                     Weems and Edwards, 2001; Kellam and Gorday, 1990;
                                                                                                 Central                                                             and Paull and Dillon, 1980.

                                                                                                 Southern
                                  FL
                                       OR
                                         ID
                                            A




        Figure	1.. Location.of.model.area,.major.structural.features,.and.24-county.coastal.Georgia.area.
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models

study area has been expanded to 42,155 mi2, extending inland         Weems and Edwards (2001) provided a regional distribution of
in Georgia and into northeastern Florida and southwestern            the sedimentary units that comprise the confined surficial and
South Carolina, and the adjacent offshore area. The Savannah–        Brunswick aquifer systems. The distribution of the confining
Hilton Head Island study area (fig. 1) encompasses about             unit above the Upper Floridan aquifer was mapped in the Hilton
3,000 mi2 and includes Chatham County and parts of Bryan,            Head Island and offshore area using seismic data (Foyle and
Effingham, and Liberty Counties in Georgia, and Beaufort             others, 2001). Falls and others (2005a) examined the distribu-
County and part of Jasper County in South Carolina and the           tion and water-bearing properties of the Lower Floridan aquifer
adjacent offshore area. Payne and others (2005) and Provost          throughout the region. The framework and ground-water chem-
and others (2006) describe the topography, climate, and land         istry of the Upper Floridan aquifer system and overlying confin-
use in the regional and Savannah–Hilton Head study areas.            ing unit offshore from Hilton Head Island were examined by
      The GaEPD subdivided the 24-county coastal area into           drilling and sampling four test wells (Falls and others 2005b).
three subareas—the northern, southern, and central subareas—         The present-day salinity distribution on Hilton Head Island was
to facilitate implementation of the State’s water-management         estimated with the installation and monitoring of specific-con-
practices (fig. 1). The northern subarea is northwest of the Gulf    ductance monitors in wells (Camille Ransom III, South Carolina
Trough, a prominent geologic feature that represents a zone of       Department of Health and Environmental Control, written com-
low permeability in the Floridan aquifer system. The southern        mun., 2004). Leeth and others (2005) used population projections
subarea lies south of what the GaEPD has called the “Satilla         by Regional Economic Models, Inc. (REMI), and Camp Dresser
Line,” a postulated hydrologic boundary based on a change in         and McKee (2001) used County Comprehensive Water Supply
the configuration of the potentiometric surface of the Upper         Plans (CWSP), to estimate projected water use for 24 coastal
Floridan aquifer, and by linear changes depicted on aeromag-         counties in Georgia. After the abrupt shutdown of Durango
netic, aeroradioactivity, gravity, and isopach maps (William H.      Paper Company wells in Camden County, Ga., the USGS moni-
McLemore, Georgia Environmental Protection Division, Geologic        tored water-level recovery and ground-water salinity, and Peck
Survey Branch, oral commun., January 6, 2000). The central           and others (2005) analyzed the hydrologic response.
subarea lies between the northern and southern subareas, and
includes the largest concentration of pumping in the coastal area—
the Savannah, Brunswick, and Jesup pumping centers (fig. 1).         Method	of	Study
      The Floridan aquifer system is the principal source of
                                                                          Several pumpage scenarios were developed to examine
water for all uses in the coastal area. The aquifer consists of
                                                                     ground-water flow system characteristics and the limitations
the predominantly carbonate Upper Cretaceous to Oligocene
                                                                     of the models, and to address the effect of anticipated future
Upper and Lower Floridan aquifers (Miller, 1986; Krause
                                                                     demands on the system. The regional MODFLOW ground-
and Randolph, 1989). Secondary sources of water include the
                                                                     water flow model was used to simulate all of the pumpage
shallow surficial and Brunswick aquifer systems, consisting of
                                                                     scenarios, whereas only scenarios that affected the hydrology in
Miocene to Holocene siliciclastic units (Clarke, 2003).
                                                                     the Savannah–Hilton Head Island area were simulated using the
      The total estimated pumpage in the model area during
                                                                     SUTRA solute-transport model. Results from simulations using
2000 was about 815 million gallons per day (Mgal/d) (table 1).
                                                                     the MODFLOW model include simulated-head distributions,
Most of the pumping occurred in coastal counties (fig. 2),
                                                                     head differences relative to the Base Case simulation (2000
most notably Duval County, Florida (Fla.), for which the esti-
                                                                     conditions), and flow-budget differences relative to the Base
mated pumpage was about 145 Mgal/d. For Chatham County,
                                                                     Case simulation. Results from simulations using the SUTRA
Ga., estimated pumpage during 2000 was about 71 Mgal/d,
                                                                     solute-transport model include simulated chloride distribution
which is the largest estimated pumpage for counties in Georgia.
                                                                     and calculated heads in the Savannah–Hilton Head Island area.
The Upper Floridan aquifer is the most heavily pumped aqui-
fer, from which an estimated 680 Mgal/d was pumped during
2000 (Payne and others, 2005). During 2000, estimated Lower          Acknowledgments
Floridan aquifer pumpage was 130 Mgal/d, most of which was
in Duval County, Fla., and estimated pumpage from the Bruns-              The authors wish to thank Caryl J. Wipperfurth of the
wick aquifer system was less than 1 Mgal/d. The single largest       U.S. Geological Survey for preparation of illustrations and
concentration of pumping in Georgia is in Jesup, Wayne               Patricia L. Nobles of the U.S. Geological Survey for prepara-
County, Ga. (fig. 1), at a rate of about 60 Mgal/d during 2000       tion of the manuscript. William H. McLemore of the Georgia
in the Upper Floridan aquifer (Fanning, 2003).                       Environmental Protection Division helped design and focus
                                                                     the scenarios and Julia L. Fanning of the U.S. Geological
                                                                     Survey helped with development and implementation of the
Previous	Coastal	Sound	Science		                                     pumpage distributions for the scenarios. Michael F. Peck of
Initiative	Studies                                                   the U.S. Geological Survey provided water-level data and
                                                                     interpretation. John S. Clarke of the U.S. Geological Sur-
    The simulation results presented herein represent the            vey assisted in implementation of the scenarios and in the
culmination of studies implemented as part of the CSSI.              approach to the documentation thereof.
                                                                                                                                   Ground-Water	Flow	and	Solute-Transport	Model	 	 


                             SOUTH
          GEORGIA           CAROLINA
                                                                                                                            7.50
                                                                     0.33                                                                   6.32
                                                                                                                  SC
                                                                                                  25.59          GA
                                                                            13.73                                                9.59
            F L OR
                     ID
                                                              18.07                                                                                   0.51
                       A
                                                                                                                                          8.63
                                                                                                   4.63




                                                                                                                                                                         Mo
                                                                                                                   18.56




                                                                                                                                                                         de
                                                                  2.45




                                                                                                                                                                          lb
                                                                                                                                                                              ou
                                                                                 4.70




                                                                                                                                                                              nd
                                                                                                                                                     21.70




                                                                                                                                                                               ar
                                   7.66                9.25                                                                                  3.35




                                                                                                                                                                                   y
                                                                       1.21                  3.16               6.01              4.65

                           13.27                                        1.80
                                           4.36                                                         1.84
           21.62
                                                                1.21             6.99                                                                               an




                                                                                                                                                               ce
                                                                                                 3.81                      1.63          71.38




                                                                                                                                                              O
                                                                                                                                                             ic
                            17.27




                                                                                                                                                         nt
              9.86                                     4.41




                                                                                                                                                       la
                                                                                                                          15.72




                                                                                                                                                      At
                                                                  4.31
                      3.01                8.16                                       4.17                      0.76

                                      7.21               16.96                                          63.47
                                                                              4.04                                          0.86
                              4.19                                                                                                                  EXPLANATION
                                                                                          6.22                                               Total pumpage by county, 2000 —
                                          6.10           2.91                                                            56.28                 In million gallons per day
                                                                                                    1.30                                             Less than 5
                                                                            8.45
                                                 1.97                                                                                                5 to 10
                                                                1.44                         1.25                 50.61                              10 to 30
                                                                                                                                                     30 to 100
                                                                2.88                                            47.38                                Greater than 100
                                                                                     GA
                                                        N                            FL
                                                                        6.04
                                                                                     2.42                        144.59


                                                   0                   25                   50 MILES
                                                                                                                        Base from U.S. Geological Survey
                                                   0            25          50 KILOMETERS                               1:100,000-scale digital data

              Figure	2.. Pumpage.distribution.by.county.for.2000..Values.represent.sum.of.estimated.pumpage.
              for.the.Brunswick.aquifer.system,.Upper.Floridan.aquifer,.and.Lower.Floridan.aquifer.



Ground-Water	Flow	and	Solute-                                                                                         a variety of saltwater intrusion mechanisms. These simulators
                                                                                                                      share a common geographic information system (GIS)-based
Transport	Models                                                                                                      interface (Argus® ONE) that facilitates the transfer of model
                                                                                                                      input between the two types of models (Winston, 2000;
     The two types of models used in the study, constructed                                                           Winston and Voss, 2004).
using MODFLOW-2000 and SUTRA, address different                                                                             The regional MODFLOW model is calibrated for 1980
objectives at different scales. MODFLOW-2000 (Harbaugh                                                                and 2000 pumpage assuming steady-state flow. As the
and others, 2000) is a finite-difference, constant-density flow                                                       MODFLOW model was being constructed and calibrated, the
simulator that is widely used and is appropriate for model-                                                           common GIS-based interface was used to enable transfer of
ing regional ground-water flow systems. SUTRA (Voss and                                                               model datasets to a SUTRA model based on the same hydro-
Provost, 2003) is a two- or three-dimensional, finite-element,                                                        geologic information. Throughout the initial development
ground-water flow and solute- or energy-transport simulator                                                           process, hydraulic properties, model layering, and boundary
capable of explicitly simulating the effects of variable fluid                                                        conditions for the MODFLOW and SUTRA models were
density on the distribution and movement of saline water for                                                          kept as mutually consistent as possible. After the regional
                                                                                                                                                                                                                                                                                                                                 	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	
Table	1.. Simulated.pumpage.by.scenario.and.difference.from.Base.Case.(estimated.2000.pumpage).by.county.—Continued
[Difference is scenario pumpage minus Base Case (estimated 2000) pumpage; values rounded to the 0.1 million gallons per day; sum of Base Case pumpage and difference may not equal scenario
pumpage because of rounding. Scenarios: A, 1997 pumpage; B, Durango Paper Company pumping eliminated; C1, Chatham County pumping eliminated; C1, southern Beaufort County pumping eliminated;
D1, Regional Economic Models, Inc. projection; D2, County Comprehensive Water-Supply Plans, projection. –, minus; Fla., Florida; Ga., Georgia, S.C., South Carolina]
                                                                    Simulated	pumpage	by	scenario	and	difference	from	Base	Case	(estimated	2000	pumpage),	in	million	gallons	per	day

                        Base		                A                              B                             C1                             C2                      D1,	2010                   D1,	2020                   D1,	20                   D2,	2010                    D2,	2020                    D2,	20
State   County          Case	




                                                   Difference




                                                                                 Difference




                                                                                                                Difference




                                                                                                                                               Difference




                                                                                                                                                                          Difference




                                                                                                                                                                                                     Difference




                                                                                                                                                                                                                                Difference




                                                                                                                                                                                                                                                            Difference




                                                                                                                                                                                                                                                                                        Difference




                                                                                                                                                                                                                                                                                                                    Difference
                                   Scenario




                                                                  Scenario




                                                                                                Scenario




                                                                                                                               Scenario




                                                                                                                                                              Scenario




                                                                                                                                                                                         Scenario




                                                                                                                                                                                                                    Scenario




                                                                                                                                                                                                                                               Scenario




                                                                                                                                                                                                                                                                           Scenario




                                                                                                                                                                                                                                                                                                       Scenario
                      pumpage



Fla.    Baker            2.4       2.4           0.0              2.4          0.0              2.4           0.0              2.4             0.0            2.4          0.0           2.4          0.0           2.4          0.0           2.4           0.0           2.4           0.0           2.4           0.0
        Columbia         6.0       6.6            .5              6.0          0.0              6.0           0.0              6.0             0.0            6.0          0.0           6.0          0.0           6.0          0.0           6.0           0.0           6.0           0.0           6.0           0.0
        Duval          144.6     149.9           5.3            144.6          0.0            144.6           0.0            144.6             0.0          144.6          0.0         144.6          0.0         144.6          0.0         144.6           0.0         144.6           0.0         144.6           0.0
        Hamilton         0.5       0.5         – 0.0              0.5          0.0              0.5           0.0              0.5             0.0            0.5          0.0           0.5          0.0           0.5          0.0           0.5           0.0           0.5           0.0           0.5           0.0
        Nassau          47.4      46.8         – 0.6             47.4          0.0             47.4           0.0             47.4             0.0           47.4          0.0          47.4          0.0          47.4          0.0          47.4           0.0          47.4           0.0          47.4           0.0
Ga.     Appling          4.2       2.5         – 1.7              4.2          0.0              4.2           0.0              4.2             0.0            4.2        – 0.0           4.4          0.2           4.7          0.5           7.7           3.5           8.6           4.4          10.0           5.9
        Atkinson         2.9       1.6         – 1.3              2.9          0.0              2.9           0.0              2.9             0.0            2.9          0.0           2.9          0.0           2.9          0.0           2.9           0.0           2.9           0.0           2.9           0.0
        Bacon            4.0       2.2         – 1.8              4.0          0.0              4.0           0.0              4.0             0.0            2.4        – 1.6           2.6        – 1.5           2.8        – 1.2           2.5         – 1.5           3.2         – 0.9           3.5         – 0.6
        Ben Hill         8.2      12.3           4.1              8.2          0.0              8.2           0.0              8.2             0.0            8.2          0.0           8.2          0.0           8.2          0.0           8.2           0.0           8.2           0.0           8.2           0.0
        Berrien          6.1       5.3         – 0.8              6.1          0.0              6.1           0.0              6.1             0.0            6.1          0.0           6.1          0.0           6.1          0.0           6.1           0.0           6.1           0.0           6.1           0.0
        Bleckley         7.7       2.7         – 4.9              7.7          0.0              7.7           0.0              7.7             0.0            7.7          0.0           7.7          0.0           7.7          0.0           7.7           0.0           7.7           0.0           7.7           0.0
        Brantley         1.3       1.9           0.6              1.3          0.0              1.3           0.0              1.3             0.0            1.6          0.3           1.7          0.4           1.9          0.6         121.8        120.4          138.3        137.0          188.4        187.1
        Bryan            1.6       1.7           0.1              1.6          0.0              1.6           0.0              1.6             0.0            2.5          0.9           2.7          1.1           3.1          1.5           5.2           3.6          10.4           8.8          19.4         17.8
        Bulloch          6.0       5.4         – 0.7              6.0          0.0              6.0           0.0              6.0             0.0           10.5          4.5          11.3          5.3          12.6          6.6          27.0         21.0           23.9         17.9           31.3         25.3
        Burke           25.6       9.5        – 16.1             25.6          0.0             25.6           0.0             25.6             0.0           27.8          2.2          29.2          3.6          30.9          5.3          35.3           9.7          40.2         14.6           50.5         25.0
        Camden          50.6      46.0         – 4.6             14.9        –35.7             50.6           0.0             50.6             0.0           42.0        –8.6           44.8        –5.8           51.1          0.5          54.6           4.0          59.4           8.8          66.5         15.9
        Candler          3.2       1.9         – 1.3              3.2          0.0              3.2           0.0              3.2             0.0            2.7        – 0.4           2.9        – 0.3           3.0        – 0.1           0.2         –2.9            0.2         –2.9           22.8         19.7
        Charlton         1.3       1.0         – 0.3              1.3          0.0              1.3           0.0              1.3             0.0            1.5          0.2           1.6          0.3           1.8          0.5          52.5         51.3           52.8         51.6           53.5         52.2
        Chatham         71.4      74.4           3.1             71.4          0.0              0.0         –71.4             71.4             0.0           75.2          3.8          81.1          9.7          92.0        20.7           72.6           1.2          75.4           4.1          79.2           7.8
        Clinch           1.4       1.0         – 0.4              1.4          0.0              1.4           0.0              1.4             0.0            1.4          0.0           1.4          0.0           1.4          0.0           1.4           0.0           1.4           0.0           1.4           0.0
        Coffee          17.0       8.0          –8.9             17.0          0.0             17.0           0.0             17.0             0.0           17.0          0.0          17.0          0.0          17.0          0.0          17.0           0.0          17.0           0.0          17.0           0.0
        Crisp            9.9      11.8           2.0              9.9          0.0              9.9           0.0              9.9             0.0            9.9          0.0           9.9          0.0           9.9          0.0           9.9           0.0           9.9           0.0           9.9           0.0
        Dodge            4.4       4.7           0.4              4.4          0.0              4.4           0.0              4.4             0.0            4.4          0.0           4.4          0.0           4.4          0.0           4.4           0.0           4.4           0.0           4.4           0.0
        Dooly           21.6      10.5        – 11.1             21.6          0.0             21.6           0.0             21.6             0.0           21.6          0.0          21.6          0.0          21.6          0.0          21.6           0.0          21.6           0.0          21.6           0.0
        Echols           2.9       1.8          –1.1              2.9          0.0              2.9           0.0              2.9             0.0            2.9          0.0           2.9          0.0           2.9          0.0           2.9           0.0           2.9           0.0           2.9           0.0
        Effingham        4.7       4.5         – 0.2              4.7          0.0              4.7           0.0              4.7             0.0            5.5          0.9           6.0          1.3           6.8          2.1          15.0         10.4           17.7         13.0           24.3         19.7
        Emanuel          4.7       5.1           0.3              4.7          0.0              4.7           0.0              4.7             0.0            5.1          0.4           5.4          0.7           5.8          1.1           3.8         – 0.9           3.9         – 0.8          10.4         – 0.8
        Evans            1.8       1.5         – 0.3              1.8          0.0              1.8           0.0              1.8             0.0            3.3          1.5           3.6          1.8           4.1          2.2           2.1           0.2           2.2           0.4          29.9         28.1
        Glascock         0.3       0.3              0.0           0.3            0.0            0.3             0.0            0.3             0.0            0.3         0.0            0.3         0.0            0.3         0.0            0.3          0.0            0.3          0.0            0.3          0.0
        Glynn           61.2      61.8              0.6          61.2            0.0           61.2             0.0           61.2             0.0           63.2         2.0           67.6         6.4           77.3        16.1           67.1          5.9           72.1         10.9           78.9         17.7
        Irwin            7.2       6.6            – 0.6           7.2            0.0            7.2             0.0            7.2             0.0            7.2         0.0            7.2         0.0            7.2         0.0            7.2          0.0            7.2          0.0            7.2          0.0
        Jeff Davis       4.3       3.5            – 0.8           4.3            0.0            4.3             0.0            4.3             0.0            4.3         0.0            4.3         0.0            4.3         0.0            4.3          0.0            4.3          0.0            4.3          0.0
Table	1.. Simulated.pumpage.by.scenario.and.difference.from.Base.Case.(estimated.2000.pumpage).by.county.—Continued
[Difference is scenario pumpage minus Base Case (estimated 2000) pumpage; values rounded to the 0.1 million gallons per day; sum of Base Case pumpage and difference may not equal scenario
pumpage because of rounding. Scenarios: A, 1997 pumpage; B, Durango Paper Company pumping eliminated; C1, Chatham County pumping eliminated; C1, southern Beaufort County pumping eliminated;
D1, Regional Economic Models, Inc. projection; D2, County Comprehensive Water-Supply Plans, projection. –, minus; Fla., Florida; Ga., Georgia, S.C., South Carolina]
                                                                   Simulated	pumpage	by	scenario	and	difference	from	Base	Case	(estimated	2000	pumpage),	in	million	gallons	per	day

                        Base		                A                             B                             C1                             C2                      D1,	2010                   D1,	2020                   D1,	20                  D2,	2010                   D2,	2020                D2,	20
State   County          Case	




                                                  Difference




                                                                                Difference




                                                                                                               Difference




                                                                                                                                              Difference




                                                                                                                                                                         Difference




                                                                                                                                                                                                    Difference




                                                                                                                                                                                                                              Difference




                                                                                                                                                                                                                                                         Difference




                                                                                                                                                                                                                                                                                   Difference




                                                                                                                                                                                                                                                                                                            Difference
                                   Scenario




                                                                 Scenario




                                                                                               Scenario




                                                                                                                              Scenario




                                                                                                                                                             Scenario




                                                                                                                                                                                        Scenario




                                                                                                                                                                                                                   Scenario




                                                                                                                                                                                                                                             Scenario




                                                                                                                                                                                                                                                                        Scenario




                                                                                                                                                                                                                                                                                                Scenario
                      pumpage



Ga.     Jefferson        13.7      8.6          –5.1            13.7            0.0           13.7             0.0           13.7            0.0            13.7          0.0          13.7          0.0          13.7         0.0          13.7          0.0          13.7         0.0         13.7         0.0
        Jenkins           4.6      3.6         – 1.0             4.6            0.0            4.6             0.0            4.6            0.0             5.1          0.5           5.4          0.7           5.7         1.1           6.7          2.0           9.1         4.5         13.3         8.6
        Johnson           2.4      2.1         – 0.3             2.4            0.0            2.4             0.0            2.4            0.0             2.4          0.0           2.4          0.0           2.4         0.0           2.4          0.0           2.4         0.0          2.4         0.0
        Lanier            2.0      2.0           0.1             2.0            0.0            2.0             0.0            2.0            0.0             2.0          0.0           2.0          0.0           2.0         0.0           2.0          0.0           2.0         0.0          2.0         0.0
        Laurens           9.2      6.8          –2.5             9.2            0.0            9.2             0.0            9.2            0.0             9.2          0.0           9.2          0.0           9.2         0.0           9.2          0.0           9.2         0.0          9.2         0.0
        Liberty          15.7     16.1           0.4            15.7            0.0           15.7             0.0           15.7            0.0            18.1          2.3          19.5          3.8          22.3         6.6          22.2          6.5          24.1         8.4         27.0       11.3
        Long              0.8      0.3         – 0.5             0.8            0.0            0.8             0.0            0.8            0.0             0.9          0.2           1.0          0.3           1.2         0.4           4.9          4.1           4.8         4.0          5.4         4.6
        Mcintosh          0.9      1.1           0.2             0.9            0.0            0.9             0.0            0.9            0.0             1.2          0.3           1.3          0.4           1.5         0.6           2.3          1.5           3.0         2.2          3.9         3.0
        Montgomery        1.8      2.7           0.9             1.8            0.0            1.8             0.0            1.8            0.0             1.8          0.0           1.8          0.0           1.8         0.0           1.8          0.0           1.8         0.0          1.8         0.0
        Pierce            6.2      3.4          –2.8             6.2            0.0            6.2             0.0            6.2            0.0             5.8        – 0.4           6.2        – 0.0           6.6         0.4           5.7        – 0.5           6.8         0.6          7.9         1.7
        Pulaski          13.3      9.9          –3.4            13.3            0.0           13.3             0.0           13.3            0.0            13.3          0.0          13.3          0.0          13.3         0.0          13.3          0.0          13.3         0.0         13.3         0.0
        Screven          18.6      7.6        – 10.9            18.6            0.0           18.6             0.0           18.6            0.0            20.1          1.5          21.1          2.5          22.4         3.8          24.6          6.1          34.9        16.3         49.0       30.4
        Tattnall          3.8      3.9           0.1             3.8            0.0            3.8             0.0            3.8            0.0             6.4          2.6           6.8          3.0           7.3         3.5          34.3        30.5           53.6        49.8         60.9       57.1
        Telfair           4.4      7.2           2.7             4.4            0.0            4.4             0.0            4.4            0.0             4.4          0.0           4.4          0.0           4.4         0.0           4.4          0.0           4.4         0.0          4.4         0.0
        Tift              4.2      4.5           0.3             4.2            0.0            4.2             0.0            4.2            0.0             4.2          0.0           4.2          0.0           4.2         0.0           4.2          0.0           4.2         0.0          4.2         0.0




                                                                                                                                                                                                                                                                                                                         Ground-Water	Flow	and	Solute-Transport	Model	 	 
        Toombs            7.0      4.4          –2.5             7.0            0.0            7.0             0.0            7.0            0.0             6.8        – 0.2           7.2          0.2           7.7         0.7          17.2        10.2           20.9        13.9         26.4       19.4
        Treutlen          1.2      1.4           0.2             1.2            0.0            1.2             0.0            1.2            0.0             1.2          0.0           1.2          0.0           1.2         0.0           1.2          0.0           1.2         0.0          1.2         0.0
        Turner            3.0      3.4           0.4             3.0            0.0            3.0             0.0            3.0            0.0             3.0          0.0           3.0          0.0           3.0         0.0           3.0          0.0           3.0         0.0          3.0         0.0
        Ware              8.5      6.0          –2.5             8.5            0.0            8.5             0.0            8.5            0.0             8.0        – 0.5           8.6          0.2           9.5         1.0          11.7          3.2          13.9         5.4         19.5       11.0
        Washington       18.1     16.9         – 1.2            18.1            0.0           18.1             0.0           18.1            0.0            18.1          0.0          18.1          0.0          18.1         0.0          18.1          0.0          18.1         0.0         18.1         0.0
        Wayne            63.5     63.6           0.1            63.5            0.0           63.5             0.0           63.5            0.0            65.0          1.5          72.3          8.8          85.6        22.1          64.0          0.5          60.5        –3.0         62.7       – 0.8
        Wheeler           1.2      2.6           1.4             1.2            0.0            1.2             0.0            1.2            0.0             1.2          0.0           1.2          0.0           1.2         0.0           1.2          0.0           1.2         0.0          1.2         0.0
        Wilcox           17.3      9.9          –7.4            17.3            0.0           17.3             0.0           17.3            0.0            17.3          0.0          17.3          0.0          17.3         0.0          17.3          0.0          17.3         0.0         17.3         0.0
S.C.    Allendale         9.6      9.9           0.3             9.6            0.0            9.6             0.0            9.6            0.0             9.6          0.0           9.6          0.0           9.6         0.0           9.6          0.0           9.6         0.0          9.6         0.0
        Bamberg           6.3      4.0          –2.3             6.3            0.0            6.3             0.0            6.3            0.0             6.3          0.0           6.3          0.0           6.3         0.0           6.3          0.0           6.3         0.0          6.3         0.0
        Barnwell          7.5      4.9          –2.6             7.5            0.0            7.5             0.0            7.5            0.0             7.5          0.0           7.5          0.0           7.5         0.0           7.5          0.0           7.5         0.0          7.5         0.0
        Beaufort         21.7     30.5           8.8            21.7            0.0           21.7             0.0            4.9         – 16.8            21.7          0.0          21.7          0.0          21.7         0.0          21.7          0.0          21.7         0.0         21.7         0.0
        Colleton          0.5      0.5         – 0.0             0.5          0.0              0.5           0.0              0.5            0.0             0.5         0.0            0.5         0.0            0.5         0.0            0.5    0.0                 0.5        0.0     0.5             0.0
        Hampton           8.6      6.0         –2.6              8.6          0.0              8.6           0.0              8.6            0.0             8.6         0.0            8.6         0.0            8.6         0.0            8.6    0.0                 8.6        0.0     8.6             0.0
        Jasper            3.4      2.1         – 1.2             3.4          0.0              3.4           0.0              3.4            0.0             3.4         0.0            3.4         0.0            3.4         0.0            3.4    0.0                 3.4        0.0     3.4             0.0
ALL                     815.2    741.7        –73.5            779.5        –35.7            743.9         –71.4            798.4         –  6.8
                                                                                                                                           1               829.0        13.8          858.3        43.1          911.8        96.6         1,105.3 290.0              1,184.3      369.1 1,388.9           573.7
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

MODFLOW model and its SUTRA-based counterpart were                     Hydrologic-Unit.Layering
satisfactorily calibrated, the SUTRA model was refined and
recalibrated in the Savannah–Hilton Head Island study area                 Both models comprise seven hydrologic units (fig. 3).
to create a transport model suitable for simulating observed           These include, in descending order:
saltwater intrusion at and near Hilton Head Island. The                         • the surficial aquifer system (unit 1),
SUTRA model was calibrated to estimated chloride values
during 2000, 2002, 2003, and 2004, assuming a transient stress                  • Brunswick aquifer system confining unit (unit 2),
response, using variable time-step lengths. Payne and others
                                                                                • Upper and Lower Brunswick aquifers grouped
(2005) and Provost and others (2006) describe details of the
                                                                                  together to form the Brunswick aquifer system
model development process for the MODFLOW model and
                                                                                  (unit 3),
the SUTRA model, respectively.
                                                                                • Upper Floridan aquifer confining unit (unit 4),
Model	Framework                                                                 • Upper Floridan aquifer (unit 5),
      In general, the framework, including hydrologic unit lay-                 • Lower Floridan aquifer confining unit (unit 6), and
ering, distribution of hydraulic conductivity and permeability,
pumpage distribution, and model extent are common to both                       • Lower Floridan aquifer (unit 7).
models. The model boundaries cover approximately the same              In areas where the Brunswick aquifer system is absent, the Upper
area, comprising about 42,155 mi2 (fig. 1). The models gener-          Floridan aquifer is separated from the surficial aquifer system by a
ally differ in discretization, calibration conditions, and to some     composite of confining units 2, 3, and 4. Payne and others (2005)
degree, in distribution of hydraulic properties and boundary           and Provost and others (2006) describe in detail the thickness,
conditions. Provost and others (2006) describe in detail simi-         extent, and other hydraulic properties of these units.
larities and differences between the two models.
                                                                                                                     Simulated
                                                                     Onshore                        Offshore         hydrologic
                                                                                                                        unit

                                                                   SURFICIAL AQUIFER SYSTEM                            Unit 1

                                                                                                                       Unit 2

                                                                           BRUNSWICK                                   Unit 3
                                                                         AQUIFER SYSTEM
                                                                                                                       Unit 4

                                                  UPPER FLORIDAN AQUIFER                                               Unit 5

                                                                                                                       Unit 6


                                                  LOWER FLORIDAN AQUIFER                                               Unit 7




                                                        EXPLANATION
                       Aquifer

                       Confining unit

                       General-head (MODFLOW) or specified-pressure (SUTRA) boundary — Represents
                          boundary condition that provides flow to underlying confined aquifers. For MODFLOW,
                          flow is restricted by a conductance term, which is equivalent to hydraulic conductivity
                         Onshore area —Water-table head (MODFLOW) or hydrostatic freshwater pressure (SUTRA)
                         Offshore area —Equivalent freshwater head of overlying saltwater column (MODFLOW)
                                           or hydrostatic seawater pressure (SUTRA)
                       No-flow boundary

                       No-flow or fixed-head (MODFLOW) or specified-pressure (SUTRA) boundary

               Figure	.. Schematic.diagram.showing.model.aquifers.and.confining.units.and.boundary.conditions.for.
               MODFLOW.and.SUTRA.models.(modified.from.Payne.and.others,.2005;.Provost.and.others,.2006).
                                                                              Ground-Water	Flow	and	Solute-Transport	Models	 	 


Hydraulic.Properties                                                         In the MODFLOW model, the southern and southwest-
                                                                       ern lateral boundaries are set as specified head for the units
      The models are designed to have a similar distribu-              representing the Upper Floridan aquifer, the Lower Floridan
tion of hydraulic properties that control ground-water flow.           aquifer, and the intervening confining unit. The head is set
MODFLOW uses hydraulic conductivity, and SUTRA uses                    as uniform for each vertical stack of cells, using values for
permeability and additional hydraulic parameters that control          Upper Floridan aquifer head estimated from potentiometric-
solute transport. The hydraulic conductivity distribution from         surface maps. For the SUTRA model, a corresponding pres-
the calibrated MODFLOW model was converted directly into               sure was calculated and applied to nodes representing the
permeability for the SUTRA model. During calibration of the            Upper Floridan aquifer at this boundary. Pressure is set at
SUTRA model, the permeability distribution was modified for            each of the nodes in a vertical stack assuming hydrostatic
the Savannah–Hilton Head Island study area. The hydraulic              conditions in the middle of the Upper Floridan aquifer.
conductivity distribution of the MODFLOW model is shown                      In the MODFLOW model, the top boundary is set as a
in figure 4 to illustrate generally the distribution of hydraulic      head-dependent flux (or general-head) boundary condition,
properties in the models.                                              with a controlling specified head and a conductance term that
                                                                       regulates the flux into the top layer of the model. The control-
Spatial.Discretization                                                 ling head is the water-table altitude in the onshore area, and
                                                                       the freshwater equivalent of sea-level altitude (NAVD 88) in
      The finite-difference technique used by MODFLOW                  the offshore area. In the onshore area, the conductance was
requires that the simulated area be divided into discrete cells,       calibrated to limit the amount of recharge entering the system
with uniform properties throughout each cell. The MODFLOW              in any given grid cell to less than the maximum estimated
model is horizontally discretized using a variably spaced              recharge from baseflow estimates, 10 inches (Payne and others,
grid, with cell sizes ranging from about 4,000 by 5,000 feet (ft)      2005). For the purpose of simplification, the conductance
to 16,500 by 16,500 ft (fig. 5A). Grid density is higher at            imposed in the offshore area is large, posing minimal resis-
Savannah and Brunswick to enable simulation of steeper head            tance to flow in or out of the system, because little is known
gradients near areas of concentrated pumping and to facilitate         about hydraulic properties in the offshore area.
linkage with smaller-scale, solute-transport models being                    The top boundary for the SUTRA model is set as speci-
developed in those areas. Each hydrologic unit is represented          fied pressure, assuming a freshwater hydrostatic water table
with one layer of grid cells in the vertical dimension.                in the onshore area, and seawater hydrostatic sea level in the
      The finite-element technique used by SUTRA (Voss and             offshore area. The permeability distribution of the confining
Provost, 2003) requires that the simulated area be divided into        units above the Upper Floridan aquifer was adjusted in the
discrete elements, with nodes at each corner of an element. In         SUTRA model to account for the resistance associated with
the SUTRA model, the finite-element mesh is refined laterally          the general-head boundary of the MODFLOW model.
in the Savannah–Hilton Head Island study area to allow more
detailed representation of the pumping and head distributions
and coarsened elsewhere to minimize the number of elements             Pumpage.Distribution
and nodes, and thus the computational demands of the model                   Pumpage data and the method used to derive the spatial
(fig. 5B). The lateral discretization is further refined in selected   distribution of pumpage for model calibration (Taylor and
areas where saltwater intrusion into the Upper Floridan aquifer        others, 2003) are generally the same for both models, with the
has been observed (Camille Ransom III, South Carolina                  following exceptions: (1) the pumpage distribution is derived
Department of Health and Environmental Control, written                for more years for the SUTRA model than for the MODFLOW
commun., 2004). The Upper Floridan aquifer is discretized              model; and (2) the placement of pumping locations differs in
vertically into 10 elements, and the Lower Floridan aquifer is         the two models because of differences in spatial discretiza-
discretized vertically into 4 elements. The remaining units are        tion (figs. 6 and 7). For the steady-state MODFLOW model,
each discretized vertically into two elements. Along any given         county aggregate and site-specific data were used to estimate
vertical column of nodes, the vertical spacing between nodes is        average annual pumpage for 1980, 1997, and 2000. For the
uniform within each hydrologic unit. The mesh is constructed           transient SUTRA model, estimated industrial and public-supply
with 4,093 elements and 4,126 nodes in the horizontal dimen-           pumpage values were used to estimate the pumpage distributions
sion, and 24 elements and 25 nodes in the vertical direction.          for 1915, 1920, 1930, 1937, 1940, 1955, 1965, 1970, and 1975;
Element sizes range from about 0.003 mi2 to 774 mi2.                   county aggregate and site-specific data were used to estimated
                                                                       pumpage distributions for 1980, 1985, 1990, 1995, September
Boundary.Conditions                                                    1998, and 2000. Pumpage distributions were linearly interpo-
                                                                       lated for intervening years. Pumpage is assigned to hydrologic
     The bottom boundary for each model is a no-flow                   units 3 (Brunswick aquifer system), 5 (Upper Floridan aqui-
boundary. The lateral boundaries on all sides of each model,           fer), and 7 (Lower Floridan aquifer) for both models. Payne
except for the southern and southwestern sides, also are no-           and others (2005) and Provost and others (2006) describe in
flow boundaries.                                                       more detail pumpage distributions used for model calibration.
10	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models

    Upper Brunswick confining unit
    (unit 2)                                                                 Model b
                                                                                     oundary
                                    SC
                               GA
                                                              C2
                               C1                   C5
                                                                                                                        Surficial aquifer system
                                                                                              C4




                                                                              At
                                                                                                                                 (unit 1)




                                                                                la
                                                                                   tic
                                                                                                        N




                                                                                 n
                                                                                         O
                                                                                             ce
                                                                                                  an
                                                        C3
                                                        GA
                                                         FL

    Brunswick aquifer system                                                                                                                      Hydraulic
                                                                                                                              Unit     Zone      conductivity
    (unit 3)                                                                                                                                    (feet per day)
                                                               C2                                                               1      N/A          70
                             C1
                                                                                                                                2,4    C1            0.00017
                                                              B1                                  C4                                   C2            0.2
                                                                                                                                       C3            0.00001
                                                                                                                                       C4            0.0001
                                           C3                                                                                          C5            0.0001

                                                                                                                                3      B1           50
                                                                                                                                       C1            0.00017
    Upper Floridan confining unit                                                                                                      C2            0.2
                                                                                                                                       C3            0.00001
    (unit 4)                                                                                                                           C4            0.0001

                                                              C2                                                                5      F1          34
                               C1                   C5                                                                                 F2           2
                                                                                                                                       F3         100
                                                                                                       C4                              F4          70
                                                                                                                                       F5         394
                                                   C3                                                                                  F6       2,819
                                                                                                                                       F7         150
                                                                                                                                       F8       2,727
                                                                                                                                       F9         100
                                                                                                                                       F10         56
    Upper Floridan aquifer                                                                                                             F11         94
                                                                                                                                       F12         25
    (unit 5)
                                         F10                   F3                                                               6      N/A*          0.02

                                                   F4                           F12                                             7      N/A*         10

                             F1                                                                        F9
                                                   F5
                                                                                                                            *Not applicable— only one zone per unit

                                                                        F7
                               F2                                       F8
                                                   F6                            F11
                   EXPLANATION
          C1    Hydraulic-property zone
                Model-calibrated hydraulic
                 conductivity, in feet per day —                                                            Lower Floridan confining unit (unit 6)
                                                                               SOUTH
                 For purposes of simulation,              GEORGIA             CAROLINA
                 vertical and horizontal
                 conductivities are equal
                 0.00001 to 0.00017

                 0.02 to 0.2                                                                                   Lower Floridan aquifer (unit 7)

                 2 to 94                                      F L ORI
                                                                        DA

                 100 to 394

                 2,500 to 3,200


        Figure	.. Schematic.diagram.showing.simulated.hydraulic-property.zones.by.model.unit.for.the.MODFLOW.model.
        (modified.from.Payne.and.others,.2005).
                                                               Ground-Water	Flow	and	Solute-Transport	Models	 	 11

                                                 EM




     A. MODFLOW                           SC
                                         GA                                                                               SOUTH
                                                                                                            GEORGIA      CAROLINA




                                                                                                                             Savannah

                                                                                                                            Brunswick

                                                                                                                        Model
EM




                                                                                                                       boundary




                                                                                                   )
                                                                                               (EM
                                                                                            RY
                                                                                           NDA
                                                                                       BOU
                                                                                      LOW
                                                                                                             EXPLANATION




                                                                                     O-F
                                           GA                                                                MODFLOW




                                                                                 —N
                                          FL                                                                  Finite-difference grid cell




                                                                                DEL
                                                                                                              Active




                                                                                 O
                                                                              FM
                                                                                                              No-flow boundary



                                                                           EO
                                                                                                               (all units)

                                                                          EDG
                                                                                                              Specified-head boundary
                                                                                                               (units 5 – 7);
                                                                                                               No-flow boundary
                                                                                                               (units 1 – 4)


     B. SUTRA                                                                                                SUTRA
                                                                                                              Finite-element mesh

                                                                                                              No-flow node
                                                                                                               (all units)
                                                                                                              Specified-pressure node
                                                                                                               (units 5 –7);
                                                                                                               No-flow node;
                                                                                                               (units 1– 4)
                                                                      n
                                                                    ea
                                                                    Oc




                                                                                             N
                                                                tic
                                                              lan
                                                             At




                                                                                      0                25        50 MILES

                                                                                      0        25           50 KILOMETERS

                                                                                      Base from U.S. Geological Survey
                                                                                      1:100,000-scale digital data



     Figure	.. (A).Finite-difference.grid.from.MODFLOW.model.and.(B).finite-element.mesh.from.SUTRA.model.
     (modified.from.Payne.and.others,.2005;.Provost.and.others,.2006).
12	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

Several of the scenarios presented in this report are based on          A. Brunswick aquifer
2000 pumpage distributions (figs. 6 and 7).                                system (unit 3)
     For the MODFLOW model, the sum of site-specific and                                                              BRYAN
nonsite-specific pumpage values for 1980 and 2000 were                                                                   0.02
assigned to the model grid cell in which their respective
assigned locations and aquifers were situated. For the SUTRA
model, site-specific pumpage associated with a given well was                                                    LIBERTY
assigned to the vertical string of nodes that lies closest to the                                                       0.04
well (as measured within the horizontal plane) and is divided                                 SOUTH
equally among the nodes that lie between the top and bottom                GEORGIA           CAROLINA

surfaces of the aquifer to which the pumpage is attributed.
Outside of Jasper and Beaufort Counties, S.C., nonsite-
specific pumpage was located at the same points as in the                             Map                                0.01
                                                                                      area
                                                                                                                        MCINTOSH




                                                                                                        dary
MODFLOW model and was assigned to nodes in the same




                                                                                                    boun del
                                                                                                      Mo
manner as site-specific pumpage. Within Jasper and Beaufort                 F L ORI
                                                                                      DA
Counties, S.C., the total nonsite-specific pumpage for each




                                                                                                                                          n
                                                                                                                                     Ocea
county was redistributed among the nodes of the finite-element                                                   GLYNN
                                                                           Approximate extent
mesh within the counties in the study area in proportion to the                                                      0.11




                                                                                                                                    ntic
                                                                              of Brunswick
area associated with each node. The area associated with each                aquifer system




                                                                                                                                   Atla
node was estimated by dividing the model volume associated
with the node by the vertical node spacing within the aquifer.
                                                                                                         CAMDEN

Model	Calibration                                                                            N
                                                                                                               0.06

      The regional MODFLOW model was calibrated to two
assumed stressed, steady-state conditions for 1980 and 2000,
                                                                       0           10         20 MILES
using water-level data for the Brunswick aquifer system and
the Upper and Lower Floridan aquifers. The SUTRA model
                                                                       0      10       20 KILOMETERS
was constructed as similarly as possible to the MODFLOW
                                                                       Base from U.S. Geological Survey
model and was initially calibrated assuming a uniform density          1:100,000-scale digital data
distribution and steady-state flow conditions to Upper Floridan
aquifer water levels in Beaufort, Hampton, and Jasper Coun-                                      EXPLANATION
ties, S.C., for September 1998 stress conditions. The SUTRA                           Total estimated pumpage per county —
model was then modified to account for variable-density solute                         In million gallons per day,
transport, run as a transient simulation from a predevelopment                         Brunswick aquifer system (unit 3)
(1885) steady-state flow field to 2004, and calibrated to esti-                                  Less than 0.025
mated chloride values during 2000, 2002, 2003, and 2004.                                         0.025 to 0.1
      After both models were calibrated and the scenario
                                                                                                 0.1 to 0.15
simulations were run, an error was discovered in pumpage
attributed to Durango Paper Company wells in St. Marys,
                                                                    Figure	.. Distribution.of.ground-water.pumpage.by.model.unit.
Camden County, Ga. This error results in a 20-percent over-
                                                                    for.the.MODFLOW.model,.2000,.for.the.(A).Brunswick.aquifer.
estimate in the sum of pumpage for these wells during 2000,
                                                                    system.(unit.3),.(B).Upper.Floridan.aquifer.(unit.5),.(C).Upper.
compared with reported values. To test the effect of this error
                                                                    Floridan.aquifer.(unit.5)—enlarged.view,.and.(D).Lower.Floridan.
on model and scenario results, the 2000 MODFLOW model
                                                                    aquifer.(unit.7)..Values.rounded.to.0.01.million.gallons.per.day,.
was corrected and recalibrated. The resulting corrected and
                                                                    values.may.differ.from.values.shown.in.table.2.and.figure.11.
recalibrated model simulated heads a maximum of 2–3 ft
                                                                    because.of.rounding.(modified.from.Payne.and.others,.2005).
higher than the original model in the area of the Durango
wells. The calculated water-level recovery associated with
the elimination of pumping at the Durango wells, however,
was approximately the same in the uncorrected and corrected
models. Thus, although the absolute simulated water levels
differ for the uncorrected and corrected models, the responses
to the scenarios, in terms of change in water levels, were prin-
cipally the same for the uncorrected and corrected models, and
the original calibrated (uncorrected) model was used.
                                                                                  Ground-Water	Flow	and	Solute-Transport	Models	 	 1




                                      B. Upper Floridan aquifer (unit 5)
                    SOUTH
  GEORGIA          CAROLINA


                                                                         SC
                                                                        GA

    F L OR
             ID
               A




                                                                                                                           Mo
                                                                                                                              d el
                                                                                                                                bo
                                                                                                                                  un
                                                                                                                                     da
                                                                                                                                     ry
                                                                                                                           an




                                                                                                                      ce
                                                                                                                      O
                                                                                                                   ic
                                                                                                                 nt
                                                                                                                 la
                                                                                                             At
                                                                                                  EXPLANATION
                                                                                              Pumpage per grid cell—
                                                                                               In million gallons per day,
                                                                                               during 2000, Upper Floridan
                                                                                               aquifer (unit 5)

                                                                                                          0 to 0.5
                                                                                                          0.5 to 1
                                                                                                          1 to 5

                                                       GA                                                 5 to 10
                                  N                     FL                                                10 to 51




                              0                25            50 MILES
                                                                              Base from U.S. Geological Survey
                              0           25        50 KILOMETERS             1:100,000-scale digital data


Figure	.. Distribution.of.ground-water.pumpage.by.model.unit.for.the.MODFLOW.model,.2000,.for.the.
(A).Brunswick.aquifer.system.(unit.3),.(B).Upper.Floridan.aquifer.(unit.5),.(C).Upper.Floridan.aquifer.(unit.5).—
enlarged.view,.and.(D).Lower.Floridan.aquifer.(unit.7).(modified.from.Payne.and.others,.2005)—.continued.
1	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

              C. Upper Floridan aquifer (unit 5) —enlarged view


                                                                                                    EXPLANATION

                                                                                S
                                                                                             Pumpage per grid cell—
                                                                               GA C           In million gallons per day,
                                                                                              during 2000, Upper Floridan
                                                                                              aquifer (unit 5)

                                                                                                       0 to 0.5
                                                                                                       0.5 to 1
                                                                                                       1 to 5




                                                                            n
                                                                          ea
                                                                                                       5 to 10




                                                                          Oc
                                                                      tic
                                                                                                       10 to 51



                                                                     an
                                                                    Atl




                                   SOUTH
           GEORGIA                CAROLINA



                          Area
                         of map           Savannah




                                                                                                                                                   c ean
                         above
             bo
             Mo nda




                                     Brunswick
                u
                 de ry




                                                                                                                                       A tlantic O
                                     St Marys
                   l




                                     Fernandina Beach
             F L ORI                 Jacksonville
                       DA
                                                                                                       GA
                            Area of map                                                                FL
                              at right




                                                                     Approxim
                                                         N                   ate mode
                                                                                          l boundary


                                                        Base modified from U.S. Geological Survey                0       5   10   15   20 MILES
                                                        1:100,000-scale digital data
                                                                                                                 0   5   10 15 20 KILOMETERS

         Figure	.. Distribution.of.ground-water.pumpage.by.model.unit.for.the.MODFLOW.model,.2000,.for.the.(A).Brunswick.
         aquifer.system.(unit.3),.(B).Upper.Floridan.aquifer.(unit.5),.(C).Upper.Floridan.aquifer.(unit.5)—enlarged.view,.and..
         (D).Lower.Floridan.aquifer.(unit.7).(modified.from.Payne.and.others,.2005)—continued.
                                                                                   Ground-Water	Flow	and	Solute-Transport	Models	 	 1

      D. Lower Floridan aquifer (unit 7)


                                                                                                EXPLANATION
                                                                       S                  Pumpage per grid cell—
                                                                      GA C                 In million gallons per day,
                                                                                           during 2000, Lower Floridan
                                                                                           aquifer (unit 7)

                                                                                                   0 to 0.5
                                                                                                   0.5 to 1
                                                                                                   1 to 5




                                                                   n
                                                                 ea
                                                                                                   5 to 10




                                                                 Oc
                                                             tic
                                                                                                   10 to 51




                                                            an
                                                           Atl




                          SOUTH
  GEORGIA                CAROLINA



                 Area
                of map           Savannah




                                                                                                                                              cean
                above
    bo
    Mo nda




                            Brunswick
       u
        de ry




                                                                                                                                   Atlantic O
                            St Marys
          l




                            Fernandina Beach
    F L ORI                 Jacksonville
              DA
                                                                                                  GA
                   Area of map
                                                                                                  FL
                     at right




                                                N           Approxima
                                                                             te model b
                                                                                       oundary

                                               Base modified from U.S. Geological Survey                    0       5   10   15   20 MILES
                                               1:100,000-scale digital data
                                                                                                            0   5   10 15 20 KILOMETERS

Figure	.. Distribution.of.ground-water.pumpage.by.model.unit.for.the.MODFLOW.model,.2000,.for.the.(A).Brunswick.
aquifer.system.(unit.3),.(B).Upper.Floridan.aquifer.(unit.5),.(C).Upper.Floridan.aquifer.(unit.5)—enlarged.view,.and..
(D).Lower.Floridan.aquifer.(unit.7).(modified.from.Payne.and.others,.2005)—continued.
1	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

                    A. Brunswick aquifer system (unit 3)


                                                                                SC
                                                                              GA




                                                                                                                           M
                                                                                                                            od
                                                                                                                            el
                                                                                                                              bo
                                                                                                                                 un
                                                                                                                                  da
                                                                                                                                   ry
                                                                                                                  Approximate
                                                                                                                   limit of Brunswick
                                                                                                                   aquifer system




                                                                                                                      an
                                                                                                                  Oce
                                                                                                                ntic
                                                                                                               Atla
                                                               GA
                             N                                 FL




                                                           0             30             60 MILES

                                                           0        30        60 KILOMETERS

                                                           Base from U.S. Geological Survey
                                                           1:100,000-scale digital data


                                                                                     EXPLANATION
                                                        SOUTH
                                                                                 Pumpage at mesh node —
                                       GEORGIA         CAROLINA                   In million gallons per day
                                                                                  during 2000, Brunswick
                                                                                  aquifer system (unit 3)
                                                    Map                                 0 to 0.0005
                                                    area
                                                                                        0.0005 to 0.001
                                                                                        0.001 to 0.005
                                          FLO
                                                R                                       0.005 to 0.01
                                                ID




                                                                                        0.01 to 0.04
                                                  A




                  Figure	.. Distribution.of.ground-water.pumpage.by.model.unit.for.the.SUTRA.model,.2000,..
                  for.the.(A).Brunswick.aquifer.system.(unit.3);.(B).Upper.Floridan.aquifer.(unit.5),.model.area;..
                  (C).Upper.Floridan.aquifer.(unit.5),.study.area;.(D).Lower.Floridan.aquifer.(unit.7),.model.area;..
                  and.(E)..Lower.Floridan.aquifer.(unit.7),.study.area.
                                                                       Ground-Water	Flow	and	Solute-Transport	Models	 	 1


B. Upper Floridan aquifer (unit 5) — model area


                                                               SC
                                                             GA




                                                                                                  M
                                                                                                   od
                                                                                                   el
                                                                                                    bo
                                                                                                        un
                                                                                                        da
                                                                                                         ry
                                                                                             an
                                                                                         Oce
                                                                                       ntic
                                                                                      Atla
                                              GA
            N                                 FL




                                          0             30            60 MILES

                                          0        30        60 KILOMETERS

                                          Base from U.S. Geological Survey
                                          1:100,000-scale digital data


                                                                    EXPLANATION
                                                               Pumpage at mesh node —
                                       SOUTH
                      GEORGIA         CAROLINA                  In million gallons per day
                                                                during 2000, Upper Floridan
                                                                aquifer (unit 5)
                                   Map                                 0 to 0.01
                                   area
                                                                       0.01 to 0.10
                                                                       0.10 to 1.0
                         FLO
                               R                                       1.0 to 10
                               ID




                                                                       10 to 84
                                 A




    Figure	.. Distribution.of.ground-water.pumpage.by.model.unit.for.the.SUTRA.model,.2000,..
    for.the.(A).Brunswick.aquifer.system.(unit.3);.(B).Upper.Floridan.aquifer.(unit.5),.model.area;..
    (C).Upper.Floridan.aquifer.(unit.5),.study.area;.(D).Lower.Floridan.aquifer.(unit.7),.model.area;..
    and.(E).Lower.Floridan.aquifer.(unit.7),.study.area—continued.
1	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models

  C. Upper Floridan aquifer (unit 5) —study area

                      SC
                 GA                                                                                                            SOUTH
                                                                                                            GEORGIA           CAROLINA

                                               JASPER
                                                                                                                         Study
                                                                                                                         area
                                                                           BEAUFORT



                                                                                                                               Model
                                                                                                                             boundary
       EFFINGHAM




            CHATHAM              Savannah




            BRYAN
  N




      LIBERTY

  0         5              10     15 MILES


  0     5       10     15 KILOMETERS
  Base from U.S. Geological Survey
  1:100,000 and 1:250,000-scale data




                                EXPLANATION                                                                                                                    d
                                                                                                                                                          an
                                                                                                                                                d   Isl
                            Pumpage at mesh node —                                                                                           ea
                                                                                                                                        nH
                                                                                                                                                                        n




                             In million gallons per day
                                                                                                                                                                        ea




                                                                                                                                  lto
                             during 2000, Upper Floridan                                                                     Hi
                                                                                                                                                                       Oc




                             aquifer (unit 5)
                                                                                                                                                                   ic
                                                                                                                                                           nt




                                   0 to 0.01
                                                                                                                                                          la
                                                                                                                                                     At




                                   0.01 to 0.10
                                   0.10 to 1.0
                                   1.0 to 10
                                   10 to 84                                                                  0       1            2            3                   4    5 MILES

                                                                                                             0   1       2    3          4      5 KILOMETERS



                           Figure	.. Distribution.of.ground-water.pumpage.by.model.unit.for.the.SUTRA.model,.2000,..
                           for.the.(A).Brunswick.aquifer.system.(unit.3);.(B).Upper.Floridan.aquifer.(unit.5),.model.area;..
                           (C).Upper.Floridan.aquifer.(unit.5),.study.area;.(D).Lower.Floridan.aquifer.(unit.7),.model.area;..
                           and.(E).Lower.Floridan.aquifer.(unit.7),.study.area—continued.
                                                                       Ground-Water	Flow	and	Solute-Transport	Models	 	 1

D. Lower Floridan aquifer (unit 7) —model area


                                                              SC
                                                             GA




                                                                                                  M
                                                                                                   od
                                                                                                   el
                                                                                                    bo
                                                                                                        un
                                                                                                        da
                                                                                                         ry
                                                                                             an
                                                                                         Oce
                                                                                       ntic
                                                                                      Atla
                                              GA
            N                                 FL




                                          0             30            60 MILES

                                          0        30        60 KILOMETERS

                                          Base from U.S. Geological Survey
                                          1:100,000-scale digital data


                                                                   EXPLANATION
                                       SOUTH                   Pumpage at mesh node —
                     GEORGIA          CAROLINA                  In million gallons per day
                                                                during 2000, Lower Floridan
                                                                aquifer (unit 7)
                                   Map                                 0 to 0.01
                                   area
                                                                       0.01 to 0.10
                                                                       0.10 to 1.0
                         FLO
                               R                                       1.0 to 10
                               ID




                                                                       10 to 38
                                 A




  Figure	.. Distribution.of.ground-water.pumpage.by.model.unit.for.the.SUTRA.model,.2000,..
  for.the.(A).Brunswick.aquifer.system.(unit.3);.(B).Upper.Floridan.aquifer.(unit.5),.model.area;..
  (C).Upper.Floridan.aquifer.(unit.5),.study.area;.(D).Lower.Floridan.aquifer.(unit.7),.model.area;..
  and.(E).Lower.Floridan.aquifer.(unit.7),.study.area—continued.
20	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

        E. Lower Floridan aquifer (unit 7) — study area

                            SC                                                                                             SOUTH
                       GA                                                                                   GEORGIA       CAROLINA



                                             JASPER                                                                   Study
                                                                                                                      area

                                                                                BEAUFORT

                                                                                                                          Model
                                                                                                                        boundary


             EFFINGHAM                                                                                      EXPLANATION
                                                                                                        Pumpage at mesh node —
                                                                                                         In million gallons per day
                                                                                                         during 2000, Lower Floridan
                                                                                                         aquifer (unit 7)
                                                                                         Hilton
                                                                                       Head                     0 to 0.01
                                                                                     Island                     0.01 to 0.10
                                                                                                                0.10 to 1.0
                                                                                                                1.0 to 10
                  CHATHAM        Savannah



                                                                                       n
                                                                                      ea
                                                                                  Oc                  Figure	.. Distribution.of.ground-water.
                                                                                                      pumpage.by.model.unit.for.the.SUTRA.
                                                                                ic


                                                                                                      model,.2000,.for.the.(A).Brunswick.aquifer.
                                                                            nt
                                                                           la




                                                                                                      system.(unit.3);.(B).Upper.Floridan.aquifer.
                                                                           At




                                                                                                      (unit.5),.model.area;.(C).Upper.Floridan.
                 BRYAN
                                                                                                      aquifer.(unit.5),.study.area;.(D).Lower.
         N
                                                                                                      Floridan.aquifer.(unit.7),.model.area;.and.
                                                          0       5          10            15 MILES
                                                                                                      (E).Lower.Floridan.aquifer.(unit.7),.study.
                                                                                                      area—continued.
             LIBERTY                                      0   5       10    15 KILOMETERS


        Base from U.S. Geological Survey
        1:100,000 and 1:250,000-scale data


Base	Case	Simulations:	Representation	of	the	                               an apparent similarity in predicted potentiometric surfaces for
                                                                            the Upper and Lower Floridan aquifers. Although the general
Flow	System	during	2000                                                     similarity of simulated water levels indicates an interaquifer
      Simulation results for all scenarios are compared with                leakage response, Lower Floridan aquifer water-level data with
those calculated for 2000, herein called the “Base Case.” For               which to calibrate the model are sparse, and there are few or no
the MODFLOW model, Base Case results are the product of                     data to estimate the hydraulic conductivity of the Lower Flori-
a steady-state simulation for 2000 pumping conditions. For                  dan aquifer or the overlying confining unit (Payne and others,
the SUTRA model, Base Case results are the product of a                     2005). Additional information on the hydraulic properties of
transient simulation using a time-varying, estimated pumping                the Lower Floridan aquifer and the overlying confining unit are
history from predevelopment to 2000, evaluated during 2000.                 necessary to simulate more accurately the interaction between
      The regional MODFLOW model was calibrated to 2000                     the Upper and Lower Floridan aquifers. Details on calibrated
conditions. Major features in the potentiometric surface in the             model fit are provided in Payne and others (2005). In the Savan-
model area simulated by the MODFLOW model include the fol-                  nah–Hilton Head Island study area, the predominant feature in
lowing: (1) a large cone of depression in the Chatham County,               the potentiometric surface simulated by the SUTRA model is
Ga., area, centered at Savannah; (2) a smaller cone of depres-              the cone of depression centered in Savannah (fig. 9). Smaller
sion at Duval County, Fla., with the depression extending north             potentiometric depressions and mounds superimposed on this
to the southern part of Camden County, Ga.; (3) broadly spaced              surface represent localized pumping and recharge areas, respec-
potentiometric contours in the southwestern part of the model               tively. Differences between potentiometric surfaces simulated
area indicating a low head gradient; and (4) closely spaced                 by the regional MODFLOW model and the Savannah–Hilton
potentiometric contours in the center of the model in the Gulf              Head Island solute-transport model are the result of different
Trough area, representing a steep head gradient (fig. 8). Results           spatial discretization, calibration, and fundamental differences
for the Base Case simulation, as from all scenarios, also show              in simulators (Provost and others, 2006).
                                                                                                                                                                                     Ground-Water	Flow	and	Solute-Transport	Models	 	 21

A. Brunswick aquifer system
   (unit 3)
                                                        SC                                                                                                                                                         EXPLANATION
                                                      GA
                                                                                                                                                                                         50        Simulated potentiometric contour —




                                                                                                                                                                 Mo
                                                                                                                                                                                                     Shows simulated potentiometric surface




                                                                                                                                                                   de
                                                                                                                                                                                                     during 2000, in feet. Hachures indicate




                                                                                                                                                                     lb
                                                                                                                                                                       ou
                                                                                                                                                                                                     depression. Contour intervals 10 and




                                                                                                                                                                         nd
                                                                                                                                                                                                     50 feet. Datum is NAVD 88




                                                                                                                                                                           ar
                                                                                                                                                                              y
                                               0
                                             15
                                                           50 0

                                               0
                                                             3
                                             10


                                                      40
                                                                                                                                                             Approximate
                                                               20                                                                                             limit of Brunswick
                                                                 10                                                                  – 50
                                                                                                 –30                                                          aquifer system
                                                                                                 –20                                 – 40
                                                                                                 –10

                                                                                                                     0




                                                                                                                                                         an
                                                                                                                                                     Oce
                                                               10




                                                                                                                                                                       B. Upper Floridan aquifer

                                                                                                                                              ntic
                                                                                                                                             Atla
                                                                                                                                                                          (unit 5)
                                                                                                                                                                                    35
                                                                                                                                                                                        0
                                                                                                                                                                                                                        200

                                                                                                                                                                                            300                                 150
                                        GA
              N                         FL                                                                                                                                                   250                                            0
                                                                                                                                                                                                                                10


                                                                                                                                                                                                                                    50
                                                                                                                                                                                                                                                             30
                                                                                                                                                                     250                                                                                      10
                              0              25           50 MILES
                                                                                                                                                                              00                                                            l            l                       l       l
                                                                                                                                                                             2
                                                                                                                                                                                                                                l
                                                                                                                                                                                                                                    l
                                                                                                                                                                                                                                             l           l               l                       l l
                                                                                                                                                                      200
                                                                                                                                                                                                                                            l        l                               l
                                                                                                                                                                                                                                                                                                                  0
                                                                                                                                                                                                                                                                 l




                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                        l                                                    l
                                                                                                                                                                                                                                                                                                      l




                                                                                                                                                                                                                                l
                                                                                                                                                                                                                                                                             l

                              0         25     50 KILOMETERS




                                                                                                                                                                                                                                    l




                                                                                                                                                                                                                                                                                 l
                                                                                                                                                                                                                    l
                                                                                                                                                                                                                            l




                                                                                                                                                                                                                                                                                         l
                                                                                                                                                                                                                                                                                     l
                                                                                                                                                                                                                                l




                                                                                                                                                                                                                                                                                             l
                                                                                                                                                                                                                                      –100




                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                     –5




                                                                                                                                                                                                                                                                                     l
                                                                                                                                                                                                                                l
                                                                                                                                                                                                                    l
                                                                                                                                                                                                                                   – 0




                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                                                                                          l




                                                                                                                                                                                                                                                                                         l
                          Base from U.S. Geological Survey




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C. Lower Floridan aquifer                                                                                                                                                                               30
   (unit 7)                                                                                                                                                                                                                 0                                                                    10
                     35
                          0
                                                                    200
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                              300                                     150
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                                                                                                                                                                                                   GEORGIA         CAROLINA
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                                                           0        l                                                                            l
                                  40                                            l            l                   l               l l0
                0
             10
             50




                                                   20
                                                     30




                                                                                                                                                                                                                                Savannah
                                                                                                                                             10

                                                                                                                                                                                                    F L OR
                   50




                                                                                                                                                                                                             ID
                                                                                                                                                                                                               A



                                                                                                                                                                                   Figure	.. Simulated.potentiometric.surfaces.generated.by.
                                                                0                                                                                                                  the.MODFLOW.model,.2000,.for.the.(A).Brunswick.aquifer.
                                                                                                                                                                                   system.(unit.3);.(B ).Upper.Floridan.aquifer.(unit.5);.and..
                                                                    –1
                                                                        0




                                    50
                                                                                    10                                                                                             (C).Lower.Floridan.aquifer.(unit.7).(modified.from.Payne..
                                                                                                 20
                                                     –20
                                                           –30                                                                                                                     and.others,.2005).
22	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	


                                                                                                                                                                                        0
                                                         20




                                                                                                                                                                                                   0
                                                                                                                                                                                                                                                    SOUTH
                                     30            GA
                                                          SC
                                                                                  10                                                                                                                                                GEORGIA        CAROLINA
                                    EFFINGHAM
                                                                                              0                               JASPER                                                                                                          Study
                                                                                                                                                                                                                                              area

                                                                                                              –10
                                                                                                                                                                                                       BEAUFORT
                                                                                                                                                                                                                                                    Model
Figure	.. Potentiometric.                                                                                        –2                                                                             Po
                                                                                                                                                                                                   rt                                             boundary
                                                                                                                          0                                                                           Ro
surface.of.the.Upper.Floridan.                                    l
                                                                                  l                   l                                                                                                 ya
                                                                                                                                                                                                          l   So
                                                                                                                                                                                                                un
aquifer.generated.by.the.                                                                                                                                                                                         d




                                                                                                                                                                                                   nd
                                                     l                                                                    l
                                                                                                              –4
                                                                          l                       l




                                                                                                                                                                                                 sla
                                                                                                                0                                                                                                                         EXPLANATION
                                                              l                                                                               l
SUTRA.model,.2000.(modified.




                                                                                                                                                                                               dI
                                                                                              l

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                                                                                                              –6
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                                                                                                                                                                                            nH
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from.Provost.and.others,.2006).                                                                                           0
                                             l
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                                                                                                                                                                                                                                        Simulated potentiometric




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                                     BRYAN                                                                                                                                                                                                depression. Datum is
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                                                                                                                                                                                                              Oc
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                                                                                                                                                                                                                          0         5        10        15 MILES




                                                                                                                                                                                                        lan
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                                                                                                                                                                                                    At
                                                                                                                                                                                                                          0     5    10    15 KILOMETERS
                                          LIBERTY                                                                                                                                                                         Base from U.S. Geological Survey
                                                                                                                                                                                                                          1:100,000 and 1:250,000-scale digital data




Table	2.. Flow-budget.components.for.the.Base.Case,.2000.
[Results from MODFLOW model; in million gallons per day; —, not applicable]

                                           Inflow                                                                                                                                                                         Outflow

    Model		          From		           From		                From		                                                                                                                     To		                 To		                 To		
     unit         general-head	   general-head	           specified-	                                                                                                             general-head	        general-head	          specified-		         Discharge		
                                                                                                                                              Total	                                                                                                                Total	
                   boundary,		     boundary,	               head		                                                                                                                 boundary,		          boundary,		             head		              to	wells
                    onshore         	offshore             boundary                                                                                                                  onshore              offshore             boundary
       1              286.29         23.76                         —                                                                          310.05                                 115.67                   15.51              —                      —          131.18
       2               46.58          —                            —                                                                           46.58                                   3.62                     —                —                      —            3.62
       3               —              —                            —                                                                           —                                        —                       —                —                      0.24         0.24
       4               —              —                            —                                                                           —                                        —                       —                —                      —            —
       5              141.32          —                           712.39                                                                      853.71                                  22.28                     —               267.77                669.43       959.48
       6               —              —                             0.00                                                                        0.00                                    —                       —                 0.00                  —            0.00
       7               —              —                            15.46                                                                       15.46                                    —                       —                 2.32                128.67       130.99
Total all units       474.19         23.76                        727.86                                                          1,225.81                                           141.57                   15.51             270.09                798.34      1,225.51
Percent flow           38.7           1.9                          59.4                                                             100.0                                             11.6                     1.3               22.0                  65.1         100.0



     Flow-budget components calculated from the 2000                                                                                                                              Floridan aquifer. Specifically, the model is intended to
MODFLOW simulation show major components of recharge                                                                                                                              simulate the observed occurrence of saltwater intrusion in
to and discharge from the system, and are used as a base to                                                                                                                       the Upper Floridan aquifer at the northern end of Hilton
compare with the scenarios (table 2; fig. 10). The simulated                                                                                                                      Head Island, at Pinckney Island, and near the Colleton River,
water budget for the Base Case indicates that the system is                                                                                                                       S.C., for 2000 (fig. 11), yet the model does not preclude the
primarily recharged by inflow from the specified-head bound-                                                                                                                      simulated occurrence of saltwater intrusion in other areas. In
ary in unit 5, the Upper Floridan aquifer (table 2; fig. 10).                                                                                                                     addition, the SUTRA model was used to simulate the system
Most of the outflow from the system is discharge to wells,                                                                                                                        through 2100, maintaining the 2000 pumpage from 2000 to
primarily in unit 5.                                                                                                                                                              2100. Results serve as a comparison with other scenarios for
     The SUTRA model accounts for the hypothesized down-                                                                                                                          2010, 2020, 2035 and 2100 (fig. 12). Details of the SUTRA
ward leakage of saltwater from marine and estuarine sources                                                                                                                       model calibration to September 1998 water levels, and 2000,
through the Upper Floridan confining unit and the predomi-                                                                                                                        2002, 2003, and 2004 chloride distributions are in Provost and
nantly lateral flow along head gradients in the Upper                                                                                                                             others (2006).
                                                                                                                      Ground-Water	Flow	and	Solute-Transport	Models	 	 2

                                                     0.24
                                  128.67                                           Onshore                                       Offshore
                                669.43                                                                                                       Surficial aquifer system (unit 1)
                                                                               115.67                             15.51                      Brunswick aquifer system
                                                                                                                                               confining unit (unit 2)
                                 3.62                     4.42                                                            4.61
                                                                         286.29                              23.76                           Brunswick aquifer system (unit 3)
                                                                               12.65
                            46.58                  176.28                                                             11.32                  Upper Floridan confining unit (unit 4)
                                                          13.06
                                                                         234.18                                                              Upper Floridan aquifer (unit 5)
          22.28                                                               13.06
                                                   234.36                                                                                    Lower Floridan confining unit (unit 6)

                                712.39                    62.11                                                                              Lower Floridan aquifer (unit 7)
   141.32                                                               234.36

 267.77                             .004                                       62.07
                                                   177.63

             0                           15.46
                                                                         177.59

              2.32
                                                                                         EXPLANATION
                                        Ground-water flow, in million gallons per day, and budget component
    22.28                                            62.11                                                                15.46
                                                                                                                                                              669.43
            General-head                                        Interunit                                                        Lateral specified-                           Pumpage
141.32
             boundary                                             leakage                              2.32                        head boundary
                                                 177.63



 Figure	10.. Schematic.diagram.showing.simulated.flow-budget.components.and.values.
 for.the.Base.Case.(2000).(modified.from.Payne.and.others,.2005).




                                                                                          Parris Island
                                                                                                                                                           SOUTH
                                                                                                                                            GEORGIA       CAROLINA


                                                                               Po                                                                                      Map
            ol                                                                      rt                                                                                 area
          C




                 let                                                                     Ro                                                           Study
                       on                                                                     ya                                                      area
                            R                                                                      lS
                                                                                                        ou
                                                                                                             nd

                                                                                                                                                          Model
                                                                                                                                                        boundary
                                                            I
                                                         ey




                                                                                                                                             EXPLANATION
                                                     ckn
                                                    Pin




                                                                                                                                   Simulated chloride concentration, in
                                                                                                                                     milligrams per liter, in ground water
                                                                                                                                     in the middle of the Upper Floridan
                                                                    Hilton Head Island                                               aquifer for the Base Case (2000)
                                                                                                                                     250 to 2,000
                                                                                                                                     2,000 to 6,000
                                                                                                              n
                                                                                                             ea
                                                                                                         Oc




                                                                                                                                     6,000 to 10,000
                                                                                                   tic




                                                                                                                  N                  Greater than 10,000
                                                                                              la n
                                                                                          At




                                                                                                                                   Observation well

 Base from U.S. Geological Survey                     0         1         2         3              4          5 MILES
 1:100,000 and 1:250,000-scale data
                                                      0     1       2     3    4     5 KILOMETERS


 Figure	11.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper.Floridan.aquifer.in.the.
 Hilton.Head.Island,.South.Carolina,.area.for.the.Base.Case.(2000).(modified.from.Provost.and.others,.2006).
2	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

  A. 2000 pumpage during 2010                                                                B. 2000 pumpage during 2020

                                                                Parris Island




                                                      Po
           ol                                              rt
          C




                let                                             Ro
                      on                                             ya
                           R                                              lS
                                                                               ou
                                                                                    nd
                                           I
                                        ney
                                          ck
                                      Pin




                                               Hilton Head Island
                                                                                         n
                                                                                    ea
                                                                                Oc
                                                                           tic




                                                                                                                                                                          N
                                                                      la n
                                                                 At




 Base from U.S. Geological Survey                                                                                                  0       1       2       3   4    5 MILES
 1:100,000 and 1:250,000-scale data
                                                                                                                                   0   1       2   3   4   5 KILOMETERS


  C. 2000 pumpage during 2035                                                                                   SOUTH
                                                                                                 GEORGIA       CAROLINA


                                                                                                                          Map
                                                                                                                          area
                                                                                                           Study
                                                                                                           area



                                                                                                               Model
                                                                                                             boundary


                                                                                                                   EXPLANATION
                                                                                                 Simulated chloride concentration, in milligrams per
                                                                                                   liter (mg/L), in ground water in the middle of the
                                                                                                   Upper Floridan aquifer for 2000 pumping rates

                                                                                                  250 to 2,000
                                                                                                  2,000 to 6,000
                                                                                                  6,000 to 10,000
                                                                                                  Greater than 10,000

                                                                                                 Chloride concentration, in mg/L, in ground water
                                                                                                   in the middle of the Upper Floridan aquifer
                                                                                                   for Base Case (2000)
                                                                                                              250                10,000

                                                                                                 Observation well




 Figure	12.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper.Floridan.aquifer.in.the.Hilton.Head.
 Island,.South.Carolina,.area.for.2000.pumpage.during.(A).2010,.(B).2020,.(C).2035,.and.(D).2100.(larger.area;.modified.from.
 Provost.and.others,.2006).
                                                                             Ground-Water	Flow	and	Solute-Transport	Models	 	 2


D. 2000 pumping rates during 2100
                                                                                                                            Parris
                                                                                                                            Island
                                                                ol




                                                              C
                                                                  let
                                                                     on R
                                                                                                                        Por
                                                                                                                           tR
                                                                                                                             oy
                                                                                                                               al




                                                                                                                                     So
         JASPER




                                                                                                                                       un
                                                                                                                                         d
                                                                                                       I
                                                                                                    ey
                                                                                                 ckn
                                                 BEAUFORT




                                                                                              Pin
                                                                                        d
                                                                                      un
                                                                                    So
           Center of                                                                           Hilton




                                                                                    e
                                                                                gu
           cone of                                                                           Head




                                                                              bo
                                                                                        Island




                                                                               li
           depression




                                                                            Ca
                                                              Daufuskie
                                                                Island




                                                                                                           an
                                          SC




                                                                                                           ce
                                                                                                           O
                                                                                                      ic
 N   Savannah                        GA                                                            nt
                                                                                              la

                                                                                          t
             CHATHAM                                                                    A

Base from U.S. Geological Survey 1:100,000 and 1:250,000-scale data                                    0            2          4             6 MILES
Cone of depression from Peck and others, 1999

                                                                                                       0        2       4     6 KILOMETERS
                                                                     EXPLANATION
                                               Simulated chloride concentration,            Chloride concentration, in mg/L,
                                                 in milligrams per liter (mg/L), in           in ground water in the middle
                    SOUTH
      GEORGIA      CAROLINA                      ground water in the middle of                of the Upper Floridan aquifer
                                                 the Upper Floridan aquifer                   for Base Case (2000)
                                                 for 2000 pumpage
                              Map                                                                               250                  10,000
              Study           area
                                                          250 to 2,000
              area
                                                          2,000 to 6,000                                   Observation well
                                                          6,000 to 10,000
                   Model
                 boundary                                 Greater than 10,000



Figure	12.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper.Floridan.aquifer.
in.the.Hilton.Head.Island,.South.Carolina,.area.for.2000.pumpage.during.(A).2010,.(B).2020,.(C).2035,.and.
(D).2100.(larger.area;.modified.from.Provost.and.others,.2006)—continued..The.center.of.the.Upper.
Floridan.aquifer.potentiometric.cone.of.depression.identifies.the.most.concentrated.pumping.in.the.
Savannah,.Georgia–Hilton.Head.Island.area.
2	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	


Simulation	of	Ground-Water	                                                    are presented from the SUTRA model, which is more finely
                                                                               discretized than the regional MODFLOW model. Simulated
Management	Scenarios                                                           chloride distributions for the middle of the Upper Floridan
                                                                               aquifer are presented for Scenarios A, C1, C2, D1, and D2.
      The calibrated MODFLOW and SUTRA models were                                   Pumpage distributions for specific points in time for
used to provide insight into the potential effect of six different             all scenarios were based on the approach used in Payne and
pumping scenarios on regional ground-water flow and local                      others (2005), using procedures to assign county-aggregate
ground-water flow and solute transport in the Savannah–Hilton                  and site-specific data described in Taylor and others (2003).
Head Island area. The scenarios were designed to simulate                      For the Base Case (2000) and Scenario A (1997) simulations,
(1) the flow system during 1997, and during 2002 after a                       the MODFLOW model simulated 2000 and 1997 steady-state
major industrial well shutdown in Camden County, Ga.                           conditions, respectively, and the SUTRA model simulated
(Scenarios A and B, respectively); (2) the relative effects of                 the pumping history from predevelopment (1885) to 2000
eliminating pumping in Chatham County, Ga., and Beaufort                       and 1997, respectively. For all other scenarios, the pumpage
County, S.C. (Scenarios C1 and C2, respectively); and (3) the                  distributions are based on projected changes in pumpage after
effects of projected changes in the distribution and amount of                 2000. Future water-use distributions for Scenarios D1 and D2
ground-water withdrawal during 2000–2035 using two differ-                     were based on two methods of projection (Leeth and others,
ent methods to estimate future pumping (Scenarios D1 and                       2005). Projected water use was applied to existing withdrawal
D2). A brief description of and rationale for each scenario is                 locations as used for the 2000 simulations. Thus, although
presented in table 3.                                                          withdrawal rates were changed for these scenarios for years
      Simulated-head and flow-budget components, and differ-                   after 2000, the locations of pumping are unchanged.
ences in these relative to the Base Case, are presented for all                      The simulated per-county pumpage may differ from the
scenarios. For Scenarios C1 and C2, simulated-head distribu-                   estimated per-county pumpage for the following reasons (in
tions for the Savannah–Hilton Head Island study area also                      order of overall importance of contribution to the discrepancy).




Table	.. Description.of.conditions.for.pumping.scenarios.
[Mgal/d, million gallons per day; GaEPD, Georgia Environmental Protection Division; –, minus; Ga., Georgia; S.C., South Carolina]

                                                                                   Total	difference	 Regional		      Solute-        Model	results		
                                                                                     in	pumping	      flow	model		 transport	         presented		
 Scenario      Description                     Purpose                              relative	to	the	     results	 model	results	       for	years
                                                                                     Base	Case,	       presented	  presented	
                                                                                       (Mgal/d)      (MODFLOW)      (SUTRA) 1 2000 2010 2020 20 2100

 Base Case 2000 pumpage                        Reference simulation                      0.0            X             X             X   X   X    X    X

 A            1997 pumpage                     Conditions at time of                   –73.6            X             X         X
                                                 implementation of GaEPD interim
                                                 strategy
 B            2000 pumpage minus pumpage       Model response to major change in       –35.7            X                           X
                at sites in St. Marys, Ga.      localized stress
 C1           2000 pumpage minus pumpage       Effect of pumping in Chatham            –71.4            X             X             X
                in Chatham County, Ga.            County on flow system
 C2           2000 pumpage minus pumpage in Effect of pumping in southern              –16.8            X             X             X
                southern Beaufort County, S.C. Beaufort County on flow system
 D1           2010, 2020, 2035, and 2100       Projected water use based on             96.7            X             X             X   X   X    X    X
                pumpage based on Regional        economic forecasting,             (during 2035)
                Economic Models, Inc.1           24-coastal county area, Ga.
                projections to 2035
 D2           2010, 2020, 2035, and 2100       Projected water use based on            576.8            X             X             X   X   X    X    X
                pumpage for County Compre-       county water-supply plans,        (during 2035)
                hensive Water-Supply Plan2       24-coastal county area, Ga.
                projections to 2035
     1
      Fanning (2003)
     2
      Camp Dresser and McKee, Inc. (2001)
                                                                                                                          Simulation	of	Ground-Water	Management	Scenarios	 	 2

       • Discretization sometimes causes estimated pumpage                                                         Scenario	A:	Representation	of	the		
         from one county to be applied to another during simu-
         lation, if the grid cell is on the county boundary. For
                                                                                                                   Flow	System	during	1
         example, pumpage in county X is assigned to a grid                                                              Scenario A is intended to represent the flow system during
         cell for which the centroid is in county Y, so simulated                                                  1997, when the GaEPD implemented the “Interim Strategy for
         county X pumpage is apparently less than the original                                                     Managing Saltwater Intrusion in the Upper Floridan Aquifer
         estimate and simulated county Y pumpage is more.                                                          of Southeast Georgia” (Georgia Environmental Protection Divi-
       • At the edge of the model, several counties are only                                                       sion, 1997). Simulated heads for Scenario A, generated using
         partially included in the model, so pumpage applied                                                       the MODFLOW model, were compared with available water-
         to the model in these counties is less than the total                                                     level observations for 1997, and model fit was calculated. A
         estimated. This will cause total simulated pumpage                                                        comparison of simulation results from the Base Case (repre-
         to be less than total estimated pumpage.                                                                  senting 2000 conditions) (Payne and others, 2005) with those
                                                                                                                   from Scenario A illustrates changes in the flow system result-
       • Wells that are in inactive cells of a model unit are not                                                  ing from implementation of the interim strategy. Both MOD-
         included in total pumpage applied to the model. This                                                      FLOW and SUTRA results are presented for this scenario.
         will also cause total simulated pumpage to be less                                                              During 1997, the total estimated ground-water pumpage
         than total estimated pumpage.                                                                             was about 742 Mgal/d for the model area, about 74 Mgal/d
                                                                                                                   (9 percent) less than during 2000 (table 1). During 1997–2000,
       • Rounding or truncation errors will contribute to                                                          the largest increases in pumpage generally occurred farther
         differences in precision.                                                                                 inland, and the largest decreases were closest to the coast, in
The discrepancy caused by these factors is, at most, 2 percent                                                     Beaufort County, S.C., and in Duval County, Fla. (figs. 13 and
for these scenarios. As a comparison, estimated pumpage                                                            14). The total decreases in Chatham and Glynn Counties, Ga.,
could have a margin of error of at least 10 percent (Payne and                                                     are likely the result of implementation of the interim strategy,
others, 2005).                                                                                                     which capped permitted withdrawal from the Upper Floridan
                                                                                                                   aquifer at 1997 withdrawal rates, and encouraged voluntary
                                                                                                                   reductions in ground-water pumpage by the largest users.



                                                                                                                                     2.60          2.28
                                                                                   – 0.02
                                  SOUTH                                                                                     SC
             GEORGIA             CAROLINA                                                                    16.10         GA
                                                                                            5.15                                       – 0.26
                                                                                 1.16                                                                            0.04
                                                                                                                                                  2.64
                                                                                                              1.04
                                                                                                                                                                                             Mo
                                                                                                                             10.94
                                                                                   0.34
                                                                                                                                                                                                del
                                                                                                                                                                                                    b

                                                                                                 –0.35                                                          –8.84
                                                                                                                                                                                                    ou

                  F L OR
                           ID
                                                                                                                                                                                                    nd

                             A                                            2.49                                                                          1.22
                                                                                                                                                                                                        ar


                                                        4.91                            –0.22               1.27           0.65
                                                                                                                                                                                                         y


                                                                                                                                           0.20

                                                                                         –0.93                                                                                              an
                                                 3.38
                                                               –0.38                                                                                                                   ce
                                       11.08                                                                       0.29                                                            O
                                                                               –1.35             2.55                                                                          c
                                                                                                                                                –3.06
                                                                                                                                                                         i




                                                                                                                                   –0.10
                                                                                                                                                                        nt




                                                                                                           –0.07
                                                                                                                                                                        la




                                        –1.97     7.40
                                                                                                                                                                   At




                                                                       –2.74
                                   N                                                                                               –0.42
                                                                                   0.83            1.70                   0.47
                                               –0.41       –4.12                                                                                                         EXPLANATION
                                                          0.59              8.92                                   –0.12                                       Change in pumpage from
                                                                                            1.83                                     –0.25
                                                  –0




                                                                                                                                                                Scenario A (1997) to Base
                                                    .27




       0           25              50 MILES                                                                                                                     Case (2000), by county —
                                                                                                     2.79
                                                               0.77        1.33                                                   –1.29                         In million gallons per day
       0     25         50 KILOMETERS                                                                         –0.64                                                                              Decrease in
       Base from U.S. Geological Survey                           –0.06                     2.49                                                                             – 5 to – 9 pumpage from
       1:100,000-scale digital data                                                                                                                                                              1997 to 2000
                                                                                  0.41                                      4.64                                             – 1 to – 5
                                                                                                          0.30
                                                                                                                                                                             1 to – 1
 Figure	1.. Change.in.pumpage.from ..                                           1.10
                                                                                                   GA                 0.56                                                   1 to 5
 Scenario.A.(1997).to.the.Base.Case.(2000)..
                                                                                        –0.53      FL
 by.county..(Values.represent.the.sum.of.                                                                                                                                    5 to 10
                                                                                                                                                                                                 Increase in
                                                                                                     0
 pumpage.differences.for.the.Brunswick.aquifer.                                                                             – 5.27
                                                                                                                                                                             10 to 16            pumpage from
                                                                                                                                                                                                 1997 to 2000
 system,.the.Upper.Floridan.aquifer,.and.the.
 Lower.Floridan.aquifer.)
2	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

                                             A. Upper Floridan aquifer (unit 5)
                           SOUTH                model area
         GEORGIA          CAROLINA


                                                                                  SC
                                                                              GA

           F L OR
                    ID
                      A




                                                                                                                                     Mo
                                                                                                                                        d     el
                                                                                                                                              bo
                                                                                                                                                un
                                                                                                                                                   da
                                                                                                                                                   ry
                                                                                                                                         an
                                                                                                                                    ce
                                                                                                                                O




                                                                                                                           ic
                                                                                                                         nt
                                                                                                                        la
                                                                                                                      At
                                                                                                                  EXPLANATION
                                                                                                           Change in ground-water pumpage
                                                                                                            from Scenario A (1997) to
                                                                                                            Base Case (2000), per grid cell —
                                                                                                            In million gallons per day, Upper
                                                                                                            Floridan aquifer (unit 5)

                                                                                                                      –1.0 to – 2.1            Decrease in
                                                                                                                                               pumpage from
                                                                                                                      –0.1 to – 1.0            1997 to 2000
                                                                                                                      –0.1 to 0.1

                                                                                                                      0.1 to 1.0

                                                                                                                      1.0 to 2.0               Increase in
                                                              GA                                                                               pumpage from
                                         N                    FL                                                      2.0 to 5.0               1997 to 2000
                                                                                                                      5.0 to 6.4



                                     0               25            50 MILES
                                                                                   Base from U.S. Geological Survey
                                     0          25        50 KILOMETERS            1:100,000-scale digital data

             Figure	1.. Change.in.ground-water.pumpage.from.Scenario.A.(1997).to.the.Base.Case.(2000).for.the.
             Upper.Floridan.aquifer.(A).model.area,.and.(B).enlarged.view.



Fit.of.Simulated.Heads.to.Observed.Heads..                                         residuals ranged from –29.9 to 24.0 ft, with a mean of –1.62 ft
Using.the.MODFLOW.Model                                                            and a root mean square of 8.59 ft. For the Lower Floridan
                                                                                   aquifer wells, residuals ranged from –13.7 to 17.2 ft, with a
     Although the model was not calibrated to 1997 condi-                          mean of 0.058 ft and a root mean square of 8.67 ft. Simulated
tions, available withdrawal and water-level data for 1997                          heads were within the 10-ft calibration target of observed
enabled comparison of simulated head (assuming steady-state                        values for 100 percent of the Brunswick aquifer system wells,
conditions) and observed head as a further check on the quality                    84 percent of the Upper Floridan aquifer wells, and 70 percent
of the calibrated model reported by Payne and others (2005).                       of the Lower Floridan aquifer wells. Dividing the standard
Characteristics of model fit during 1997 are shown using                           deviation of the residuals by the range of water-level varia-
water-level measurements in 44 wells (table 4), of which 3                         tion yields a model fit of 0.025 for the Upper Floridan aquifer
wells are completed in the Brunswick aquifer system (unit 3),                      and 0.037 for the Lower Floridan aquifer, indicating a good
31 wells are completed in the Upper Floridan aquifer (unit 5),                     fit of the data (Kuniansky and others, 2003). These residual
and 10 wells are completed in the Lower Floridan aquifer                           statistics compare well with those for calibrated 1980 and
(unit 7). For the Upper Floridan aquifer wells, water-level                        2000 pumping conditions (Payne and others, 2005).
                                                                     Simulation	of	Ground-Water	Management	Scenarios	 	 2

     B. Upper Floridan aquifer (unit 5) —enlarged view
                                                                                        EXPLANATION
                                                                              Change in ground-water pumpage from
                                                                               Scenario A (1997) to Base Case (2000),
                                                           S                   per grid cell— In million gallons per day,
                                                          GA C
                                                                               Upper Floridan aquifer (unit 5)

                                                                                      – 1.0 to – 2.1       Decrease in
                                                                                                           pumpage from
                                                                                      – 0.1 to – 1.0       1997 to 2000
                                                                                      – 0.1 to 0.1

                                                                                      0.1 to 1.0




                                                       n
                                                     ea
                                                                                                           Increase in




                                                     Oc
                                                                                      1.0 to 2.0           pumpage from




                                                 tic
                                                                                                           1997 to 2000




                                                an
                                                                                      5.0 to 6.4



                                               Atl




                         SOUTH
 GEORGIA                CAROLINA



                Area
               of map
               above




                                                                                                                                       cean
   bo
   Mo nda
      u
       de ry




                                                                                                                            Atlantic O
         l




   F L ORI
             DA
                                                                                        GA
                  Area of map                                                           FL
                    at right




                                                Approxima
                                    N                            te model b
                                                                           oundary


                                   Base modified from U.S. Geological Survey                    0          5   10   15    20 MILES
                                   1:100,000-scale digital data
                                                                                                0      5   10 15 20 KILOMETERS


Figure	1.. Change.in.ground-water.pumpage.from.Scenario.A.(1997).to.the.Base.Case.(2000).for.the.
Upper.Floridan.aquifer.(A).model.area,.and.(B).enlarged.view—continued.
0	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

    Table	.. Model-fit.statistics.for.simulated.heads.for.Scenario.A,.1997.pumping.conditions,.for.the.MODFLOW.model.
    [–, minus; —, too few observations to calculate]

                                                                        Brunswick		       Upper	Floridan	 Lower	Floridan	
      Calibration	statistic                                            aquifer	system		      aquifer		       aquifer		
                                                                          (unit	)           (unit	)        (unit	)
      Number of observations                                                3                  31              10
      Range of observations (feet)                                         20.9               345             246
      Minimum residual1 (feet)                                             –8.9               –29.9           –13.7
      Maximum residual (feet)                                               2.7                24.0            17.2
      Mean residual (feet)                                                 –2.4                –1.6             0.6
      Standard deviation of residuals (feet)                                —                   8.6             9.1
      Root mean square residual (feet)                                      —                   8.6             8.7
      Percentage of simulated values within 10 foot error criteria        100                  84              70
      Calibration fit: Standard deviation of residuals divided by            —                  0.025           0.037
        range of observed values (Kuniansky and others, 2003)
      1
          Residual equals simulated minus observed head.


      The spatial distribution of water-level residuals for          potentiometric gradient in the southwestern part of the model
Scenario A (fig. 15) also compares well with that for the            area; and (4) potentiometric highs north of Port Royal Sound,
calibration periods 1980 and 2000 (Payne and others, 2005),          S.C. (see locations, fig. 1).
although there are fewer total observations, and most of those             The 1997 simulated potentiometric surface for the Lower
are in counties nearest to the coast. Similar to results for the     Floridan aquifer (unit 7) (fig. 16C) is similar to that for the
calibration periods, for units 3 and 7, there are too few obser-     Upper Floridan for 1997 (fig. 16B), as well as to the simu-
vations to discern spatial patterns of the residuals. For unit 5,    lated Lower Floridan aquifer potentiometric surface for 2000
residuals in the area closest to the coast are mostly between        (fig. 8C). The simulated potentiometric surfaces are similar for
–10 and 10 ft, and residuals of larger magnitude are mostly          the Brunswick aquifer system (unit 3) for 1997 (fig. 16A) and
farther inland in the northeastern part of the model area. For       2000 (fig. 8A). The general similarity of 1997 to 2000 simu-
the 1980 and 2000 simulations, there is an observed correla-         lated water levels is the result of a similar pumpage distribu-
tion in the magnitude of residuals with physiography. The            tion on a regional scale.
magnitudes of residuals in the northwestern part of the model              During 1997–2000, simulated water levels showed a
area (north of the Gulf Trough) are largest and show the great-      combination of rises and declines in response to changing
est variability (there are fewer observations in that area, even     pumping patterns (fig. 17). During this period, total ground-
for the calibration datasets). Residuals are mostly of smaller       water use increased (table 1), however, the distribution
magnitude in the coastal area (Payne and others, 2005). The          of withdrawal changed. The largest decreases, exceeding
model-fit statistics and the spatial pattern of residuals for        1 Mgal/d, occurred in Beaufort County, S.C., Chatham and
1997 conditions indicate that Scenario A model results repre-        Glynn Counties, Ga., and Duval County Fla., and the largest
sent 1997 ground-water flow conditions satisfactorily.               increases, exceeding 5 Mgal/d, occurred in Burke, Coffee,
                                                                     Dooly, Jefferson, Screven, and Wilcox Counties, Ga. (table 1;
Ground-Water.Flow                                                    fig. 13) (see locations, fig. 1).
                                                                           Simulated drawdown exceeded 1 ft in the Upper Floridan
      The 1997 simulated potentiometric surfaces for all hydro-      aquifer across much of the model area to the northwest of
logic units (fig. 16) generated using the MODFLOW model              the Gulf Trough, with drawdown exceeding 8 ft in Bleckley,
are similar to the 2000 simulated potentiometric surfaces            Burke, Candler, Dooly, part of Effingham, Jenkins, Pulaski,
(fig. 8). The 1997 potentiometric surface for Upper Floridan         and Screven Counties, Ga. (fig. 17B). These declines gener-
aquifer (fig. 16B) also has prominent features similar to those      ally correspond to an increase in pumpage from the Upper
found on potentiometric-surface maps for May 1998 (Peck and          Floridan aquifer in these areas (fig. 13). In Beaufort and Jasper
others, 1999), September 1998 (Ransom and White, 1999),              Counties, S.C., Ben Hill, Telfair, Wheeler, and Chatham
and September 2000 (Peck and McFadden, 2004). These fea-             Counties, Ga., and Duval County, Fla., simulated water levels
tures include (1) a large cone of depression in the Savannah,        in the Upper Floridan aquifer rose several feet in response
Ga., area and smaller cones of depression in the Jesup, Ga.,         to decreased pumping (fig. 17B). Although total pumpage
Brunswick, Ga., and the St. Marys, Ga.–Fernandina Beach,             in Glynn County, Ga., decreased from 1997 to 2000 (fig. 13;
Fla., areas; (2) a steepening of the potentiometric gradient         table 1), local increases in pumpage (fig. 14) resulted in a
in the area of the Gulf Trough; (3) flattening of the regional       slight decline in water level at Brunswick, Ga. (fig. 17B).
                                                                                         Simulation	of	Ground-Water	Management	Scenarios	 	 1

A. Brunswick aquifer system                                                            C. Lower Floridan aquifer
   (unit 3)                                                                               (unit 7)




                                                  Approximate
                                                   limit of Brunswick
                                                   aquifer system




                                                                             0   25   50 MILES

                                                                             0 25 50 KILOMETERS
                                                                   Base from U.S. Geological Survey
                                                    N              1:100,000-scale digital data




                        SOUTH
      GEORGIA          CAROLINA


                                                                                        SC
                                                                                       GA

        F L OR
                 ID
                   A




                                                                                                                                  Mo
                                                                                                                                  de
                                                                                                                                   lb
                                                                                                                                       ou
                                                                                                                                       nd
 B. Upper Floridan




                                                                                                                                        ar
    aquifer (unit 5)




                                                                                                                                            y
                                                                                                                             an
                                                                                                                          ce
                                                                                                                         O
                                                                                                                    tic
                                                                                                                   lan
                                                                                                                   At




                                                                                                             EXPLANATION
                                                                                                      Water-level residuals, in feet—
                                                                                                       Represents difference of
                                                                                                       simulated minus observed
                                                                                                       water levels
                                                                                                           20 to 25
                                                                                                           10 to 20
                                                                                                           10 to –10
                                                                        GA                                 – 10 to – 20
                                                                        FL
           0                25         50 MILES                                                            – 20 to – 30

           0           25        50 KILOMETERS
           Base from U.S. Geological Survey
           1:100,000-scale digital data



           Figure	1.. Difference.between.simulated.and.observed.water.levels.(residuals).by.model.layer.
           for.Scenario.A.(1997)..Results.from.MODFLOW.model.
2	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

         A. Brunswick aquifer system
            (unit 3)
                                                             SC                                                                                                                                                        EXPLANATION
                                                           GA
                                                                                                                                                                                      50          Simulated potentiometric contour—




                                                                                                                                                                 Mo
                                                                                                                                                                                                    Shows simulated potentiometric surface




                                                                                                                                                                    d
                                                                                                                                                                    el
                                                                                                                                                                                                    for Scenario A (1997), in feet. Hachures




                                                                                                                                                                      bo
                                                                                                                                                                                                    indicate depression. Contour intervals




                                                                                                                                                                        un
                                                                                                                                                                                                    10 and 50 feet. Datum is NAVD 88




                                                                                                                                                                            da
                                                          0
                                                      10




                                                                                                                                                                            ry
                                                                                                                                           Po
                                                              50
                                                                                                                                                rt R




                                                              40
                                                                                                                                                    oyal
                                                                                                                                                         Sound
                                                                   30
                                                                        20                                                         Savannah
                                                                         10
                                                                                                      –30
                                                                                                      –20                                                    Approximate
                                                                                                                                                              limit of Brunswick




                                                                                                                           0
                                                                                                                  –1
                                                                                                                                                              aquifer system
                                                 Jesup
                                                                                                                       0




                                                                                                                                                     an
                                                                                                                                            Oce
                                                               10




                                                                                  Brunswick                                                ntic                    B. Upper Floridan aquifer
                                                                                                                                   Atla

                                                                                                                                                                      (unit 5)    350
                                                          St Marys                                                                                                                                                         200

                                    GA
                                                                                  Fernandina                                                                                               300
                                                                                  Beach
                     N              FL                                                                                                                                                     250
                                                                                                                                                                                                                                                                                                               40
                                                                                                                                                                                                                                                                                                                       20
                                                                                                                                                                                            200
                                                                                                                                                                        0
                                                                                                                                                                     25



                                                                                                                                                                                                                                                                                                                       l
                                                                                                                                                                                                    0                       5030
                                                                                                                                                                                                                                                                                                                               l
                                                                                                                                                                                                                   0
                                                                                                                                                                                                  15          10                                                           10




                                                                                                                                                                                                                                                                                                               l
                                                                                                                                                                                                                                                                                                                                   l
                              0              25            50 MILES                                                                                                                                                                                l               l           l           l          l




                                                                                                                                                                                                                                                                                                                                   l
                                                                                                                                                                                                                                    l                          l
                                                                                                                                                                                                                                                                           –10                                             l
                                                                                                                                                     250




                                                                                                                                                                                                                               l               l
                                                                                                                                                                                                                                       l




                                                                                                                                                                                                                                   l
                                                                                                                                                                                                                                                                                                               l




                                                                                                                                                                                                                            l
                              0     25           50 KILOMETERS




                                                                                                                                                                                                                                       0
                                                                                                                                                                                                                                                                                                                       –10




                                                                                                                                                                                                                                                                                                     l
                                                                                                                                                                                                                                    –3
                                                                                                                                                                                                                               l
                                                                                                                                                                                                                         l
                                                                                                                                           250




                                                                                                                                                                                                                                                                                                         l l
                                                                                                                                                                                                                                                                                                   l l l l
                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                                           –100
                                                                                                                                                                                                                                                                                                               l




                                                                                                                                                                                                                        l
                                                                                                                                                                                                                               l
                                                                                                                                                                                                                                                               –50
                          Base from U.S. Geological Survey
                                                                                                                                                                                                                                                                                                                       l
                                                                                                                                                                                                                                                                           –40


                                                                                                                                                                                                                        l

                                                                                                                                                                                                                                    l
                                                                                                                                                                                                                                        l




                                                                                                                                                                                                                                                                                                                           l
                                                                                                                                                                                                                                               l
                          1:100,000-scale digital data                                                                                                                                                                                                                                   0
                                                                                                                                                                                                                                                                                       –2 l
                                                                                                                                                                                                                       l
                                                                                                                                                                                                                                                           l




                                                                                                                                                                                                                                                                                                   l
                                                                                                                                                                                                                                                                   l




                                                                                                                                                                                                                                                                                                                       l
                                                                                                                                                                                                                                                                           l           l
                                                                                                                                                                                                                   0    l




                                                                                                                                                                                                                                                                                                                   l
                                                                                                                                                                                                                                                                                                                   l
                                                                                                                                                                                                                           l    l          l           l               l           l           l      l
                                                                                                                                                                                                                                                                                                      0
                                                                                                                                                                                                              20
                                                                                                                                                                                                         30
                                                                                                                                                                            50




         C. Lower Floridan aquifer                                                                                                                                                                                                                                                      10
            (unit 7)         350
                                                                         200
                                                                                                                                                                                 50




                              300
                                                                                                                                                                                                   40




                                  250
                                                                                                                                                40
                                                                                                                                                                                                                       l l


                                                                                                                                                                                                                                    l




                                                                                                                                                 20
                                                                                                                                                                                                                                            l




                                        0
                                                                                                                                                                                                                           l




                                   20
                                                                                                                                                                                                                                                   l




                                                                                                                                                                                                                       –10
                                                                                                                                                                                                                                                           l
                 0




                                                                                                                                                                                                                                    0
                25




                                                 0                           50
                                                                                                                                                                                                                                                           l




                                                                                                                                                                                                                       –20
                                                                                                                                                     l
                                            15           0
                                                                                                                                            l




                                                                                                                                                                                            50
                                                      10                      30                                                                                                                                                            l 10
                                                                                                                                                             l
                                                                                                                  10
                                                                                                                                           l



                                                                                                                                                             l




                                                                                                                                       l
                                                                                                                                                                                                                                            20
                                                                                              l               l        l       l
                                                                                                                  –10
                                                                                  l           l       l
                                                                                                                                                         l




                                                                                                                                                         l
                                                                              l       l
                                                                                                                                   l
                                                                                  l
                                                                             l


                                                                                  0




                                                                                                                                                l
                                                                                  –3
                                                                              l




   250
                                                                                                                                   l
                                                                        l




                                                                                                                                           l l
                                                                                                                                   l
                                                                              l
          25




                                                                                                  –100
                                                                        l




                                                                                                                                                 l
                                                                                                      –50                  0
                                                                                                                                   l
                                                                              l




                                                                                                                  –4
           0




                                                                    l




                                                                                                                                                     l
                                                                                  l




                                                                                          l                                    0
                                                                                                                                   l




                                                                                                                           –2
                                                                    l




                                                                                                  l                                                                                                                     SOUTH
                                                                                                                                                 l
                                                                                                                                   l




                                                                                                          l                    l
                                                           0                                                       l
                                                                                                                                                                                                  GEORGIA              CAROLINA
                                                                    l




                                                                         l
                                                                                                                                                l




                                                                              l                                                             l
                                                                                  l           l           l          l l
                                                                                                                   l 0
                                                     20
                     50




                                                 30




                                                                                                                           10
                                            40




                                                                                                                                                                                                       F L OR
                         50




                                                                                                                                                                                                                ID
                                                                                                                                                                                                                  A
                                                                        l l

                                                                                  l
                                                                                      l




                                                                                                                                                                                      Figure	1.. Simulated.potentiometric.surfaces.for.
                                                                                              l




                                                                    –10
                                                                                                  l
                                                                         –20




                                                                                                                                                                                      Scenario.A.(1997).for.the.(A).Brunswick.aquifer.
                                                                                  0




                                   50
                                                                                              10
                                                                                                      20                                                                              system.(unit.3),.(B).Upper.Floridan.aquifer.(unit.5),.
                                                              –30
                                                                                                                                                                                      and.(C).Lower.Floridan.aquifer.(unit.7).
                                                                            Simulation	of	Ground-Water	Management	Scenarios	 	 

A. Brunswick aquifer system
   (unit 3)
                                                                                               EXPLANATION
                                    SC
                                  GA                                                      Change in simulated head
                                                                                           between Base Case (2000)




                                                                  Mo
                                                                                           and Scenario A (1997), in feet




                                                                     d
                                                                     el
                                                                                                   8




                                                                       bo
                                                                                                   4     Increase in




                                                                         un
                                                                                                         water level




                                                                           da
                                                                                                   1




                                                                             ry
                                                                                                           –1
                                                             Approximate
                                                              limit of Brunswick                           –4
                                                              aquifer system
                                                                                                           –8       Decrease in
                                                                                                           – 12     water level
                                                                                                           – 16




                                                            an
                                                                                                           – 19




                                                        Oce
                                                      ntic
                                         Brunswick
                                                                    B. Upper Floridan aquifer
                                                     Atla              (unit 5)

                       GA
            N          FL




                   0         25   50 MILES

                   0    25    50 KILOMETERS

                 Base from U.S. Geological Survey
                 1:100,000-scale digital data




C. Lower Floridan aquifer
   (unit 7)




                                                                                                          SOUTH
                                                                                        GEORGIA          CAROLINA




                                                                                          F L OR
                                                                                                   ID
                                                                                                     A



                                                                                   Figure	1.. Change.in.simulated.head.between.the.
                                                                                   Base.Case.(2000).and.Scenario.A.(1997).for.the..
                                                                                   (A).Brunswick.aquifer.system.(unit.3),.(B).Upper.
                                                                                   Floridan.aquifer.(unit.5),.and.(C).Lower.Floridan.
                                                                                   aquifer.(unit.7).
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

      Water levels in the Brunswick aquifer system and Lower                                                             Major flow-budget components for Scenario A using the
Floridan aquifer also showed a combination of rises and                                                            MODFLOW model are shown in table 5. Simulated ground-
declines during 1997–2000 (figs. 17A and 17C). Because                                                             water pumpage increases in the model from about 727 Mgal/d
these aquifers were not used widely in the coastal area during                                                     during 1997 to about 798 Mgal/d during 2000 (tables 2 and 5).
1997–2000, most of the simulated changes were in response                                                          Figure 18 illustrates the difference in major flow-budget
to changes in pumping patterns in the Upper Floridan aquifer,                                                      components between Scenario A (1997 pumping conditions)
which induced a leakage response in the two aquifers. The                                                          and the Base Case (2000 pumping conditions). Figure 18
pattern of change in simulated water levels for the Lower                                                          shows that, as a result of the increase in pumpage, outflow
Floridan aquifer is similar to that for the Upper Floridan                                                         from the system decreased while inflow increased, primarily at
aquifer (figs. 17C and 17B). Rises and declines in water levels                                                    the onshore part of the general-head boundary, and to a lesser
in the Brunswick aquifer system also are similarly distributed                                                     degree at the specified-head boundary. The effect was more
spatially compared with those for the Upper Floridan aquifer.                                                      pronounced on the flux at the general-head boundary, because
The rise in Chatham County, however, is less pronounced                                                            the increase in pumpage was widespread, and occurred away
and extensive than elsewhere, and there is a slight rise in                                                        from the southern specified-head boundary. Within the model
water level in the southwestern part of Camden County, at the                                                      boundaries, the change in stresses also resulted in a slight
margin of the modeled extent of the Brunswick aquifer system                                                       decrease in net landward flux from 1997 (Scenario A) to 2000
(fig. 17A).                                                                                                        (Base Case) because, although the total stresses in the model
                                                                                                                   area increased, the pumpage in several of the coastal counties
                                                                                                                   decreased (figs. 13 and 19).

Table	.. Flow-budget.components.for.Scenario.A..
[Results from MODFLOW model; in million gallons per day; —, not applicable]

                                                                       Inflow                                                                              Outflow

     Model		                                   From		            From		            From		                                 To		                 To		             To		
      unit                                  general-head	    general-head	      specified-	                          general-head	        general-head	      specified-	   Discharge		
                                                                                                         Total	                                                                            Total	
                                             boundary,		      boundary,	          	head		                             boundary,		          boundary,		         head		       to	wells
                                              onshore          	offshore        boundary                               onshore              offshore         boundary
        1                                     282.24               24.66            —                306.89              117.22              15.48              —             —           132.70
        2                                      39.99               —                —                 39.99                5.20                —                —             —             5.20
        3                                       —                  —                —                 —                    —                   —                —              0.47         0.47
        4                                       —                  —                —                 —                    —                   —                —             —             —
        5                                     111.44               —              699.93             811.37               31.47                —              275.08         601.97       908.53
        6                                       —                  —                0.00               0.00                 —                  —                0.00          —             0.00
        7                                       —                  —               15.90              15.90                 —                  —                2.48         124.48       126.96
 Total all units                              433.67                24.66         715.84           1,174.16              153.89              15.48            277.57         726.93      1,173.86
 Percent flow                                  36.9                   2.1             61.0           100.0                13.1                1.3                 23.6        61.9        100.0



                                       70
                                                                                                                                                          67.46
                                       60
            MILLION GALLONS PER DAY




                                       50
                                                40.52
                                       40
                                       30
                                       20
                                                                              12.02
                                       10                                                                                                                                    4.18
                                                                                                                  0.03
                                        0
                                                              –0.89                                                                             – 0.23
                                      –10
                                                                                                                                 – 7.47
                                                                                               – 12.32
                                      –20
                                              Inflow from    Inflow from    Inflow from       Outflow to   Outflow to        Outflow to        Pumpage        Pumpage      Pumpage
                                             general-head   general-head     specified-      general-head general-head       specified-         unit 3          unit 5       unit 7
                                               boundary,      boundary,         head          boundary,    boundary,           head
                                                onshore        offshore      boundary          onshore      offshore         boundary

                                                                                                           FLOW COMPONENT

            Figure	1.. Simulated.flow.budget.indicating.changes.from.Scenario.A.(1997).conditions.to.Base.Case.(2000).conditions.
                                                                                                                                          Simulation	of	Ground-Water	Management	Scenarios	 	 


                                                                                                          Scenario
                                                                                                                                       Solute.Transport.
                                                                                                          D2 (2035)
                                                                                                                                             The simulated chloride distribution in ground water dur-
                                                            Scenario D1 (2035)                                                         ing 1997 for Scenario A, generated using the SUTRA model, is
                                                                                                                                       similar to that during 2000, with the extent of the plumes slightly
                                          Scenario C2
SCENARIO




                                                                                                                                       greater during 2000 than during 1997 (fig. 20). Although com-
               Scenario C1                                                                                                             bined pumpage decreased only about 10 Mgal/d from 1997 to
                                                                                                                                       2000 in Chatham County, Ga., and Beaufort and Jasper Counties,
                                      Scenario B                                                                                       S.C., the plumes likely will continue to grow in response to estab-
                                                 Scenario A                                                                            lished hydraulic gradients in the Upper Floridan aquifer.

                                                Base Case
                                                                                                                                       Scenario	B:	Effects	of	an	Industrial		
           0      5         10             15     20     25         30            35           40             45              50
                                                                                                                                       Well	Field	Shutdown
                             SIMULATED NET LANDWARD FLOW,
                               IN MILLION GALLONS PER DAY                                                                                   During October 2002, the Durango Paper Company in
                                                                                                                                       St. Marys, Camden County, Ga., ceased operations, result-
           Figure	1.. Comparison.of.simulated.net.landward.flow.for.the.                                                              ing in an abrupt decrease of about 36 Mgal/d in ground-water
           Base.Case.(2000).and.Scenario.A.(1997),.Scenario.B.(Durango.                                                                pumpage from the Upper and Lower Floridan aquifers (Peck
           Paper.Company.pumping.eliminated),.Scenario.C1.(Chatham.                                                                    and others, 2005). Scenario B was designed to simulate the
           County,.Georgia,.pumping.eliminated),.Scenario.C2.(southern.                                                                effect of this reduction. Regionwide pumpage data for 2002
           Beaufort.County,.South.Carolina,.pumping.eliminated),.                                                                      were not available, so the reduction in pumpage caused by
           Scenario.D1.(projection.based.on.Regional.Economic.Models,.                                                                 the shutdown was applied to the 2000 pumpage distribution.
           Inc.),.and.Scenario.D2.(County.Comprehensive.Water-Supply.                                                                  Because the observed response to the shutdown was limited
           Plans.projection).                                                                                                          mostly to the southernmost part of the model area (Peck and
                                                                                                                                       others, 2005), the scenario simulation was limited to the




                                                                                                                                                                      SOUTH
                                                                                                         Parris Island                                 GEORGIA       CAROLINA


                                                                                                                                                                                Map
                                                                                                                                                                                area
                                                                                                                                                                 Study
                                                                                               Po                                                                area
                      ol                                                                            rt
                      C




                           let                                                                           Ro
                                 on                                                                           ya
                                      R                                                                            lS
                                                                                                                        ou
                                                                                                                             nd                                      Model
                                                                                                                                                                   boundary


                                                                                                                                                               EXPLANATION
                                                                      I




                                                                                                                                               Simulated chloride concentration, in milligrams per
                                                                   ey
                                                               ckn




                                                                                                                                                 liter (mg/L), in ground water in the middle of the
                                                              Pin




                                                                                                                                                 Upper Floridan aquifer for Scenario A (1997)

                                                                                                                                                250 to 2,000
                                                                              Hilton Head Island                                                2,000 to 6,000
                                                                                                                                                6,000 to 10,000
                                                                                                                                                Greater than 10,000
                                                                                                                              n
                                                                                                                             ea
                                                                                                                        Oc




                                                                                                                                               Chloride concentration, in mg/L in, ground water
                                                                                                                                                 in the middle of the Upper Floridan aquifer
                                                                                                                   tic




                                                                                                                                  N
                                                                                                                                                 for Base Case (2000)
                                                                                                              la n
                                                                                                         At




                                                                                                                                                250

           Base from U.S. Geological Survey                    0          1            2        3              4             5 MILES
                                                                                                                                                10,000
           1:100,000 and 1:250,000-scale data
                                                               0     1        2    3       4        5 KILOMETERS                               Observation well


           Figure	20.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper.Floridan.aquifer.in.the.
           Hilton.Head.Island,.South.Carolina,.area.for.Scenario.A.(1997).(modified.from.Provost.and.others,.2006).
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models

regional, steady-state MODFLOW model; the Savannah–                      tiometric surface for the Brunswick aquifer system (fig. 22A)
Hilton Head Island transient, solute-transport SUTRA model               shows little difference from 2000 (fig. 8A).
was not used. The pumpage distribution for Scenario B is the                   For Scenario B conditions, simulated water levels in
same as for the Base Case, except that pumpage in hydrologic             all aquifers are higher relative to the Base Case in response
unit 5 (the Upper Floridan aquifer) was reduced by about                 to lower pumpage (fig. 23). The water-level difference is
36 Mgal/d in cells where wells that were turned off during               greatest near the pumping center for the Upper Floridan aqui-
October 2002 are located (figs. 21 and 6C).                              fer, where the Scenario B water level is about 29 ft higher than
      Simulated potentiometric surfaces for Scenario B                   for the Base Case. The magnitudes of water-level differences
(fig. 22) are similar to the 2000 simulated potentiometric sur-          are smaller for the Lower Floridan aquifer and Brunswick
faces (fig. 8) on a regional scale. The most notable difference          aquifer system, with simulated water levels 16 ft and 3 ft
in simulated head for the Upper and Lower Floridan aquifers              higher, respectively, than those for the Base Case. A water-
is the seaward movement of the 20-ft contour offshore of                 level difference of 1–2 ft in all aquifers for Scenario B relative to
Camden County, and the elimination of the cone of depression             the Base Case is simulated as far away from the pumping center
at St. Marys (fig. 22B). The reduction in localized pumping              as southern Chatham County, about 98 miles (mi), and extends
results in a water-level rise in the area. The simulated poten-          inland as far as the Gulf Trough, about 109 mi.




                                                                                                                                   SOUTH
                                                                                                          GEORGIA                 CAROLINA




                                                                                                            bo
                                                                                                            Mo nda
                                                                                                               u                        Brunswick
                                                                                                                de ry
                                                                                                                  l
                                                                                                                                        St Marys
                                                                                                   cean




                                                                                                                                        Fernandina Beach
                                                                                                            F L ORI                     Jacksonville
                                                                                                                        DA
                                                                                       A tlantic O




                                                                                                                             Map area

                                                        GA
                                                        FL
                                                                                                                 EXPLANATION
                                                                                                          Pumpage per grid cell—
                                                                                                           In million gallons per day,
                                                                                                           Scenario B, Upper Floridan
                                                                                                           aquifer (unit 5)

                                                                                                                         0 to 0.5
                                                                                                                         0.5 to 1

                                                                                                  N                      1 to 5
                                                                                                                         5 to 10
                                                                                                                         10 to 51

                                                                                                            Cell in which Durango
                                                                                                             Paper Company
                      Approxim                                                                               pumping was eliminated
                                ate mode
                                           l boundary


         Base modified from U.S. Geological Survey           0       5   10   15     20 MILES
         1:100,000-scale digital data
                                                             0   5   10 15 20 KILOMETERS


         Figure	21.. Distribution.of.ground-water.pumpage.for.the.Upper.Floridan.aquifer.(unit.5).in.the.southern.part.
         of.the.model.area.for.Scenario.B.(Durango.Paper.Company.pumping.eliminated).
                                                                                                                                                             Simulation	of	Ground-Water	Management	Scenarios	 	 

A. Brunswick aquifer system
   (unit 3)
                                                         SC                                                                                                                                            EXPLANATION
                                                    GA
                                                                                                                                                                       50              Simulated potentiometric contour—




                                                                                                                                                  Mo
                                                                                                                                                                                         Shows simulated potentiometric surface




                                                                                                                                                     d  el
                                                                                                                                                                                         for Scenario B, in feet. Hachures indicate




                                                                                                                                                        bo
                                                                                                                                                                                         depression. Contour intervals 10 and




                                                                                                                                                         un
                                                                                                                                                                                         50 feet. Datum is NAVD 88




                                                                                                                                                             da
                                                0
                                              10




                                                                                                                                                             ry
                                                     50




                                                          0
                                                                                                    –50




                                                     30 4
                                                                                                                                          Approximate
                                                              20




                                                                                        –40
                                                                                                                                           limit of Brunswick




                                                              10
                                                                                                –30                                        aquifer system
                                                                                                –20
                                                                                                                        0
                                                                            0                                   –1




                                                                                                                                      an
                                                              10




                                                                                                                                Oce
                                                                                                                            ntic
                                                                                                                                                    B. Upper Floridan aquifer
                                                                                                                        Atla                           (unit 5)   350
                                                    St Marys                                                                                                                                                 200

                               GA                                                                                                                                           300
                 N             FL                                                                                                                                           250
                                                                                                                                                                                                                                                                                              50
                                                                                                                                                                                                                                                                                                          30




                                                                                                                                                                             0
                                                                                                                                                                            20
                                                                                                                                                        0
                                                                                                                                                        25



                                                                                                                                                                                    0                         40
                          0            25           50 MILES                                                                                                                      15                           20                                                10




                                                                                                                                                                                        0
                                                                                                                                                  200
                                                                                                                                                                                                                                         l           l           l        l               l




                                                                                                                                                                                        10
                                                                                                                                                                                                                                                                 –10
                                                                                                                                                                                                                     l                           l                                                l
                                                                                                                                                                                                                                 l
                                                                                                                                                                                                                         l                                                                                    l




                                                                                                                                                                                                                 l
                                                                                                                                                                                                                                                                                                                      0
                                                                                                                                                                                                                                                                                      l




                                                                                                                                                                                                                     l
                                                                                                                                                                                                                                                                                              l
                                                                                                                                                                                                                                                                                                          l




                                                                                                                                                                                                                       0
                          0      25      50 KILOMETERS




                                                                                                                                                                                                             l
                                                                                                                                                                                                                                                                                                              –10




                                                                                                                                                                                                                 l

                                                                                                                                                                                                                     –3




                                                                                                                                                                                                                                                                                          l
                                                                                                                                                                                                                                                                                              ll
                                                                                                                                                                                                             l
                                                                                                                                                                                                                 l




                                                                                                                                                                                                                                                                                          l
                                                                                                                                                                                                                                                     –100




                                                                                                                                                                                                                                     –5
                                                                                                                                                                                                                 l
                                                                                                                                                                                                                                                                                                      l




                                                                                                                                                                                                                                                                                              l
                                                                                                                                                                                                         l l
                      Base from U.S. Geological Survey                                                                                                                                                                                                                    0




                                                                                                                                                                                                                 l



                                                                                                                                                                                                                                             0
                                                                                                                                                                                                                                                                                                                  l




                                                                                                                                                                                                                                                                                              l
                                                                                                                                                                                                                                                                     –4




                                                                                                                                                                                                                     l
                                                                                                                                              0
                                                                                                                                                                                                                             l




                                                                                                                                                                                                                                                                                                  l
                                                                                                                                                                                                                                                                            0
                                                                                                                                          25
                                                                                                                                                                                                                                     l
                                                                                                                                                                                                                                                                          –2 l


                                                                                                                                                                                                             l
                      1:100,000-scale digital data




                                                                                                                                                                                                                                                                                                                  l
                                                                                                                                                                                                                                                                                              l
                                                                                                                                                                                                                                                 l
                                                                                                                                                                                                                                                     l




                                                                                                                                                                                                                                                                                          l
                                                                                                                                                                                                                                                             l




                                                                                                                                                                                                                                                                                                              l
                                                                                                                                                                                                   0

                                                                                                                                                                                                         l
                                                                                                                                                                                                                                                                     l        l
                                                                                                                                                                                                                 l                                                                                    l l



                                                                                                                                                                                                             l
                                                                                                                                                                                                                         l                                                        l           l0
                                                                                                                                                                                                                                             l           l           l
                                                                                                                                                                                             20
                                                                                                                                                             50




                                                                                                                                                                                         30




C. Lower Floridan aquifer                                                                                                                                                                                                                                                                         10
                                                                                                                                                                                   40




                                                                                                                                                                                                        10
   (unit 7)         350
                                                                   200
                                                                                                                                                                  50




                          300
                           250
                                                                                                                                50
                                                                                                                                  30
                                                                                                                                                                                                                     0
                              0
                              20
            0




                                                                                                                                                                                                        –10
            25




                                   0                               40                                                                                                        50
                               15                                   20                                  10                                l                                                                                      10
                                         0




      200
                                                                                                            l
                                                                                                                                                                                                                                                     20
                                                                                    l           l                   l       l     l
                                       10




                                                                        l
                                                                            l
                                                                                    l       l       l
                                                                                                        –10                               l                                                            –20
                                                                   l
                                                                        l




                                                                                                                                  l
                                                                          0



                                                                                                                        l
                                                                        –3
                                                                   l
                                                               l




                                                                                                                                l l
                                                                                                                        l




                                                                                            –100
                                                                   l

                                                                                    –5
                                                               l




                                                                                                                        l




                                                                                                          0
                                                                    l


                                                                                       0




                                                                                                                                 l




                                                                                                        –4
                                                                        l




                                                                                                                            l




250
                                                              l




                                                                                l
                                                                                                                                      l




                                                                                                             0
                                                                                                           –2 l
                                                                                        l
                                                                                                                                                                                                        SOUTH
                                                                                                                        l




                                                                                                l
                                                                                                                                  l
                                                              l




                                                                                                        l       l
                                                    0                                                                                                                              GEORGIA             CAROLINA
                                                                                                                                l




                                                                    l
                                                                  l




                                                                                l           l           l       l           0
                                            20




                                         30
                 50




                                                                                                                    10
                                    40




                                                              10

                                                                                                                                                                                        F L OR
                     50




                                                                                                                                                                                                 ID
                                                                                                                                                                                                   A




                                                              0
                                                                  –1
                                                                    0




                              50
                                                                                    10
                                                                                                20
                                                        –30 –20


 Figure	22.. Simulated.potentiometric.surfaces.for.Scenario.B.(Durango.Paper.Company.pumping.eliminated).for.the.
 (A).Brunswick.aquifer.system.(unit.3),.(B).Upper.Floridan.aquifer.(unit.5),.and.(C).Lower.Floridan.aquifer.(unit.7).
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

      A. Brunswick aquifer system
         (unit 3)
                                                                                                                      EXPLANATION
                                          SC
                                        GA                                                          Change in simulated head between
                                                                                                     Scenario B and Base Case (2000),




                                                                        Mo
                                                                                                     in feet




                                                                           d
                                                                                                               0




                                                                           el
                                                                             bo
                                                                                                                        1




                                                                               un
                                                                                 da
                                                                                                                        2




                                                                                   ry
                                                                                                                        4
                                                                   Approximate                                                   Increase in
                                                                    limit of Brunswick                                  5        water level
                                                                    aquifer system
                                                                                                                        10
                                                                                                                        15
                                                                                                                        29




                                                                 an
                                                             Oce
                                                           ntic           B. Upper Floridan aquifer
                                                          Atla

                                                                             (unit 5)

                             GA
                  N          FL




                         0         25   50 MILES

                         0    25    50 KILOMETERS

                       Base from U.S. Geological Survey
                       1:100,000-scale digital data




      C. Lower Floridan aquifer
         (unit 7)




                                                                                                                       SOUTH
                                                                                                     GEORGIA          CAROLINA




                                                                                                       F L OR
                                                                                                                ID
                                                                                                                  A



                                                                                         Figure	2.. Change.in.simulated.head.between.Scenario.B.
                                                                                         (Durango.Paper.Company.pumping.eliminated).and.the.Base.
                                                                                         Case.(2000).for.the.(A).Brunswick.aquifer.system.(unit.3);..
                                                                                         (B).Upper.Floridan.aquifer.(unit.5);.(C).Lower.Floridan.aquifer.
                                                                                         (unit.7);.(D).Upper.Floridan.aquifer.(unit.5),.enlarged.view;.
                                                                                         and.(E).Lower.Floridan.aquifer.(unit.7),.enlarged.view.
                                                                         Simulation	of	Ground-Water	Management	Scenarios	 	 


   D. Upper Floridan aquifer (unit 5)                     E. Lower Floridan aquifer (unit 7)


       BRANTLEY           GLYNN                                                                                      GEORGIA




                                                an
                                            Oce
                                                                                                                        Map
                                                                                                                        area




                                          ntic
                                         Atla
                   CAMDEN
                                                                                                                        FL O
                                                                                                                               RI
                                                                                                                                    DA
    CHARLTON

                   GA
                   FL
                                                                                                                   EXPLANATION
                                                                                                               Change in simulated head
                                                                                                                between Scenario B and
           NASSAU                                                                                               Base Case (2000), in feet
                                                                                                                           0
                                                                                                                           1




                                                                                                                                         Increase in water level
                                                                                                                           2
                                                                                                                           4
                              DUVAL                                                                                        5
               N
                                                                                                                           10
                                                                                                                           15
                                                                                                                           20
                                                                                                                           29
   Base from U.S. Geological Survey                                                  0    5    10   15 MILES
   1:100,000-scale digital data
                                                                                     0 5 10 15 KILOMETERS

   Figure	2.. Change.in.simulated.head.between.Scenario.B.(Durango.Paper.Company.pumping.eliminated).and.the.Base.Case.
   (2000).for.the.(A).Brunswick.aquifer.system.(unit.3);.(B).Upper.Floridan.aquifer.(unit.5);.(C).Lower.Floridan.aquifer.(unit.7);.
   (D).Upper.Floridan.aquifer.(unit.5),.enlarged.view;.and.(E).Lower.Floridan.aquifer.(unit.7),.enlarged.view—continued.



      In Camden County, after the shutdown of wells at the           dan aquifer water level in western Camden County, Ga., and
Durango Paper Company, water levels in monitored wells               Nassau County, Fla., for Scenario B, the observed rise from
in the Brunswick aquifer system, Upper Floridan aquifer,             5 to 10 ft from 2001 through 2003 may be partly caused by
and Lower Floridan aquifer exhibited a distinct rise (fig. 24)       regional effects and not the October 2002 shutdown. Some
(Peck and others, 2005). With increasing distance from the           of the discrepancy between simulated and observed water
pumping center, the water-level rise decreased (fig. 25). At         levels also may occur because the model does not attempt to
the center of pumping, the water level in the Upper Floridan         simulate regional pumping conditions accurately after October
aquifer rose an estimated 140 ft on average; observed water          2002. The proximity of the Scenario B pumpage change to the
levels rose 21.5, 18, and 11.5 ft at locations 1.1, 2, and 4.6 mi    southern specified-head boundary in the MODFLOW model
from the pumping center, respectively (Peck and others,              may diminish the simulated response, particularly in the direc-
2005). The center of the measured rise in water level is offset      tion of that boundary. Discretization of the model also may
from the pumping center because wells within the plant were          contribute to the discrepancy between observed water levels
unavailable for water-level measurement, and the contours are        and simulated results.
based on the distribution of available data. If the difference            Observed water levels for the Brunswick aquifer system and
in simulated water levels for Scenario B and the Base Case is        the Lower Floridan aquifer also indicate a distinct water-level
considered a simulated recovery in response to the shutdown          recovery caused by the shutdown, although fewer observa-
of the wells at the Durango Paper Company, then the model            tions exist to evaluate the effects. At a location 0.2 mi from the
generally predicts a smaller recovery, by a maximum of about         center of pumping, observed water levels in the Upper Bruns-
10 ft, than was observed for water levels (fig. 25). Away from       wick aquifer recovered 12.9 ft during the 8 months following
the pumping center, observed water-level changes resulting           the shutdown. The water level in a Lower Floridan aquifer well
from the shutdown are difficult to discern from regional rises       located the same distance from the pumping center rose 18.2 ft
that began during 2000 (fig. 24; Peck and others, 2005). Close       during this time period. The magnitude of simulated recovery
to the pumping center, some of the observed recovery may             of 14 ft in the Lower Floridan aquifer is somewhat less than
be attributed to the regional trend. For example, although the       observed, and the simulated maximum recovery of 2 ft in the
model simulates a rise of less than 3 ft in the Upper Flori-         Brunswick aquifer system also is less than observed.
0	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

                                                 – 30
                                                                                                                                                                        Lower Floridan




                                                                                                                               Industrial shutdown
                                                                                                                                                                        aquifer, 33D073
          ABOVE (–) AND BELOW NAVD 88, IN FEET   – 25


                                                 – 20
                                                                                                                                                                        Upper Floridan
               DAILY MEAN WATER LEVEL




                                                                                                                                                                        aquifer, 33D069
                                                 –15


                                                 –10                                                                                                                    Upper Brunswick
                                                         Blank where                                                                                                     aquifer, 33D071
                                                         data missing
                                                  –5


                                                   0


                                                   5


                                                  10


                                                  15
                                                                    2000                2001                        2002                                                 2003


                                                        Figure	2.. Hydrographs.for.St..Marys.                                                                 CAMDEN
                                                                                                                                                                            Naval
                                                        well.cluster.(33D071.and.33D073).and.                                                                             Submarine




                                                                                                                                                                                              A t l a n t i c O c ean
                                                                                                       GEORGIA                                       Kingsland              Base
                                                        nearby.National.Park.Service.well.(33D069),.                                                 95
                                                                                                                                                                          Kings Bay
                                                                                                           Map
                                                        Camden.County,.Georgia,.2000–2003.                 area
                                                                                                                                                                     St Marys
                                                                                                                                                          GA      33D071
                                                        (modified.from.Peck.and.others,.2005).                             N                                      33D073
                                                                                                                                                          FL
                                                                                                         FLO
                                                                                                               RI                                                        33D069
                                                                                                                    D
                                                                                                                                                     17        NASSAU
                                                                                                                    A




                                                                                                                                   Base from U.S.                           0     1.5   3 MILES
                                                                                                                                   Geological Survey
                                                                                                                                   1:100,000 and 1:250,000-                 0 1.5 3 KILOMETERS
                                                                                                                                   scale digital data




      Simulated pumpage for Scenario B is lower by about                                               County, Ga., area, pumping has resulted in the development
36 Mgal/d than simulated pumpage for the Base Case (tables 2                                           of a large cone of depression in the Upper Floridan aquifer
and 6). This resulted primarily in a decrease in the inflow from                                       potentiometric surface, which extends into Beaufort County,
and an increase in the outflow to the specified-head boundary                                          S.C. (figs. 8 and 9). In addition, increase in pumpage at
(fig. 26). The effect was larger on flows at the specified-head                                        Hilton Head Island since the 1960s likely has resulted in
boundary because the change in pumpage was localized and                                               localized drawdown of the Upper Floridan aquifer potentio-
proximal to the southern specified-head boundary. Within the                                           metric surface, as indicated by localized fluctuation of water
model boundaries, the decrease in pumpage also resulted in a                                           levels in response to pumpage fluctuations (Hayes, 1979).
decrease in net landward flow relative to the Base Case (fig. 19).                                     To separate the effects of these two causes of drawdown,
                                                                                                       Scenario C1 simulates the flow system for a hypothetical
                                                                                                       pumping history to 2000, assuming pumping never occurred
Scenarios	C1	and	C2:	Relative	Effects	of	                                                              in Chatham County, Ga., and Scenario C2 simulates the flow
Pumping	in	Chatham	County,	Georgia,	and	                                                               system for a hypothetical pumping history to 2000, assuming
Southern	Beaufort	County,	South	Carolina                                                               pumping never occurred in southern Beaufort County, S.C.
                                                                                                       Results from these two simulations during 2000 are compared
     Scenarios C1 and C2 illustrate the relative effects of                                            with each other and with those for the Base Case simulations
pumping in Chatham County, Ga., and southern Beaufort                                                  to evaluate the individual and combined effects of stresses on
County, S.C., on ground-water levels and saltwater distribu-                                           the ground-water flow system and the transport of saltwater in
tion and movement in that area. In the Savannah, Chatham                                               these areas.
                                                                        Simulation	of	Ground-Water	Management	Scenarios	 	 1

         82º                                                                                                                    81º30'

                                                                                                                                                     GLYNN
           CHARLTON

  31º

                                                              5.08
                                                                                                                  5.33
                                             6.59
                                                                                                                       6.33
                                                                                                                        6.1                        6.55

                                                                       CAMDEN



                                                                                                                                           7.48 7.31
                                                                                                             4.03
                                                               5.93                         10         10 9.17                               8.54
                                                  5.85
                                                                             4.41                          8.72     10.7
                                   GA                                                              7.67                          12.18

                                   FL                                 6.45                  7.33                         20                 10.14
                                                                                                                         30                 11.3
                                                                              7.51         7.49     12.5
                                                                                                                       20
                                                                                                  18.77




                                                                                                           15
                                                                                                                            St Marys

                                                                                                                            22.91
                                                                                                                            21.1
                                                                                                                  10                               8.34
                                                                                                                                             Fernandina
                                                                                    7.91                                                     Beach




                                                                                                                                                          Atlantic Ocean
                                                                                                                                7.49
                                                                                                                                            4.68
                                NASSAU
                                                                                                   5
                                                                                      4
                                                                      3




                                                                                                                                              2.56
                                           6.81
                                                          2




                                                                                                                                    5.94

                                                  1




30º30'


                                                                                                 DUVAL




         Base from U.S. Geological Survey                                                    0                              5                  10 MILES
         1:100,000- and 1:250,000-scale digital data
                                                                                             0               5                  10 KILOMETERS


                                                                             EXPLANATION
                                           10            Line of equal water-level change— Shows approximate
                                                           change in water level in the Upper Floridan aquifer,
               GEORGIA                                     September 2001 through May 2003. Interval 10 feet

                                           10            Change in simulated head in the Upper Floridan aquifer
                                                          between Scenario B and Base Case (2000 ) —
                                                          Intervals 1 and 5 feet
                                           7.91
           FLORIDA                                       Observation well and water-level change in feet,
                     Map                                  September 2001 through May 2003
                     area
                                                         Pumping center

    Figure	2.. Observed.water-level.change.from.September.2001.through.May.2003.in.wells.
    completed.in.the.Upper.Floridan.aquifer.(Peck.and.others,.2005).and.simulated.change.in.
    potentiometric.surface.for.Scenario.B.(Durango.Paper.Company.pumping.eliminated).in.
    Camden.County,.Georgia,.and.Nassau.County,.Florida.
2	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	




Table	.. Flow-budget.components.for.Scenario.B.
[Results from MODFLOW model; in million gallons per day; —, not applicable]


                                                                       Inflow                                                                                    Outflow

    Model		                                From		               From		             From		                                To		                 To		                  To		
     unit                               general-head	       general-head	        specified-	                        general-head	        general-head	           specified-	    Discharge		
                                                                                                    Total	                                                                                      Total	
                                         boundary,		         boundary,	            head		                            boundary,		          boundary,		              head		        to	wells
                                          onshore             	offshore          boundary                             onshore              offshore              boundary
       1                                    285.83              22.98                —              308.81            115.92                   16.07                 —             —           131.98
       2                                     46.51               —                   —                  46.51              3.63                 —                    —             —             3.63
       3                                       —                 —                   —                  —                   —                   —                    —              0.24         0.24
       4                                       —                 —                   —                  —                   —                   —                    —             —             —
       5                                    141.29               —                 688.99           830.29             22.29                    —                 276.60          633.72       932.61
       6                                       —                 —                    0.00               0.00               —                   —                    0.00          —             0.00
       7                                       —                 —                  14.60               14.60               —                   —                    2.79         128.67       131.45
Total all units                             473.63              22.98              703.60         1,200.21            141.84                   16.07              279.38          762.63      1,199.92

Percent flow                                 39.5                1.9                58.6            100.0              11.8                     1.3                 23.3           63.6        100.0




                                      20
                                                                                                                                     9.29
                                      10
           MILLION GALLONS PER DAY




                                                                                                 0.27               0.55                                0                            0
                                       0
                                               –0.56          –0.78
                                     – 10
                                     – 20
                                     – 30                                       – 24.26
                                     – 40                                                                                                                             –35.71
                                     – 50
                                     – 60
                                     – 70
                                              Inflow from    Inflow from    Inflow from       Outflow to         Outflow to       Outflow to          Pumpage        Pumpage      Pumpage
                                             general-head   general-head     specified-      general-head       general-head      specified-            unit 3         unit 5       unit 7
                                               boundary,      boundary,         head          boundary,          boundary,          head
                                                onshore        offshore      boundary          onshore            offshore        boundary

                                                                                                          FLOW COMPONENT

                                            Figure	2.. Simulated.flow.budget.for.Scenario.B.(Durango.Paper.Company.pumping.eliminated),.
                                            relative.to.the.Base.Case.(2000).
                                                                       Simulation	of	Ground-Water	Management	Scenarios	 	 


Scenario.C1:.Elimination.of.Pumping.in.                             C1 and the Base Case are even higher for the Lower Floridan
                                                                    aquifer, with a maximum simulated difference of about 160 ft
Chatham.County,.Georgia                                             (fig. 29C). These large differences result from reductions
      The pumpage distributions for Scenario C1 simulations         in drawdown in the Lower Floridan aquifer attributable to
are the same as for the Base Case simulations, except that          (1) elimination of pumpage from the Lower Floridan aquifer,
pumpage in all hydrologic units is eliminated within Chatham        and (2) elimination of pumpage from the Upper Floridan aqui-
County, Ga., from predevelopment to 2000. Table 1 shows             fer, which reduces leakage from the Lower Floridan aquifer. In
a reduction in total pumpage of more than 71 Mgal/d in the          the Lower Floridan aquifer, a smaller reduction in pumpage is
model area during 2000 for Scenario C1 relative to the Base         required to produce the same reduction in drawdown as in the
Case. Simulated water levels, water-level differences, flow-        Upper Floridan aquifer, because the Lower Floridan aquifer is
budget components, and chloride distributions are evaluated         assigned a lower hydraulic conductivity. The simulated water
for Scenario C1 simulations during 2000.                            levels in the Brunswick aquifer system are about 80 ft higher
                                                                    for Scenario C1 than those for the Base Case because of
Ground-Water.Flow                                                   reduced leakage to the Upper Floridan aquifer (fig. 29A).
                                                                          Pumpage for Scenario C1 is about 72 Mgal/d lower than
      Simulated potentiometric surfaces are presented for the
                                                                    for the Base Case (tables 2 and 7). This relative decrease
Upper and Lower Floridan aquifers and the Brunswick aquifer
                                                                    in pumpage resulted in a decrease in the inflow to and an
system, as generated by the regional MODFLOW model, and
                                                                    increase in the outflow from all model boundaries (fig. 30).
for the Upper Floridan aquifer as generated by the SUTRA
                                                                    The change in boundary fluxes is evenly distributed among the
model of the Savannah–Hilton Head Island. Because of its
                                                                    boundaries, compared to other simulation results. Although the
finer resolution and calibration to local data, the SUTRA
                                                                    change in stress for Scenario C1 is localized in the northern
model provides a more precise depiction of ground-water flow
                                                                    part of the model area, its effect is widespread—the magnitude
in the Savannah–Hilton Head Island area.
                                                                    of change in flux at the southern specified-head boundary is
      The simulated potentiometric surfaces for Scenario C1
                                                                    similar to the magnitude of change in flux at the general-head
(figs. 27 and 28A) differ distinctly from the simulated poten-
                                                                    boundary. Within the model boundaries, the decrease in pump-
tiometric surfaces for the Base Case (figs. 8 and 9). The most
                                                                    age also resulted in a notable decrease in net landward flow,
notable differences are the elimination of the large cones of
                                                                    relative to the Base Case (fig. 19), because of the proximity of
depression centered at Savannah, Ga., in the Upper Floridan
                                                                    the large change in stresses to the coastal area.
aquifer, Lower Floridan aquifer, and Brunswick aquifer sys-
tem. The elimination of pumpage in Chatham County reveals
the presence of a simulated cone of depression in the Upper         Solute.Transport
and Lower Floridan aquifers at Hilton Head Island (figs. 27               Results of the Scenario C1 solute-transport simulation
and 28A). The maximum depth of the depression in the Upper          using the SUTRA model show the development of chlo-
Floridan aquifer is almost 8 ft below NAVD 88 on the southern       ride plumes in the Upper Floridan aquifer in the same three
part of the island (fig. 28A).                                      locations as for the Base Case: Colleton River, Pinckney
      Because of the lower simulated pumpage, simulated             Island, and the northern end of Hilton Head Island (fig. 31).
water levels in all aquifers for Scenario C1 are higher relative    The plumes originating at Colleton River and Pinckney
to those for the Base Case (figs. 28B and 29). The water-           Island are of lesser extent for Scenario C1 than for the Base
level difference in the Upper Floridan aquifer is greatest near     Case. The plume at the northern end of Hilton Head Island
the pumping center at Savannah, where the water level is            is approximately the same shape and only slightly smaller
about 140 ft higher for Scenario C1 than for the Base Case          overall for Scenario C1 than for the Base Case. This indicates
(fig. 28B). This simulated trend extends at least as far north as   that pumping in Chatham County has a greater influence on
the northern part of Hilton Head Island, where the water level      chloride transport from the two western source areas than on
for Scenario C1 is about 2 ft higher than that for the Base         chloride transport from the source area on the northern end of
Case (fig. 28B). Differences in water level between Scenario        Hilton Head Island.
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

      A. Brunswick aquifer system
         (unit 3)
                                                                  SC                                                                                               EXPLANATION
                                                                GA
                                                                                                                                    50             Simulated potentiometric contour—




                                                                                                                Mo
                                                                                                                                                     Shows simulated potentiometric surface




                                                                                                                   d
                                                                                                                   el
                                                                                                                                                     for Scenario C1, in feet. Hachures indicate




                                                                                                                    bo
                                                                                                                                                     depression. Contour intervals 10 and




                                                                                                                       un
                                                                                                                                                     50 feet. Datum is NAVD 88




                                                                                                                          da
                                                            0




                                                                                                                           ry
                                                          10

                                                                  50
                                                                  40 30                                 Approximate
                                                                                       Savannah          limit of Brunswick



                                                                           20
                                                                                                         aquifer system




                                                                                           10



                                                                                                       an
                                                                                                   Oce
                                                                                                 ntic              B. Upper Floridan aquifer
                                                                                                Atla
                                                                                                                      (unit 5)   350
                                                                                                                                                                      200

                                        GA                                                                                               300
                         N              FL                                                                                               250
                                                                                                                                                                                            50
                                                                                                                                                                                              30




                                                                                                                                          0
                                                                                                                                         20
                                                                                                                    250




                                                                                                                                                0                           40
                                  0                 25          50 MILES                                                                      15


                                                                                                                                                        0
                                                                                                             200

                                                                                                                                                    10
                                                                                                                                                                                20
                                  0         25       50 KILOMETERS                                                                                                                                 0
                              Base from U.S. Geological Survey
                                                                                                            0                                                      10
                              1:100,000-scale digital data                                              25


                                                                                                                                                           20                                10
                                                                                                                            50




                                                                                                                                                         30
      C. Lower Floridan aquifer
                                                                                                                                                   40




                                                                                                                                                                   10
         (unit 7)         350
                                                                           200
                                                                                                                               50




                                      300
                                      250
                                                                                                 50
                                                                                                                                                                    l l l




                                                                                                   30
                                                                                                                                                                            0
                                                                                                                                                                                l




                                            0
                                       20
                                                                                                                                                                                 l




                                                                                                                                                                    0
                                                                                                                                                                   –10
                                                                                                                                                                                    l
                     0
                   25




                                                                                                                                                                                    l l




                                                                                                                                          50                        –20
                                        15
                                                0                            40                                                                                                  l l
                                                                                                                                                                                10




                                                                                                                                                                            l
             200
                                                      0




                                                                                                                                                                                       20
                                                                                  20
                                                    10




                                                                                                  0




         0
       25                                                                                                                                                           SOUTH
                                                                                                                                               GEORGIA             CAROLINA
                                                           20                                     10
                         50




                                                         30            10
                                                40




                                                                                                                                                    F L OR
                                                                                                                                                             ID
                             50




                                                                                                                                                               A


                                                                              l
                                                                        l


                                                                                  l
                                                                             0
                                                                       l



                                                                                  l




                                                                       –10
                                                                                      l
                                                                        –20



                                                                                       l




                                       50                                                                                 Figure	2.. Simulated.potentiometric.surfaces.for.Scenario.C1.
                                                                              10 0
                                                                                      l




                                                                                  l

                                                                 –30
                                                                                          20                              (Chatham.County,.Georgia,.pumping.eliminated).for.the..
                                                                                                                          (A).Brunswick.aquifer.system.(unit.3),.(B).Upper.Floridan.
                                                                                                                          aquifer.(unit.5),.and.(C).Lower.Floridan.aquifer.(unit.7).
                                                                                                                                                               Simulation	of	Ground-Water	Management	Scenarios	 	 

A. Upper Floridan aquifer (unit 5)— Simulated potentiometric surface                                                                                      B. Upper Floridan aquifer (unit 5)— Change in simulated head

                SC                                                                       4                                   02              2
           GA                                                                                                                                                                          10




                                                                                     2
   40


                                                                                                                                                                                                  8
                                                                                                                                                                                                       6
           30




                                                                                                                                                                                                               4
                           18
                                                            JASPER                                                                                               20




                                                                                                                                                                                                                        2
                                     12
 EFFINGHAM                      14
                 20




                                                8
                                                                                                                 BEAUFORT
                     16




                                                       6
                                 10
                                                                                                                         Po
                                                                                                                           rt
                                                                                         4                                    Ro
                                                                                                                                y



                                                              4
                                                                                                                           2 al          So                           40
                                                                                                                                           un




                                                                         2
                                                                                                         l               l
                                                                                                                                             d
                                                                                                                                 l       l
                                                                                             l                           l
                                                                                                             l
                                                                                 l                   l




                                                                                                                             l
                                                                                                      –4




                                                                                                                                             l
                                                                                                                                 Hilton
                                                                                     l                                   l

                                                                     l
                                                                                                 l
                                                                                                                               Head                                        60                                                      1




                                                                                                                              l
                                                                         l
                                                                                                     –6




                                                                                                                         l
                                                                                     l
                                                                         l




                                                                                                                                          l
                                                                     l
                                                                                                                             Island




                                                                                                                             l
                                                                                                                     l
                                                                                                                 l




                                                                                                                                                                                80
                                                                                                         l
                                                                                     l           l
                                                                         l

            CHATHAM



                                                                                                                         l


                                                                                                                                         l
                                                                     l


                                                                             l                                       l                                                                                                             3
                                                                                         l           –2                              l
                                                                                                                             l                                                                                                     5
                                                                     l



                                                                             l               l 0 l
                           Savannah                                                                                                                                                   100                                          7
                                                                                                                                                                                       90                                         9
                                                                                                             2
                                                               Tybee I                                                                                                                  70
                                                     ay
                                                        I                                                4
                                                                                                                                         an
                                                aw                                                                                                                                           50
                                           id
                                      Sk
                                                                                                                                     Oce



                                                                                                     6
            BRYAN
                                                                                                                              ic




                                                                                                                                                  N                                               30
                                                                                                                             ant




                                                                                     8
                                                                                                                         Atl




                                                                                                                                                                                                                                       10
                                                       12       10
 LIBERTY
                      14

Base from U.S. Geological Survey                                                                                                                                                                           0       5        10   15 MILES
1:100,000-scale digital data
                                                                                                                                                                EXPLANATION                                0   5   10   15 KILOMETERS



                                                                                 Change in simulated head                                                                  10        Line of equal change in simulated
                                                                                  between Scenario C1 and                                                                              head between Scenario C1 and
                                  SOUTH                                           Base Case (2000), in feet                                                                            Base Case (2000) — Intervals
            GEORGIA              CAROLINA
                                                                                                                                                                                       1 and 10 feet
                                                                                                                              0
                                                                                                                              10                                           10        Simulated potentiometric contour —
                          Map
                          area                                                                                                20                                                       Shows simulated potentiometric
                                                                                                                              40                                                       surface for Scenario C1.
                                                                                                                                                 Increase in                           Hachures indicate depression.
                                                                                                                              60                 water level
                              Model
                                                                                                                                                                                       Contour intervals 2 and 10 feet.
                            boundary                                                                                          80                                                       Datum is NAVD 88
                                                                                                                              100
                                                                                                                              160


Figure	2.. Results.using.solute-transport.model.for.(A).simulated.potentiometric.surface.for.Scenario.C1.(Chatham.County,.
Georgia,.pumping.eliminated).and.(B).change.in.simulated.head.between.Scenario.C1.and.the.Base.Case.(2000).for.the.
Upper.Floridan.aquifer.in.Chatham.County,.Georgia,.and.southern.Beaufort.County,.South.Carolina.
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models

       A. Brunswick aquifer system
          (unit 3)                                                                                           EXPLANATION
                                           SC                                                    Change in simulated head between
                                         GA                                                       Scenario C1 and Base Case (2000),
                                                                                                  in feet




                                                                         Mo
                                                                                                             0




                                                                            d
                                                                            el
                                                                                                                    10




                                                                              bo
                                                                                un
                                                                                                                    20




                                                                                  da
                                                                                                                            Increase in




                                                                                    ry
                                                                                                                    40
                                                                                                                            water level
                                                                    Approximate
                                                                                                                    60
                                                                     limit of Brunswick                             80
                                                                     aquifer system
                                                                                                                    100
                                                                                                                    160




                                                                  an
                                                              Oce
                                                            ntic           B. Upper Floridan aquifer
                                                           Atla
                                                                              (unit 5)

                              GA
                   N          FL




                          0         25   50 MILES

                          0    25    50 KILOMETERS

                        Base from U.S. Geological Survey
                        1:100,000-scale digital data




       C. Lower Floridan aquifer
          (unit 7)




                                                                                                                  SOUTH
                                                                                                GEORGIA          CAROLINA




                                                                                                  F L OR
                                                                                                           ID
                                                                                                             A




                                                                                     Figure	2.. Change.in.simulated.head.between.Scenario.C1.
                                                                                     (Chatham.County,.Georgia,.pumping.eliminated).and.the.Base.
                                                                                     Case.(2000).for.the.(A).Brunswick.aquifer.system.(unit.3),..
                                                                                     (B).Upper.Floridan.aquifer.(unit.5),.and.(C).Lower.Floridan.
                                                                                     aquifer.(unit.7).
                                                                                                                 Simulation	of	Ground-Water	Management	Scenarios	 	 

Table	.. Flow-budget.components.for.Scenario.C1.
[Results from MODFLOW model; in million gallons per day; —, not applicable]

                                                                        Inflow                                                                         Outflow

     Model		                                From		               From		             From		                         To		                To		                   To		
      unit                               general-head	       general-head	        specified-	                 general-head	       general-head	            specified-	    Discharge		
                                                                                                   Total	                                                                                   Total	
                                          boundary,		         boundary,	            head		                     boundary,		         boundary,		               head		        to	wells
                                           onshore             	offshore          boundary                      onshore             offshore               boundary

        1                                     262.58            14.69                —            277.28        122.96                   18.87                —              —            141.83
        2                                      45.26              —                  —             45.26             3.98                 —                   —              —               3.98
        3                                       —                 —                  —              —                —                    —                   —               0.24           0.24
        4                                       —                 —                  —              —                —                    —                   —              —               —
        5                                     140.77              —                695.41         836.18         22.63                    —                 275.78          600.76        899.17
        6                                       —                 —                   0.00             0.00          —                    —                   0.00           —               0.00
        7                                       —                 —                 14.99          14.99             —                    —                   2.74          125.43        128.17
Total all units                               448.61            14.69              710.40       1,173.71        149.57                   18.87              278.52          726.44       1,173.40

Percent flow                                   38.2               1.3               60.5          100.0          12.7                     1.6                23.7            61.9         100.0




                                       20
                                       10                                                       8.00                          8.43
                                                                                                              3.36
            MILLION GALLONS PER DAY




                                                                                                                                                   0
                                        0
                                      – 10                                                                                                                                     – 3.23
                                                               –9.07
                                      – 20
                                                                                 –17.46
                                      – 30      – 25.58
                                      – 40
                                      – 50
                                      – 60
                                                                                                                                                       – 68.67
                                      – 70
                                               Inflow from    Inflow from    Inflow from      Outflow to   Outflow to       Outflow to          Pumpage        Pumpage        Pumpage
                                              general-head   general-head     specified-     general-head general-head      specified-            unit 3         unit 5         unit 7
                                                boundary,      boundary,         head         boundary,    boundary,          head
                                                 onshore        offshore      boundary         onshore      offshore        boundary

                                                                                                         FLOW COMPONENT

                                             Figure	0.. Simulated.flow.budget.for.Scenario.C1.(Chatham.County,.Georgia,.pumping.eliminated),.
                                             relative.to.the.Base.Case.(2000).
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	


                                                                                                                                           SOUTH
                                                                             Parris Island                                 GEORGIA        CAROLINA


                                                                                                                                                     Map
                                                                                                                                                     area
                                                                                                                                      Study
                                                                  Po                                                                  area
              ol                                                       rt
             C




                   let                                                      Ro
                         on                                                      ya
                              R                                                       lS
                                                                                           ou
                                                                                                nd                                        Model
                                                                                                                                        boundary


                                                                                                                                    EXPLANATION
                                               I
                                                                                                                    Simulated chloride concentration, in milligrams per
                                           ney

                                                                                                                      liter (mg/L), in ground water in the middle of the
                                             ck
                                         Pin



                                                                                                                      Upper Floridan aquifer for Scenario C1

                                                                                                                     250 to 2,000
                                                          Hilton Head Island                                         2,000 to 6,000
                                                                                                                     6,000 to 10,000
                                                                                                                     Greater than 10,000
                                                                                                 n
                                                                                                ea
                                                                                            Oc

                                                                                                                    Chloride concentration, in mg/L, in ground water
                                                                                                                      in the middle of the Upper Floridan aquifer
                                                                                      tic




                                                                                                     N
                                                                                                                      for Base Case (2000)
                                                                                 la n
                                                                             At




                                                                                                                     250

    Base from U.S. Geological Survey
                                                                                                                     10,000
                                           0          1       2        3              4          5 MILES
    1:100,000 and 1:250,000-scale data
                                           0      1       2   3   4     5 KILOMETERS                                Observation well


    Figure	1.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper.Floridan.aquifer.in.the.Hilton.Head.
    Island,.South.Carolina,.area.for.Scenario.C1.(Chatham.County,.Georgia,.pumping.eliminated).




Scenario.C2:.Elimination.of.Pumping.in.Southern.                                                           aquifer for Scenario C2, relative to the Base Case, as indicated
                                                                                                           by a southwestern shift in the position of the 0- and –10-ft
Beaufort.County,.South.Carolina                                                                            contours at Hilton Head Island (figs. 33A and 9). The resulting
      Pumpage distributions for Scenario C2 simulations are                                                extent of the simulated Upper Floridan aquifer potentiometric
the same as for the Base Case simulations, except that pump-                                               cone of depression centered at Savannah is somewhat smaller
age in all hydrologic units is eliminated within southern                                                  for the Scenario C2 than for the Base Case. Elsewhere, the
Beaufort County, S.C. (southwest of Port Royal Sound; fig. 1)                                              extent and magnitude of this cone of depression is generally
from predevelopment to 2000. Table 1 shows a reduction                                                     the same for Scenario C2 as for the Base Case.
in total pumpage of about 17 Mgal/d for the model area for                                                       Simulated water levels in all aquifers for Scenario C2,
Scenario C2 during 2000 relative to the Base Case. Simulated                                               are higher relative to those for the Base Case (figs. 33B and
water levels, water-level differences, flow-budget compo-                                                  34), mostly in the coastal area of Beaufort and Jasper Coun-
nents, and chloride distributions are evaluated for Scenario C2                                            ties, S.C., and eastern Chatham County, Ga. This relative
simulations during 2000.                                                                                   increase in Upper Floridan aquifer water levels is great-
                                                                                                           est at the northern end of Hilton Head Island (about 11 ft),
Ground-Water.Flow                                                                                          and extends to Savannah, where the maximum increase is
      At a regional scale, simulated potentiometric surfaces                                               about 2 ft (fig. 33B). The simulated water-level increase in
for Scenario C2 generated using the steady-state MODFLOW                                                   the Lower Floridan aquifer is similar to that for the Upper
model (fig. 32) appear similar to simulated potentiometric                                                 Floridan aquifer, and is generally less than 2 ft and limited to a
surfaces for Base Case (fig. 8). Differences are more apparent                                             small part of Jasper and Beaufort Counties for the Brunswick
at the scale of the Savannah–Hilton Head Island study area, as                                             aquifer system (fig. 34). Because the presence and hydrau-
shown for results generated using the transient SUTRA model                                                lic characteristics of these units are poorly defined in South
(fig. 33A). Elimination of pumpage in southern Beaufort                                                    Carolina, these results are less reliable than those for the
County results in higher water levels in the Upper Floridan                                                Upper Floridan aquifer.
                                                                                                                                                           Simulation	of	Ground-Water	Management	Scenarios	 	 

A. Brunswick aquifer system
   (unit 3)
                                                          SC                                                                                                                                             EXPLANATION
                                                        GA
                                                                                                                                                                           50          Simulated potentiometric contour—




                                                                                                                                                      Mo
                                                                                                                                                                                         Shows simulated potentiometric surface




                                                                                                                                                         d el
                                                                                                                                                                                         for Scenario C2, in feet. Hachures indicate




                                                                                                                                                           bo
                                                                                                                                                                                         depression. Contour intervals 10 and




                                                                                                                                                             un
                                                                                                                                                                                         50 feet. Datum is NAVD 88




                                                                                                                                                                 da
                                                    0
                                               10




                                                                                                                                                                 ry
                                                        50




                                                         40
                                                         30 0
                                                                                          –5
                                                                                             0                                                 Approximate
                                                             2




                                                                                      –40
                                                                              –20
                                                                                                                                                limit of Brunswick
                                                                                              –30                                               aquifer system
                                                           10                 –1
                                                                                      0

                                                                                                          0




                                                                                                                                   an
                                                                                                                               Oce
                                                           10




                                                                                                                           ntic
                                                                                                                                                       B. Upper Floridan aquifer

                                                                                                                       Atla
                                                                                                                                                          (unit 5)   350
                                                                                                                                                                                                              200

                              GA
                 N               FL                                                                                                                                             250
                                                                                                                                                                                                                                                                                  50
                                                                                                                                                                                                                                                                                    30




                                                                                                                                                                                 0
                                                                                                                                                                                20
                                                                                                                                                             0
                                                                                                                                                           25

                                                                                                                                                                                      0                          40
                        0                 25            50 MILES                                                                                                                  15                              20                                     10




                                                                                                                                                                                          0
                                                                                                                                                     200
                                                                                                                                                                                                                                 l                   l l
                                                                                                                                                                                                                                                         –10
                                                                                                                                                                                                                                                                     l




                                                                                                                                                                                          10
                                                                                                                                                                                                                             l           l           l                       l




                                                                                                                                                                                                                     l
                                                                                                                                                                                                                                                                                              0




                                                                                                                                                                                                                     l

                                                                                                                                                                                                                                 0
                        0         25       50 KILOMETERS




                                                                                                                                                                                                                                                                     l
                                                                                                                                                                                                                         –3




                                                                                                                                                                                                                                                                             l l
                                                                                                                                                                                                             l
                                                                                                                                                                                                                     l




                                                                                                                                                                                                                                                                         l
                                                                                                                                                                                                                                             –100




                                                                                                                                                                                                                                     –5
                                                                                                                                                                                                             l
                                                                                                                                                                                                                     l




                                                                                                                                                                                                                                                                             l
                                                                                                                                                                                                                                                                                      l
                      Base from U.S. Geological Survey                                                                                                                                                                                                           0




                                                                                                                                                                                                                                         0
                                                                                                                                                                                                                                                             –4




                                                                                                                                                                                                                     l
                                                                                                                                                                                                                             l
                                                                                                                                                 0




                                                                                                                                                                                                                                                                             l
                                                                                                                                                                                                         l
                                                                                                                                               25                                                                                    l




                                                                                                                                                                                                                                                                     0


                                                                                                                                                                                                                                                                                          l
                      1:100,000-scale digital data




                                                                                                                                                                                                                                                                 –2
                                                                                                                                                                                                                                                 l




                                                                                                                                                                                                                                                                             l
                                                                                                                                                                                                                                                         l               l
                                                                                                                                                                                                    0                                                            l




                                                                                                                                                                                                         l




                                                                                                                                                                                                                                                                                      l
                                                                                                                                                                                                                 l
                                                                                                                                                                                                                         l                   l               l           l       0l
                                                                                                                                                                  50




C. Lower Floridan aquifer                                                                                                                                                                                                                                            10
   (unit 7)         350                                                                                                                                                                                      0
                                                                   200
                                                                                                                                                                      50




                            300
                            250
                                                                                                                               50                                                                        0
                                                                                                                                 30
                                                                                                                                                                                                             0
                             0
                            20
            0
            25




                                                                                                                                                                                                             –20




                                      0                                40                                                                                                        50                      –10
                                 15                                      20                       10                                       0                                                                                     10
                                           0




      200
                                                                              l                   l
                                                                                                  –1                                                                                                                                                 20
                                          10




                                                                                  l           l
                                                                                                               l           l
                                                                        l l
                                                                                                     0
                                                                                                                               l
                                                                              0
                                                                l
                                                                       l
                                                                         –3



                                                                                                                   l
                                                                                                                           l




                                                                                          –100
                                                               l
                                                                       l

                                                                                  –5




                                                                                                                       l




                                                                                                               0
                                                                                                                                       l
                                                                                      0




                                                                                                          –4
                                                               l




                                                                                                                       l
                                                                         l




  0                                                                           l                                        0
25                                                                                        l                    –2                                                                                         SOUTH
                                                                                                                                   l
                                                                                                                       l
                                                               l




                                                                                                      l            l
                                                    0              l                                                               l                                                   GEORGIA           CAROLINA
                                                                         l                                                 l
                                                                                                                           0
                                                                                      l               l            l
                                               20
                 50




                                           30
                                  40




                                                                                                               10

                                                                                                                                                                                          F L OR
                 50




                                                                                                                                                                                                   ID
                                                                                                                                                                                                     A



                                                                                                                                                                      Figure	2.. Simulated.potentiometric.surfaces.
                                                               –10                                                                                                    for.Scenario.C2.(southern.Beaufort.County,.South.
                                                                                                                                                                      Carolina,.pumping.eliminated).for.the.(A).Brunswick.
                                                                           0
                                                                –20




                             50
                                                                                  10
                                                                                              20                                                                      aquifer.system.(unit.3),.(B).Upper.Floridan.aquifer.
                                                         –30
                                                                                                                                                                      (unit.5),.and.(C).Lower.Floridan.aquifer.(unit.7).
0	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	




  A. Upper Floridan aquifer (unit 5)— Simulated potentiometric contour                                                                                                                                  B. Upper Floridan aquifer (unit 5)— Change in simulated head
                SC                                                                                                                                                     0
                        20




                                                                                                                                                                              0
           GA
      30




                                                                                                                          0




                                                                                                                                                                                                                             1
                10                                                                                                                           JASPER
                                                                                                                               0


   EFFINGHAM            –10




                                                                                                                                                                                                                                      2
                                                                                                                                                                               BEAUFORT
                                                                                                                                                                              Po
                                                                                                                                                                                rt
                                                                                             –2                                                                                    Ro
                                                                                                                                                                                     ya
                                         l                       l                                           0                                                                         l   So
                         l                                                                                                                                                                   un
                                                                                                                                                                                               d
                                                                                         l
                   0
                 –3
                                                 l                   l
                                 l                                                                               l
                                                                                                                                                                                                                                                4
                                                                                     l
                                                                                                                                                                                   Hilton
                                                     l
                                   0
                                                                         l
                l




                                                                                                             l
                                 –5
                        l




                                                                                                                                 l
                                                         l                                           l
                                                                                                                                                                                 Head                                                                             11




                                                                                                                                                                                                                                                        6
                                                                         l




                                                                                                                                                                                                                                                        8
                                                                                                                            l
                                                                                                                                                                                                                                                                10
                                                                                             l
                                             l




                                                                                                                                                                               Island
                                                                     l                                                                           l
                    l




                                                                                                                     l
           l




                                                                             l
                                                                                                             l
                                     –9 0




                                                                                                                                     l




                                                                                                     l
                                                                                                                                                                                                                                                                 9
                                                                 l            l
                        l
                                     l

                                         7




                                                                                                                           l




                                                                                                 l
                                     –



                                                                                                         l
                                                                                                                     l
                                       0
                l




                                                                                                                                                     l
                                                                                                                                         l




                                                                                                                                                                                                                                                                7
         l




                                 l




                                                                                                         l
                        l




                                                                                                                 l


                                                                                                                                     l
                                         l




                                                                                                                            l




                                                             –10
               CHATHAM
                                                     l




                                                                         0
                l




                                                                                                             l
                                                                                                                          l
                                 l




                                                                                                                                                         l
                                                                                                                                                     l
                                         l




                                                         – 80 Savannah
                                                                                                                                l
                                                         l
           l


                        l




                                                                                                                                      l




                                                                                                                                                                                                                                                                 5
                                                                                                         l
                                                                                                         l
                                             l
                                     l




                                                               l
                l




                                                                                                                 l
                                                                                                                           l



                                                                                                                                                     l
                                                                                                                                                         l




                                                                 l                               l
                         l


                                                     l




                                                                             l
                                                                                                                                     l
                                     l




                                             –6
           l




                                                                                                                 l
                    l




                                                                                                         l
                                                         0
                                                                                                                                                 l




                                                                                 l
                                 l




                                                                                                                          l




                                                                                                                                                         l




                                             l                                                                   l                       l                   Tybee I
                        l




                                                                                                                                                                                                                                                                     3
                l




                      –
                    l 40                                         l                   l
                                                                                                                                l I
                                                                                                                                                                                    an




                                             l                                                               l                   y
                                                                                                                                                     l




                             l                               l                   l                                               a
                                                                                                                             w
                                                                                                                                                                                   Oce




                                                                                                                      i   da                 l
                                             l                                                                   Sk        l
      BRYAN                                                          l                           l
                             –20
                                                                                                                                                                                ic




                        –1
                                                                                                                                                                              ant




                             0                                                                                                                                                                     N
                                                                                                                                                                             Atl




   LIBERTY

  Base from U.S. Geological Survey                                                                                                                                                                                                                  0       5            10   15 MILES
  1:100,000-scale digital data
                                                                                                                                                                                                             EXPLANATION                            0   5   10         15 KILOMETERS



                                                                                                                                                                       Change in simulated head                        10      Line of equal change in simulated
                                                                                                                                                                        between Scenario C2 and                                  head between Scenario C2 and
                                                                      SOUTH                                                                                             Base Case (2000), in feet                                Base Case (2000 ) —Interval 1 foot
                GEORGIA                                              CAROLINA
                                                                                                                                                                                0
                                                                                                                                                                                   2                                   10      Simulated potentiometric contour —
                                                                                                                                                                                                                                 Shows simulated potentiometric
                                     Map                                                                                                                                           4
                                                                                                                                                                                              Increase in                        surface for Scenario C2. Hachures
                                     area                                                                                                                                          6
                                                                                                                                                                                              water level                        indicate depression. Contour interval
                                                                                                                                                                                   8                                             10 feet. Datum is NAVD 88
                                                           Model
                                                                                                                                                                                   10
                                                         boundary
                                                                                                                                                                                   11

  Figure	.. Results.using.solute-transport.model.for.(A).simulated.potentiometric.surface.for.Scenario.C2.(southern..
  Beaufort.County,.South.Carolina,.pumping.eliminated).and.(B).change.in.simulated.head.between.Scenario.C2.and.the.
  Base.Case.(2000).for.the.Upper.Floridan.aquifer.in.Chatham.County,.Georgia,.and.southern.Beaufort.County.
                                                                             Simulation	of	Ground-Water	Management	Scenarios	 	 1

A. Brunswick aquifer system
   (unit 3)                                                                                                 EXPLANATION
                                    SC                                                      Change in simulated head between
                                  GA
                                                                                             Scenario C2 and Base Case (2000),
                                                                                             in feet




                                                                  Mo
                                                                                                         0




                                                                     d
                                                                     el
                                                                       bo
                                                                                                                 1




                                                                         un
                                                                                                                 2




                                                                           da
                                                                             ry
                                                                                                                 4
                                                                                                                        Increase in
                                                             Approximate                                         6      water level
                                                              limit of Brunswick
                                                              aquifer system                                     8
                                                                                                                 10
                                                                                                                 12




                                                           an
                                                       Oce
                                                     ntic
                                                                    B. Upper Floridan aquifer
                                                    Atla
                                                                       (unit 5)

                       GA
            N          FL




                   0         25   50 MILES

                   0    25    50 KILOMETERS

                 Base from U.S. Geological Survey
                 1:100,000-scale digital data




C. Lower Floridan aquifer
   (unit 7)




                                                                                                              SOUTH
                                                                                            GEORGIA          CAROLINA




                                                                                             F L OR
                                                                                                      ID
                                                                                                        A



                                                                                   Figure	.. Change.in.simulated.head.between.
                                                                                   Scenario.C2.(southern.Beaufort.County,.South.
                                                                                   Carolina,.pumping.eliminated).and.the.Base.Case.
                                                                                   (2000).for.the.(A).Brunswick.aquifer.system.(unit.3),.
                                                                                   (B).Upper.Floridan.aquifer.(unit.5),.and.(C).Lower.
                                                                                   Floridan.aquifer.(unit.7).
2	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models

      Major components of the simulated water budgets for                                                      Solute.Transport.
Scenario C2 using the MODFLOW model are summarized in
                                                                                                                    Results of the solute-transport simulation using SUTRA
table 8. The pumpage changes from about 798 Mgal/d for the
                                                                                                               for Scenario C2 show that all chloride plumes developed to
Base Case to about 782 Mgal/d for Scenario C2 (tables 2 and 8).
                                                                                                               a lesser extent than for the Base Case simulation (fig. 36).
The primary response in boundary fluxes to this pumpage is a
                                                                                                               Comparison of Scenarios C1 and C2 indicates that, in general,
decrease in the inflow from the general-head boundary (fig. 35).
                                                                                                               elimination of all pumping in southern Beaufort County, S.C.
The change in pumpage is small and localized, and it has a
                                                                                                               (Scenario C2) has a smaller effect on plume development
minimal effect on the flux at the southern specified-head bound-
                                                                                                               than elimination of all pumping in Chatham County, Ga.
ary. The decrease in flow from the offshore general-head bound-
                                                                                                               (Scenario C1; fig 31). Elimination of pumping in Chatham
ary indicates a reduced potential for saltwater intrusion relative
                                                                                                               County has a greater effect on plume development west
to the Base Case. This decrease is less than that for Scenario C1
                                                                                                               of Hilton Head Island. At the northern end of Hilton Head
by about 6 Mgal/d, indicating that pumpage at Chatham County
                                                                                                               Island, the effect of both scenarios on plume development
may have the potential to draw a larger amount of saltwater into
                                                                                                               is small, although the effect of Scenario C2 appears to be
the Upper Floridan aquifer than pumping at southern Beaufort
                                                                                                               slightly greater (compare figs. 31 and 36).
County. Within the model boundaries, the decrease in pump-
age also resulted in a slight decrease in landward flux from the
offshore area, relative to the 2000 Base Case (fig. 19).


Table	.. Flow-budget.components.for.Scenario.C2..
[Results from MODFLOW model; in million gallons per day; —, not applicable]

                                                                        Inflow                                                                         Outflow

      Model		                                From		              From		              From		                          To		              To		                To		
       unit                               general-head	      general-head	         specified-	                  general-head	     general-head	         specified-	        Discharge		
                                                                                                     Total	                                                                                 Total	
                                           boundary,		        boundary,	             head		                      boundary,		       boundary,		            head		            to	wells
                                            onshore            	offshore           boundary                       onshore           offshore            boundary
         1                                    274.37            20.29                  —            294.67          116.11                15.66               —               —            131.76
         2                                     46.54             —                     —             46.54            3.62                 —                  —               —              3.62
         3                                      —                —                     —              —               —                    —                  —                0.24          0.24
         4                                      —                —                     —              —               —                    —                  —               —             —
         5                                    141.29             —                   712.18         853.48           22.29                 —                267.86           653.08        943.24
         6                                      —                —                     0.00           0.00            —                    —                  0.00            —              0.00
         7                                      —                —                    15.46          15.46            —                    —                  2.33           128.65        130.98
 Total all units                              462.20            20.29                727.64       1,210.14          142.02                15.66             270.19           781.98       1,209.85
 Percent flow                                  38.2               1.7                 60.1          100.0            11.7                  1.3               22.3             64.6         100.0



                                        20
                                        10
                                                                                                  0.45            0.14          0.10
             MILLION GALLONS PER DAY




                                                                                                                                                   0
                                         0
                                                                                   – 0.22                                                                                       – 0.01
                                       – 10                    – 3.47
                                       – 20     – 11.99
                                                                                                                                                                 – 16.35
                                       – 30
                                       – 40
                                       – 50
                                       – 60
                                       – 70
                                               Inflow from    Inflow from        Inflow from    Outflow to     Outflow to    Outflow to          Pumpage       Pumpage         Pumpage
                                              general-head   general-head         specified-   general-head   general-head   specified-            unit 3        unit 5          unit 7
                                                boundary,      boundary,             head       boundary,      boundary,       head
                                                 onshore        offshore          boundary       onshore        offshore     boundary

                                                                                                          FLOW COMPONENT
                                              Figure	.. Simulated.flow.budget.for.Scenario.C2.(southern.Beaufort.County,.South.Carolina,.
                                              pumping.eliminated),.relative.to.the.Base.Case.(2000).
                                                                                                                 Simulation	of	Ground-Water	Management	Scenarios	 	 


                                                                                                                                              SOUTH
                                                                                Parris Island                                    GEORGIA     CAROLINA


                                                                                                                                                        Map
                                                                                                                                                        area
                                                                                                                                         Study
                                                                      Po                                                                 area
              ol                                                           rt
             C




                   let                                                          Ro
                         on                                                          ya
                              R                                                           lS
                                                                                               ou
                                                                                                    nd                                       Model
                                                                                                                                           boundary


                                                                                                                                       EXPLANATION

                                               I
                                           ney                                                                         Simulated chloride concentration, in milligrams per
                                                                                                                         liter (mg/L), in ground water in the middle of the
                                             ck
                                         Pin


                                                                                                                         Upper Floridan aquifer for Scenario C2

                                                                                                                        250 to 2,000
                                                          Hilton Head Island                                            2,000 to 6,000
                                                                                                                        6,000 to 10,000
                                                                                                                        Greater than 10,000

                                                                                                     n
                                                                                                    ea
                                                                                               Oc
                                                                                                                       Chloride concentration, in mg/L, in ground water
                                                                                                                         in the middle of the Upper Floridan aquifer
                                                                                          tic


                                                                                                         N
                                                                                     la n




                                                                                                                         for Base Case (2000)
                                                                                At




                                                                                                                        250
    Base from U.S. Geological Survey       0          1       2        3              4             5 MILES             10,000
    1:100,000 and 1:250,000-scale data
                                           0      1       2   3   4        5 KILOMETERS                                Observation well

    Figure	.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper.Floridan.aquifer.in.the.Hilton.Head.
    Island,.South.Carolina,.area.for.Scenario.C2.(southern.Beaufort.County,.South.Carolina,.pumping.eliminated).




Comparison.of.Head.Gradients.in.the.Savannah–                                                                       The water-level profile for Scenario C1 indicates a head
                                                                                                              gradient similar to predevelopment with direction of flow from
Hilton.Head.Island.Area,.Scenarios.C1.and.C2
                                                                                                              Savannah toward the southern part of Hilton Head Island.
      Simulated water-level profiles for the Upper Floridan                                                   There is also a small cone of depression at Hilton Head Island,
aquifer, along a cross section from Savannah to Port Royal                                                    however, indicating a flow direction from Port Royal Sound
Sound that is approximately parallel to the flow direction,                                                   southwestward toward the southern part of Hilton Head Island.
were generated using results from SUTRA model simulations                                                     The water levels for Scenario C1 are lower than for predevel-
(fig. 37). The water-level profiles for predevelopment condi-                                                 opment conditions and higher than for the Base Case. Fig-
tions and Base Case conditions represent end members, and                                                     ure 37 indicates that eliminating pumping in Chatham County
those for Scenarios C1 and C2 represent intermediate mem-                                                     would have a discernible effect (about 1 ft) as far away as the
bers for which some pumping has been removed. The intent is                                                   northern end of Hilton Head Island.
to examine the relative effects of pumping at Chatham County,                                                       For Scenario C2, the simulated water-level profile is simi-
Ga., and southern Beaufort County, S.C. on water levels, head                                                 lar to that for the 2000 Base Case, indicating a flow direction
gradients, and flow directions in the Savannah–Hilton Head                                                    from Port Royal Sound toward Savannah. The head gradients
Island area.                                                                                                  are similar for Scenario C2 and the Base Case close to Savan-
      For predevelopment conditions, the simulated poten-                                                     nah. Closer to and at Hilton Head Island, water-level profiles
tiometric surface is above NAVD 88 along the entire profile,                                                  for Scenario C2 and the Base Case diverge, and simulated water
and the head gradient indicates a general flow direction from                                                 levels on Hilton Head Island are higher for Scenario C2 than
Savannah toward Port Royal Sound. For Base Case conditions                                                    for the Base Case. No localized cone of depression at Hilton
(2000), the profile shows distinctly one side of the potentio-                                                Head Island is present, as local pumping was eliminated for this
metric cone of depression centered at Savannah, indicating a                                                  scenario. The water levels for Scenario C2 are lower than for
steep head gradient with direction of flow from Hilton Head                                                   predevelopment conditions, and higher than for the Base Case
Island toward Savannah. The profile also shows a smaller                                                      2000 pumping conditions. Figure 37 indicates that eliminating
depression, superimposed on the larger feature, at Hilton                                                     pumping in southern Beaufort County would have a discernible
Head Island.                                                                                                  effect (about 5 ft) as far away as the Jasper–Beaufort county line.
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	




                                                            Savannah
                                                                                                                                                                    Port
                                                                                                                                                                   Royal
                                                                                                                                      Hilton Head Island           Sound
                                                      60


                                                      40
        SIMULATED UPPER FLORIDAN HEAD, IN FEET




                                                      20                                                                          A


                                                 NAVD 88


                                                     – 20


                                                     – 40                                                                                  GA           SC

                                                                                                                                            Map
                                                     – 60                                                                                   area




                                                                                                                                                                    Po
                                                                                                                                                                      rt
                                                                                                                                                                         Ro
                                                                                                                                                                              ya
                                                                                                                                                         SC   A                    lS
                                                                                                                                                                                        oun
                                                                                                                                                    GA                                        d
                                                     – 80
                                                                                                                                                                        Hilton
                                                                                                                                                                      Head
                                                                                                                                           Savannah               Island
                                                    –100                                          Predevelopment




                                                                                                                                                                           n
                                                                                                  Base Case (2000 pumping)




                                                                                                                                                                         ea
                                                                                                                                                                        Oc
                                                    –120




                                                                                                                                                                   tic
                                                                                                  Scenario C1




                                                                                                                                                                  lan
                                                                                                                                                              At
                                                    –140                                          Scenario C2


                                                    –160
                                                               0             5              10             15                20                    25         30                                  35

                                                                             DISTANCE ALONG CROSS SECTION FROM SAVANNAH, GEORGIA, IN MILES


                                                               Figure	.. Cross.section.of.potentiometric.surfaces.for.the.Upper.Floridan.aquifer.for.predevelopment,.
                                                               Base.Case.(2000),.Scenario.C1.(Chatham.County,.Georgia,.pumping.eliminated),.and.Scenario.C2.
                                                               (southern.Beaufort.County,.South.Carolina,.pumping.eliminated)..Point.A.indicates.location.where.
                                                               the.model.predicts.an.equivalent.effect.on.water.levels.by.pumping.at.Chatham.County,.Georgia,.and.
                                                               southern.Beaufort.County,.South.Carolina..Results.from.SUTRA.model.




      Comparison of the water-level profiles for Scenarios C1                                                   Scenarios	D1	and	D2:	Projected		
and C2 shows a location on Hilton Head Island (point A,                                                         Pumping	to	20
fig. 37) where simulated water levels are the same for both
scenarios. The model predicts that pumping at Chatham                                                                Scenarios D1 and D2 represent the flow-system response
County and pumping at southern Beaufort County have an                                                          to projected pumpage in the 24-county coastal area of
equal effect on water levels at point A during 2000.                                                            Georgia, based on two different water-use estimates (Leeth
      The model results indicate that, south of point A, elimi-                                                 and others, 2005) (fig. 38). Pumpage for Scenario D1 is
nating pumping in Chatham County would result in a greater                                                      based on regional economic forecasting, and pumpage for
water-level increase relative to the Base Case than eliminating                                                 Scenario D2 is based on county water-supply plans. Both
pumping in southern Beaufort County.                                                                            scenarios assume no changes from 2000 pumpage in the
      North of point A, eliminating pumping in southern                                                         model area outside of the 24-county coastal area of Georgia.
Beaufort County would result in a greater water-level increase                                                  These simulations are used to evaluate the response of the
relative to the Base Case than eliminating pumping in                                                           ground-water flow system to anticipated future needs, as well
Chatham County.                                                                                                 as the suitability of the model to evaluate substantial increases
                                                                                                                ground-water withdrawals. Results are presented from both
                                                                                                                the regional MODFLOW model and the SUTRA model of the
                                                                                                                Savannah–Hilton Head Island.
                                                                                                                              Simulation	of	Ground-Water	Management	Scenarios	 	 


Scenario.D1:.Projection.Based.on.Regional.                                                                                 Floridan aquifers on a county-by-county basis, as described
                                                                                                                           previously. Some of the nonsite-specific pumpage is attributed
Economic.Models,.Inc.                                                                                                      to aquifers that are not simulated; thus, values for projected
      The pumpage distribution for Scenario D1 is based on                                                                 use per county are often less than the total provided in the
the estimated change in ground-water usage for the years                                                                   forecast. During 2000 to 2035, the simulated pumpage
2010, 2020 and 2035, in the 24-county coastal area (fig. 1) in                                                             increased by about 97 Mgal/d (12 percent) for Scenario D1.
Georgia, as a function of population and employment projec-                                                                      The regional MODFLOW model was run assuming
tions by Regional Economic Models, Inc. (REMI) (Leeth and                                                                  steady-state conditions for 2035. To simulate solute transport
others, 2005) (fig. 39; table 1). A consistent approach based                                                              using the SUTRA model, projected pumpage for the years
on historical trends and economic parameters was used to                                                                   2010, 2020, and 2035 were added to the pumping history from
evaluate population projections for each county. Estimates of                                                              1885 to 2000, and the pumpage for the intervening years was
water use were derived from projected population and indus-                                                                linearly interpolated. Pumpage after 2035 was assumed to
trial growth. To distribute the pumpage, the estimated ground-                                                             remain at 2035 levels for simulations through 2100. Water
water usage component was extracted from the overall usage                                                                 levels and flow-budget components are evaluated for steady-
projections. For each county, projected ground-water usage for                                                             state conditions during 2035. Water-level changes are
specific years was distributed to the Upper and Lower Flori-                                                               evaluated for steady-state conditions during 2000 and 2035.
dan aquifers based on methods described in Payne and others                                                                Chloride distributions are evaluated for 2010, 2020, 2035, and
(2005), with the Upper Floridan aquifer comprising the great-                                                              2100, and are compared with chloride distributions for the
est proportion. Pumpage projections were not applied to the                                                                Base Case, and with chloride distributions for simulations for
surficial or Brunswick aquifer system. In some cases, water-                                                               which pumpage remains at 2000 levels through 2100.
use projections based on REMI estimate higher pumpage
for 2000 than is used by the models because pumpage in the                                                                 Ground-Water.Flow
surficial aquifer system and aquifers deeper than the Floridan
aquifer system are not accounted for in the models. Projected                                                                    For Scenario D1, the simulated potentiometric surfaces
values for public-supply and industrial use were applied to                                                                generated using the MODFLOW model for the Upper and
existing public supply and industrial wells in the Upper and                                                               Lower Floridan aquifers during 2035 (fig. 40) are similar and
Lower Floridan aquifers, when available, based on the propor-                                                              show large potentiometric cones of depression centered over
tions during 2000. The remaining per-county pumpage values                                                                 Chatham County, Ga., with a smaller cones of depression in
for other water-use categories were applied to the nonsite-                                                                the Nassau–Duval county area of northern Florida. The simu-
specific distribution for the Upper Floridan and Lower                                                                     lated potentiometric surface of the Brunswick aquifer system
                                                                                                                           shows a cone of depression at Chatham County, Ga., which
                                                                                                                           results from leakage caused by pumping from the Upper Flori-
                                          1,600                                                                            dan aquifer. Relative to the Base Case, the 0-ft potentiometric
                                                                                         I
                                                                                  M
                                                                                RE                                         contour in the Upper and Lower Floridan aquifer potentio-
WITHDRAWALS, IN MILLION GALLONS PER DAY




                                                                           on
                                                                   se
                                                                      d         io
                                                                                     n                                     metric surfaces has expanded inland in Georgia into Evans
                                                                 ba        ect
                                                             n          oj                                                 County, southwestward into Wayne County, and southward to
                                                          tio         pr
                                          1,200       jec        SP                                                        Glynn County, to coalesce with smaller cones of depression
                                                  Pro       CW
                                                                  Agriculture                                              at Jesup and Brunswick (fig. 40). The cones of depression
                                                                  Public supply                                            in the Nassau–Duval county area also expanded northward
                                                                  Industrial                                               in response to increased pumping in Georgia. Water-level
                                           800
                                                                                                                           declines also resulted in a landward shift in the position of the
                                                                                                                           20-ft contour of the Upper Floridan aquifer potentiometric
                                                                                                                           surface. During 2000, this contour was offshore of Camden
                                                                                                                           County, whereas for Scenario D1 during 2035, the contour
                                                                                                                           shifted westward and onshore.
                                           400
                                                                                                                                 For Scenario D1, simulated water levels decline from
                                                                                                           Not projected




                                                                                                                           2000 to 2035 for all aquifer units (fig. 41). The extent of the
                                                                                                                           decline is beyond the 24-county area, although the effects are
                                                                                                                           most substantial within the 24-county area.
                                             0                                                                                   Maximum simulated water-level declines for Scenario D1
                                                  2000                          2010         2020   2035            2050
                                                                                                                           from 2000 to 2035 occur in the Savannah–Chatham County,
Figure	.. Projected.ground-water.use.in.the.24-county.coastal.                                                           and Statesboro–Bulloch County, Ga., areas (fig. 41). This
area.of.Georgia,.2000–2050,.by.category.(Regional.Economic.                                                                pattern is similar for all of the three simulated aquifer units
Models,.Inc..[REMI];.County.Comprehensive.Water-Supply.Plans.                                                              represented, with a maximum drawdown of about 34, 44, and
[CWSP]).(modified.from.Leeth.and.others,.2005).                                                                            43 ft for the Brunswick aquifer system, and the Upper and
                                                                                                                           Lower Floridan aquifers, respectively.
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	




                                             A. Upper Floridan aquifer (unit 5)
                           SOUTH                model area
         GEORGIA          CAROLINA


                                                                                  SC
                                                                              GA

           F L OR
                    ID
                      A




                                                                                                                                    Mo
                                                                                                                                       d
                                                                                                                                      el
                                                                                                                                       bo
                                                                                                                                         un
                                                                                                                                           da
                                                                                                                                           ry
                                                                                                                                n




                                                                                                                           a
                                                                                                                           ce
                                                                                                                       O
                                                                                                                      ic
                                                                                                                      nt
                                                                                                                  la
                                                                                                                 At
                                                                                                                 EXPLANATION
                                                                                                      Difference in pumpage between
                                                                                                       Scenario D1 during 2035 and
                                                                                                       Base Case (2000), per grid cell—
                                                                                                       In million gallons per day,
                                                                                                       Upper Floridan aquifer (unit 5)

                                                                                                                Less than – 5              Decrease in
                                                                                                                                           pumpage
                                                                                                                – 1.0 to 1.0

                                                                                                                1.0 to 5.0
                                                              GA
                                                                                                                                           Increase in
                                         N                    FL                                                5.0 to 10.0                pumpage
                                                                                                                Greater than 15.0



                                     0               25            50 MILES
                                                                                   Base from U.S. Geological Survey
                                     0          25        50 KILOMETERS            1:100,000-scale digital data


            Figure	.. Distribution.of.the.difference.between.ground-water.pumpage.for.Scenario.D1.(projection.based.
            on.Regional.Economic.Models,.Inc.).during.2035.and.the.Base.Case.(2000).in.the.Upper.Floridan.aquifer..
            (A).model.area.(24-county.coastal.area.outlined.in.magenta),.and.(B).enlarged.view.
                                                                   Simulation	of	Ground-Water	Management	Scenarios	 	 

  B. Upper Floridan aquifer (unit 5) —enlarged view



                                                                                   EXPLANATION
                                                         S
                                                        GA C          Difference in pumpage between
                                                                       Scenario D1 during 2035 and
                                                                       Base Case (2000), per grid cell—
                                                                       In million gallons per day,
                                                                       Upper Floridan aquifer (unit 5)

                                                                               Greater than – 5          Decrease in
                                                                                                         pumpage




                                                     n
                                                                               –1.0 to 1.0




                                                   ea
                                                  Oc
                                               tic
                                                                               1.0 to 5.0                Increase in




                                              an
                                                                                                         pumpage



                                            Atl




                       SOUTH
GEORGIA               CAROLINA



              Area
             of map




                                                                                                                                   cean
             above
 bo
 Mo nda
    u




                                                                                                                       A tlantic O
     de ry
       l




 F L ORI
           DA
                                                                                    GA
                Area of map                                                         FL
                  at right




                                              Approxima
                                  N                            te model b
                                                                         oundary


                                 Base modified from U.S. Geological Survey                   0       5     10     15   20 MILES
                                 1:100,000-scale digital data
                                                                                             0   5   10 15 20 KILOMETERS

 Figure	.. Distribution.of.the.difference.between.ground-water.pumpage.for.Scenario.D1.(projection.based.
 on.Regional.Economic.Models,.Inc.).during.2035.and.the.Base.Case.(2000).in.the.Upper.Floridan.aquifer..
 (A).model.area.(24-county.coastal.area.outlined.in.magenta),.and.(B).enlarged.view—continued.
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

      A. Brunswick aquifer system
         (unit 3)
                                                                  SC                                                                                                                                                                        EXPLANATION
                                                                GA
                                                                                                                                                                                               50            Simulated potentiometric contour—




                                                                                                                                                                             Mo
                                                                                                                                                                                                               Shows simulated potentiometric surface




                                                                                                                                                                                del
                                                                                                                                                                                                               for Scenario D1 during 2035, in feet.
                                                              Statesboro




                                                                                                                                                                                   bo
                                                                                                                                                                                                               Hachures indicate depression. Contour




                                                                                                                                                                                    un
                                                                        100
                                                                                                                                                                                                               interval 10 and 50 feet. Datum is NAVD 88




                                                                                                                                                                                      da
                                                                                                                                                                                        ry
                                                               50



                                                                    0
                                                               30 4
                                                                20
                                                                            10                                                                                     Approximate




                                                                                                         – 5040
                                                                                                0                                                                   limit of Brunswick




                                                                                                            –
                                                                                                                                                                    aquifer system
                                                                                                                 –30
                                                                                                –1               –20
                                                                                                    0




                                                                                                                                                            an
                                                                                            0




                                                                                                                                                           Oce
                                                                                                                                                   ntic                       B. Upper Floridan aquifer
                                                                                                                                              Atla

                                                                                                                                                                                 (unit 5)     350
                                                                                                                                                                                                                                                        200
                                                               10




                                   GA                                                                                                                                                               300
                     N                 FL                                                                                                                                                            250
                                                                                                                                                                                                                                            0
                                                                                                                                                                                                        0                               10
                                                                                                                                                                                                      20                                                            50
                                                                                                                                                                                                              0                                                                                  30
                                                                                                                                                                             250                           15                                                                                     10
                             0                  25              50 MILES
                                                                                                                                                                                                                                        l       l                                                                 l
                                                                                                                                                                             200




                                                                                                                                                                                                                                                            l
                                                                                                                                                                                                                            l                       l                                                                     l
                                                                                                                                                                                                                                                                                                                                               0
                                                                                                                                                                                                                                                                                l            l
                                                                                                                                                                                                                                                                                             –5
                                                                                                                                                                                                                                                                    l




                                                                                                                                                                                                                                                        l
                                                                                                                                                                                                                                                                                 l       l           l

                             0         25        50 KILOMETERS
                                                                                                                                                                                                                                                                        l
                                                                                                                                                                                                                                                                                                         0




                                                                                                                                                                                                                                                l


                                                                                                                                                                                                                                                                l




                                                                                                                                                                                                                                                                                                         l
                                                                                                                                                                                                                                                        l
                                                                                                                                                                                                                                                                                                                                           l
                                                                                                                                                                                                                     l




                                                                                                                                                                                                                                                                    l
                                                                                                                                                                                                                                                                l




                                                                                                                                                                                                                                                                                                                                                   l
                                                                                                                                                                                                                                                    – 40
                                                                                                                                                                                                                                        l



                                                                                                                                                                                                                                                                l
                                                                                                                                                                                                                                                                                –100




                                                                                                                                                                                                                                                                                                                 l
                                                                                                                                                                                                                                                                                                                                   l




                                                                                                                                                                                                                                                      l l




                                                                                                                                                                                                                                                                                                                     l
                                                                                                                                                                                                                                                                    l
                           Base from U.S. Geological Survey                                                                                                                                                           l




                                                                                                                                                                                                                                                                                                                 l



                                                                                                                                                                                                                                                                                                                              l
                                                                                                                                                                                                                                                                –3




                                                                                                                                                                                                                                                                        l
                                                                                                                                                                                                                                                                            l


                                                                                                                                                                                                                                        l
                                                                                                                                                                   250




                                                                                                                                                                                                                                                                                                                  l
                                                                                                                                                                                                                                                                                                                 l
                                                                                                                                                                                                                                                                                     l                       l




                                                                                                                                                                                                                                                                                                                                                   l
                                                                                                                                                                                                                                                                                                 l
                                                                                                                                                                                                                                                                    0
                           1:100,000-scale digital data




                                                                                                                                                                                                                                                                                                                 l
                                                                                                                                                                                                                                                                                                         l
                                                                                                                                                                                                                        l
                                                                                                                                                                                                                                    l




                                                                                                                                                                                                                                                                                                                              l
                                                                                                                                                                                                                                                        l
                                                                                                                                                                                                                                                                l                                                     0
                                                                                                                                                                                                                                                                                                                 –2




                                                                                                                                                                                                                                                                                                                                                   l
                                                                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                l
                                                                                                                                                                                                                        l


                                                                                                                                                                                                       30                                   l                                                    l
                                                                                                                                                                                                                   10



                                                                                                                                                                                                                                    l               l                                                            l0
                                                                                                                                                                                                                                                                                                             l –1




                                                                                                                                                                                                                                                                                                                                           l
                                                                                                                                                                                                                                l                           l       l                    l
                                                                                                                                                                                         0




                                                                                                                                                                                                                                                                                                                                       l
                                                                                                                                                                                      10




                                                                                                                                                                                                                                            l
                                                                                                                                                                                      50




                                                                                                                                                                                                                                                                                                                              l0
                                                                                                                                                                                                                                                                                                                      l
                                                                                                                                                                                                                    20




                                                                                                                                                                                                                                                                                                     l
      C. Lower Floridan aquifer                                                                                                                                                                                                                                                  l
                                                                                                                                                                                                                                                    l



                                                                                                                                                                                                                                                                l
         (unit 7)
                                                                                                                                                                                                                                                        l




                           350
                                                                                                                                                                                                                                                        l




                                                                                            200
                                                                                                                                                                                         50




                             300
                                                                                                                                                                                                            40




                                 250                                                                                                                                                                                                                            0
                                                                        0
                                                               10




                                       0                                                                                                                    20                                                                                                  –10
                                                                                                                                                                                                                                                0




                                  20                                                                     50                                                                                                                                     –10
                                             0                                                                            30
                                                                                                                                                                                                                                                    –20




            250                            15                                                                              10                                                                         50
                                                                                                                                                                                                                                                                            10
                                                                                                                                                                                                                                                                                         20
                                                                                                                         l            l
                                                                l           l           l       l                                             l
                                                                                                                                                            l




             200                                          l
                                                                                    l




                                                                                                         –150
                                                                                                                         –5
                                                                                        l




                                                                                                                                              l

                                                                                                                                                           l




                                                                                                                                                                         l
                                                                        l




                                                                                                                              0
                                                      l




                                                                                                                                                               l
                                                                                        –40




                                                                                                                                                       l




                                                                                                                 –100
                                                                                         l
                                                                        l




                                                                                                                                                                             l
                                                                                                                                                   l
                                                          l




                                                                                                    –3
                                                                        l

                                                                                            l




      250                                                                                                    0
                                                                                                                                                       l




                                                                                                                                                                                                                                             SOUTH
                                                                                                                                                                             l
                                                                      l
                                                          l



                                                                                            l




                                                                                                                                     0




                                                                                                     l
                                                                                                                                                   l




                                                                                                                                                                                                            GEORGIA                         CAROLINA
                                                                                                                                  –2




                                  30
                                                                    l




                                                                                                                 l            l                        0
                                                 10
                                                          l




                                                                                                                                                                         l




                                                                        l
                                                                                                                                              –1
                                                              l




                                                                                    l
                                                                                                                                              l




                                                                  l                         l                                             l
                                                                    l                               l                     l
                                                                                                                                                                   l




                                                     20
                      0




                                                                                                                                                               l
                   10




                                                                                l
                   50




                                                                                                                     l
                                                                                                                                  l               0l
                                                                                     l




                                                                                                         l
                                                                                        l




                                                                                                                                                                                                                                                                           ry
                                                                                                                                                                                                                                                                    bou odel
                                                                                                                                                                                                                                                                       nda
                                                                                            l
                                   40




                                                                                                                                                                                                                                                                      M




                                                                                                                                                                                                                  F L OR
                                                                                                                                                                                                                           ID
                      50




                                                                                                                                                                                                                             A



                                                                                                                                                                                             Figure	0.. Simulated.potentiometric.surfaces.for.
                                                                                                                                                                                             Scenario.D1.(projection.based.on.Regional.Economic.
                                                                                    0




                                                                                    –10
                                                                                                                                                                                             Models,.Inc.;.24-county.coastal.area.shaded).during.
                                  50                                                                                 10
                                                                                                                     20
                                                                                                                                                                                             2035.for.the.(A).Brunswick.aquifer.system.(unit.3),..
                                                              –20
                                                                        –30                                                                                                                  (B).Upper.Floridan.aquifer.(unit.5),.and.(C).Lower.Floridan.
                                                                                                                                                                                             aquifer.(unit.7).
                                                                                  Simulation	of	Ground-Water	Management	Scenarios	 	 

A. Brunswick aquifer system (unit 3)
                                                                                                  EXPLANATION
                                           SC                                                 Change in simulated head between
                                         GA
                                                                                               Scenario D1 during 2035 and
                                                                                               Base Case (2000), in feet




                                                                       Mo
                                                                          d
                                                                                                                 0




                                                                           el
                                       Statesboro




                                                                             bo
                                                                                                                 –1




                                                                               un
                                                                                 da
                                                                                                                 –5




                                                                                   ry
                                                                                                                 – 10
                                                                  Approximate                                    – 15     Decrease in
                                                                   limit of Brunswick                                     water level
                                                                   aquifer system                                – 20
                                                                                                                 – 25
                                                                                                                 – 35




                                                                an
                                                                                                                 – 44




                                                            Oce
                                                          ntic
                                                         Atla
                                                                     B. Upper Floridan aquifer (unit 5)


                            GA
                 N          FL




                        0         25     50 MILES

                        0    25    50 KILOMETERS

                      Base from U.S. Geological Survey
                      1:100,000-scale digital data




C. Lower Floridan aquifer (unit 7)




                                                                                                                      SOUTH
                                                                                                GEORGIA              CAROLINA




                                                                                                  F L OR
                                                                                                           ID
                                                                                                             A




                                                                                   Figure	1.. Change.in.simulated.head.between.Scenario.D1.
                                                                                   (projection.based.on.Regional.Economic.Models,.Inc.;..
                                                                                   24-county.coastal.area.outlined.in.magenta).during.2035.
                                                                                   and.the.Base.Case.(2000).for.the.(A).Brunswick.aquifer.
                                                                                   system.(unit.3),.(B).Upper.Floridan.aquifer.(unit.5),.and..
                                                                                   (C).Lower.Floridan.aquifer.(unit.7).
0	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

Although by 2035 the total projected increase in pumpage                                                          confining unit in this zone was assigned a hydraulic conductivity
at Bulloch County is only one-third that at Chatham County,                                                       several orders of magnitude higher than the adjacent zones, in
the pumpage is localized at Statesboro, where the assigned                                                        order to simulate recharge of water into the Upper Floridan aqui-
hydraulic conductivity of the Upper Floridan aquifer is an                                                        fer near mounds on the potentiometric-surface map of the Upper
order of magnitude lower than that in Chatham County. Thus,                                                       Floridan aquifer and to match water-level data there (Payne and
the effect of increases in pumpage at Bulloch County between                                                      others, 2005). This area corresponds to the hydraulic conductivity
2000 and 2035 is intensified, resulting in greater drawdown.                                                      zone C2, near where zone C2 is in contact with zones C4 and C5
     One notable feature for Scenario D1 during 2000–2035 is                                                      (fig. 4). Because simulated leakage to the Upper Floridan aquifer
an area of minimal drawdown in the Upper and Lower Floridan                                                       occurs more readily in this zone, the effect of increased stress in
aquifers offshore of Hilton Head Island (fig. 41). This feature                                                   Scenario D1 is mitigated in this zone relative to adjacent zones.
corresponds to an indent in the 0-ft potentiometric contours                                                            Simulated pumpage changes from about 798 Mgal/d
of the Upper and Lower Floridan aquifers, as demonstrated in                                                      during 2000 to about 896 Mgal/d during 2035 for Scenario D1
the 2000 simulated results (fig. 8). In this area, the model units                                                (tables 2 and 9). The simulated system responds by increasing
overlying the Upper Floridan aquifer (units 2, 3, and 4) comprise                                                 inflow at all boundaries, and decreasing outflow at all boundar-
the Upper Floridan confining unit. For model calibration, the                                                     ies during 2035, relative to the Base Case (fig. 42). The change


Table	.. Flow-budget.components.for.Scenario.D1,.2035..
[Results from MODFLOW model; in million gallons per day; —, not applicable]
                                                                        Inflow                                                                         Outflow

     Model		                                    From		           From		             From		                                To		             To		            To		
      unit                                   general-head	   general-head	        specified-	                        general-head	    general-head	     specified-	   Discharge		
                                                                                                         Total	                                                                       Total	
                                              boundary,		     boundary,	            head		                            boundary,		      boundary,		        head		       to	wells
                                               onshore         	offshore          boundary                             onshore          offshore        boundary
        1                                      303.13           29.94                —               333.07                111.96         14.28             —           —             126.24
        2                                       50.53            —                   —                50.5                   3.08          —                —           —               3.08
        3                                        —               —                   —                —                    —               —                —            0.24           0.24
        4                                        —               —                   —                —                    —               —                —           —              —
        5                                      147.82            —                 749.90            897.71                 20.22          —              249.99       760.59       1,030.80
        6                                        —               —                   0.00              0.00                —               —                0.00        —               0.00
        7                                        —               —                  16.46             16.46                —               —                1.79       135.32         137.11
Total all units                                501.48           29.94              766.36          1,297.78                135.26         14.28           251.77       896.15       1,297.47
Percent flow                                    38.6              2.3               59.1             100.0                  10.4           1.1             19.4          69.1         00.0


                                      100
                                                                                                                                                           91.16

                                       80
            MILLION GALLONS PER DAY




                                       60


                                                                                 38.50
                                       40
                                                 27.29

                                       20
                                                               6.18                                                                                                      6.65
                                                                                                                                                 0
                                        0
                                                                                                                  – 1.23
                                                                                                – 6.31
                                                                                                                                      – 18.32
                                      – 20
                                               Inflow from  Inflow from     Inflow from      Outflow to   Outflow to         Outflow to     Pumpage       Pumpage      Pumpage
                                              general-head general-head      specified-     general-head general-head        specified-       unit 3       unit 5       unit 7
                                                boundary,    boundary,          head         boundary,    boundary,            head
                                                 onshore      offshore       boundary         onshore      offshore          boundary

                                                                                                           FLOW COMPONENT

                                             Figure	2.. Simulated.flow.budget.for.Scenario.D1.(projection.based.on.Regional.Economic.Models,.Inc.).
                                             during.2035,.relative.to.the.Base.Case.(2000).
                                                                         Simulation	of	Ground-Water	Management	Scenarios	 	 1

in inflow and outflow at the southern specified-head boundary is      with Scenario D1 results, using a linear interpolation between
larger than at the general-head boundary, likely because some of      2020 and 2050 to estimate water use for the year 2035. Dur-
the increase in pumpage is occurring in the southern part of the      ing 2000 to 2035, the simulated pumpage increased by about
model area, and the specified-head boundary allows uncon-             574 Mgal/d (70 percent) for Scenario D2.
strained flow into and out of the modeled system, whereas flow              Using projections for these years, the pumpage was
to and from the general-head boundary is constrained to some          distributed as described for Scenario D1, and a steady-state
degree by the general-head conductance term. The increase in          simulation was run for the year 2035 using the MODFLOW
the inflow from the general-head boundary in the offshore area        model. For the solute-transport model, the years 2010, 2020,
indicates an increase in potential to recharge saltwater. Within      and 2035 were added to the pumping history from 1885 to the
the model boundaries, the change in stresses also resulted in a       present, and pumpage for the intervening years was linearly
larger, net landward flux relative to the 2000 Base Case (fig. 19).   interpolated. Simulation results during 2100 also are shown
                                                                      for which pumpage after 2035 was assumed to remain at 2035
Solute.Transport                                                      pumpage through the year 2100.

      The simulated chloride distribution shows a steady
increase in the extent of the plumes with increasing pumpage          Ground-Water.Flow
for Scenario D1 (fig. 43). The two westernmost plumes, origi-               The increase in pumpage for Scenario D2 from 2000 to
nating at Colleton River and Pinckney Island, show a greater          2035 produced changes in the configuration of the simulated
increase in extent than the plume originating on the northern         potentiometric surfaces and widespread, substantial water-
end of Hilton Head Island, relative to the plume distribu-            level declines using the MODFLOW model (figs. 45 and 46).
tions during 2000. The plumes generally grew southwestward            As is the case for other simulations, the potentiometric surface
from the source areas with advancing time. When Scenario              of the Lower Floridan aquifer is similar to that of the Upper
D1 pumpage for 2035 is maintained until 2100, the plumes              Floridan aquifer. Likewise, depressions in the potentiometric
at Colleton River and Pinckney Island increase substantially          surface of the Brunswick aquifer system correspond to the
more than that at the northern end of Hilton Head Island by           potentiometric cones of depression in the Upper Floridan
2100 (fig. 43D). In addition, during 2100 the plumes originat-        aquifer, although the maximum depth is not as great. The large
ing at Colleton River and Pinckney Island extend farther to the       increase in pumpage resulted in expansion of the Savannah
southwest than resulting plumes when 2000 pumpage is main-            area cone of depression southward and westward (fig. 45). The
tained until 2100. The plumes at the northern end of Hilton           0-ft contour of the Savannah area cone of depression expanded
Head Island during 2100 are similar for Scenario D1 condi-            westward into Screven, northern Bulloch, Candler and Toombs
tions and for 2000 conditions maintained until 2100. This             Counties, Ga., and southward to coalesce with the 0-ft contour
indicates that the projected 97-Mgal/d increase in pumpage            of the cone of depression centered at Duval County, Fla. A
relative to 2000 pumpage (table 1) would have limited effect          deep cone of depression, with a maximum depth of 333 ft
on chloride plume development in the Hilton Head Island area.         below NAVD 88, developed in the Gulf Trough area, centered
      New plumes developed by the year 2035 beneath Broad             at Tattnall County, Ga., near the borders with Candler and
Creek in the east-central part of the Hilton Head Island, in the      Evans Counties.
area offshore to the east of Hilton Head Island, and beneath                For Scenario D2, simulated water-level declines during
May River. The SUTRA solute-transport model has been                  2000–2035 exceed 20 ft across most of the 24-county coastal
calibrated only to the chloride plumes at the northern end of         area in Georgia (fig. 46). Declines exceeding 100 ft occur in
Hilton Head Island, Pinckney Island, and the Colleton River,          the Upper Floridan aquifer in Bulloch, Candler, Emanuel,
so simulated plume development elsewhere must be inter-               Evans, Tattnall, and Toombs Counties, Ga. Maximum declines
preted with caution.                                                  exceeding 250 ft coincide with the deepest part of the cone of
                                                                      depression in the Upper and Lower Floridan potentiometric
                                                                      surfaces (fig. 45), with a maximum decline of 400 ft. In the
Scenario.D2:.County.Comprehensive.
                                                                      Brunswick aquifer system, maximum declines of more than
Water-Supply.Plan.Projection                                          100 ft occur in Bulloch and Candler Counties, also coinciding
     The pumpage distribution for Scenario D2 is based on             with the cone of depression in the Upper and Lower Floridan
estimated change in ground-water use for the years 2010, 2020         aquifer potentiometric surfaces.
and 2035, as derived from the County Comprehensive Water                    The deep potentiometric cone of depression and area
Supply Plans (CWSP) developed for each of the 24 coastal              of maximum water-level decline in the Candler, Evans, and
counties of Georgia (Camp Dresser and McKee, 2001; Leeth              Tattnall Counties, Ga., area are situated at the intersection of
and others, 2005) (figs. 1 and 44; table 1). Each county              these counties with the hydraulic-conductivity zone represent-
provided its own estimate for 2010, 2020, and 2050 based on           ing the Gulf Trough (fig. 4). The deep cone of depression and
suggested guidelines; these estimates indicate that there is          widespread water-level decline reflect substantial increases in
considerable inconsistency between counties. The analysis for         pumpage for these counties during 2000–2035 (table 1) and the
Scenario D2 herein is extended only to 2035 for comparability         very low hydraulic conductivity assigned to the Gulf Trough.
2	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

  A. Scenario D1, 2010                                                                               B. Scenario D1, 2020

                                                                        Parris Island




                                                              Po
           ol                                                      rt
         C




                let                                                     Ro
                      on                                                     ya
                           R                                                      lS
                                                                                       ou
                                                                                            nd
                                           I
                                         ney
                                          ck
                                      Pin




                                                Hilton Head Island
                                                                                                 n

                                                              ek
                                                                                            ea


                                                       C re
                                               Broad
                                                                                        Oc
                                                                                   tic




                                                                                                                                                                                N
                                                                              la n
                                                                         At




 Base from U.S. Geological Survey                                                                                                        0       1       2       3   4    5 MILES
 1:100,000 and 1:250,000-scale data
                                                                                                                                         0   1       2   3   4   5 KILOMETERS

                                                                                                                      SOUTH
  C. Scenario D1, 2035                                                                                   GEORGIA     CAROLINA


                                                                                                                                Map
                                                                                                                                area
                                                                                                                Study
                                                                                                                area



                                                                                                                     Model
                                                                                                                   boundary


                                                                                                                         EXPLANATION
                                                                                                         Simulated chloride concentration, in milligrams
                                                                                                           per liter (mg/L), in ground water in the middle
                                                                                                           of the Upper Floridan aquifer for Scenario D1

                                                                                                          250 to 2,000
                                                                                                          2,000 to 6,000
                                                                                                          6,000 to 10,000
                                                                                                          Greater than 10,000

                                                                                                         Chloride concentration, in mg/L, in ground water
                                                                                                           in the middle of the Upper Floridan aquifer
                                                                                                           for Base Case (2000)
                                                                                                                   250                 10,000
                                                                                                         Chloride concentration, in mg/L, in ground water
                                                                                                           in the middle of the Upper Floridan aquifer
                                                                                                           for 2000 pumpage during (C) 2035
                                                                                                                   250                 10,000

                                                                                                         Observation well

 Figure	.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper.Floridan.aquifer.in.the.Hilton.Head.Island,.
 South.Carolina,.area.for.Scenario.D1.(projection.based.on.Regional.Economic.Models,.Inc.).during.(A).2010,.(B).2020,.(C).2035,.and.
 (D).2100.(larger.area).
                                                                          Simulation	of	Ground-Water	Management	Scenarios	 	 


D. Scenario D1, 2100
                                                                                                                                      Parris
                                                                                                                                      Island

                                                              ol




                                                          C
                                                                let
                                                                   on R                                                          Por
                                                                                                                                    tR
                                                                                                                                      oy
                                                                                                                                        al




                                                                                                                                               So
         JASPER




                                                                                                                                                 un
                                                                                                                                                   d
                                                                                                                I
                                                                                                             ey
                                                                                                          ckn
                                                                                                       Pin
                                                        Bluffton

                                                                          May
                                                                              R   ive
                                                                                     r
                                                BEAUFORT                                                                         ee




                                                                                                                             k
                                                                            Bull                                            Cr
                                                                                                                    Broad
                                                                           Island




                                                                                              d
                                                                                            un
                                                                                          So
                                                                                         e
            Center of                                                                             Hilton




                                                                                     gu
                                                                                   bo
            cone of                                                                             Head




                                                                                li
                                                                             Ca
            depression                                                                     Island
                                                          Daufuskie
                                                            Island




                                                                                                           an
                                         SC




                                                                                                           ce
                                                                                                           O
                                                                                                      ic
                                    GA                                                             nt
 N   Savannah                                                                                 la
                                                                                          t
                                                                                         A
             CHATHAM

Base from U.S. Geological Survey 1:100,000 and 1:250,000-scale data                                            0            2            4             6 MILES
Cone of depression from Peck and others, 1999

                                                                                                               0      2           4     6 KILOMETERS
                                                                  EXPLANATION
                                   Simulated chloride concentration,                     Chloride concentration, in mg/L, in
                                     in milligrams per liter (mg/L), in                    ground water in the middle of the Upper
                  SOUTH              ground water in the middle of                         Floridan aquifer for Base Case (2000)
  GEORGIA        CAROLINA
                                     the Upper Floridan aquifer                                            250                        10,000
                                     for Scenario D1
                            Map
                            area                                                         Chloride concentration, in mg/L, in
         Study                                 250 to 2,000
         area                                                                              ground water in the middle of the
                                               2,000 to 6,000                              Upper Floridan aquifer for 2000
                                                                                           pumpage during (D) 2100
               Model
                                               6,000 to 10,000
             boundary                                                                                      250                        10,000
                                               Greater than 10,000
                                                                                                    Observation well



Figure	.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper.Floridan.aquifer.in.
the.Hilton.Head.Island,.South.Carolina,.area.for.Scenario.D1.(projection.based.on.Regional.Economic.Models,.
Inc.).during.(A).2010,.(B).2020,.(C).2035,.and.(D).2100.(larger.area)—continued.
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

                                             A. Upper Floridan aquifer (unit 5)
                           SOUTH                model area
          GEORGIA         CAROLINA


                                                                               SC
                                                                              GA

           F L OR
                    ID
                      A




                                                                                                                                   Mo
                                                                                                                                      d
                                                                                                                                     el
                                                                                                                                      bo
                                                                                                                                        un
                                                                                                                                          da
                                                                                                                                           ry
                                                                                                                               n




                                                                                                                           a
                                                                                                                          ce
                                                                                                                      O
                                                                                                                     ic
                                                                                                                     nt
                                                                                                                 la
                                                                                                                At
                                                                                                                EXPLANATION
                                                                                                     Difference in pumpage between
                                                                                                      Scenario D2 during 2035 and
                                                                                                      Base Case (2000), per grid cell—
                                                                                                      In million gallons per day,
                                                                                                      Upper Floridan aquifer, unit 5

                                                                                                             Less than – 5
                                                                                                                                    Decrease in
                                                                                                             – 5 to –1.0            pumpage

                                                                                                             –1.0 to 1.0
                                                              GA
                                                                                                             1.0 to 5.0
                                         N                    FL                                                                    Increase in
                                                                                                             5.0 to 10.0            pumpage




                                     0               25            50 MILES
                                                                                  Base from U.S. Geological Survey
                                     0          25        50 KILOMETERS           1:100,000-scale digital data


       Figure	.. Distribution.of.the.difference.between.ground-water.pumpage.for.Scenario.D2.(County.Comprehensive.
       Water-Supply.Plans.projection).during.2035.and.the.Base.Case.(2000).for.the.Upper.Floridan.aquifer.(A).model.area.
       (24-county.coastal.area.outlined.in.magenta),.and.(B).enlarged.view.
                                                                        Simulation	of	Ground-Water	Management	Scenarios	 	 

      B. Upper Floridan aquifer (unit 5) —enlarged view



                                                                                 EXPLANATION
                                                            S
                                                           GA C          Difference in pumpage between
                                                                          Scenario D2 during 2035 and
                                                                          Base Case (2000), per grid cell —
                                                                          In million gallons per day,
                                                                          Upper Floridan aquifer (unit 5)

                                                                                  – 1.0 to 1.0

                                                                                  1.0 to 5.0




                                                        n
                                                                                                     Increase in




                                                      ea
                                                      Oc
                                                                                  5.0 to 10.0        pumpage




                                                  tic
                                                 an
                                                Atl




                          SOUTH
  GEORGIA                CAROLINA



                 Area
                of map
                above




                                                                                                                                     cean
    bo
    Mo nda
       u
        de ry




                                                                                                                         A tlantic O
          l




    F L ORI
              DA
                                                                                      GA
                   Area of map                                                        FL
                     at right




                                                 Approxima
                                      N                           te model b
                                                                            oundary


                                    Base modified from U.S. Geological Survey                    0       5   10    15   20 MILES
                                    1:100,000-scale digital data
                                                                                                 0   5   10 15 20 KILOMETERS

Figure	.. Distribution.of.the.difference.between.ground-water.pumpage.for.Scenario.D2.(County.Comprehensive.
Water-Supply.Plans.projection).during.2035.and.the.Base.Case.(2000).for.the.Upper.Floridan.aquifer.(A).model.area.
(24-county.coastal.area.outlined.in.magenta),.and.(B).enlarged.view—continued.
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

   A. Brunswick aquifer system (unit 3)

                                                                                         SC                                                                                                                                                                                                        EXPLANATION
                                                                                       GA
                                                                                                                                                                                                                                       50              Simulated potentiometric contour—




                                                                                                                                                                                                                Mo
                                                                                                                                                                                                                                                         Shows simulated potentiometric surface




                                                                                                                                                                                                                   d
                                                                                                                                                                                                                   el
                                                                                                                                                                                                                                                         for Scenario D2 during 2035, in feet.




                                                                                                                                                                                                                          bo
                                                                                                                                                                                                                                                         Hachures indicate depression.




                                                                                                                                                                                                                          un
                                                                                                     0
                                                                                                  50 0

                                                                                                20 3
                                                                                                                                                                                                                                                         Contour intervals 10 and 50 feet.




                                                                                                                                                                                                                            da
                                                                                                  10



                                                                                               0 10
                                                                                                 40




                                                                                                                                                                                                                                ry
                                                                                               –20
                                                                                                                                                                                                                                                         Datum is NAVD 88




                                                                                                                                     –10
                                                                           –40                  –30

                                                                                                            –20
                                                                                                                                                                                                           Approximate
                                                                                                            –30                                                                                             limit of Brunswick
                                                                                                                                                                                                            aquifer system
                                                                                                                                                     –50
                                                                                                                                                     –40
                                                                                                                 –30

                                                                                                            –20




                                                                                                                                                                                                an
                                                                                                                                                                                               Oce
                                                                                                                                                                                          ntic
                                                                                                –10
                                                                                                                                                                                         Atla
                                                                                                                                                                                                            B. Upper Floridan aquifer (unit 5)
                                                                                                                                                                                                                                             350
                                                                                           0                                                                                                                                                                                                                   200

                                                                                                                                                                                                                                            300                                                                    1 50
                                           GA
                                           FL                                                                                                                                                                                                250                                                                       100
                        N
                                                                                                                                                                                                                                                                                                                                                                                50
                                                                                                                                                                                                                                                                                                                                                                                        30
                                                                                                                                                                                                                                                                                                                           40                                                            20
                                                                                                                                                                                                                                              150
                                                                                                                                                                                                                      0            0                                                                                                                    10
                                                                                                                                                                                                                 25
                                                                                                                                                                                                                                20                                                                                                                                  0
                                                                                                                                                                                                                                                                                           l
                                                                                                                                                                                                                                                                                   l
                                 0                     25                                  50 MILES
                                                                                                                                                                                                                                                                           l
                                                                                                                                                                                                                                                                                                                                                                                                0
                                                                                                                                                                                                                                                                   l                                                       l




                                                                                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                                           l                                           l   l   l                    l
                                                                                                                                                                                                                                                                                   –100
                                                                                                                                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                                                                                                   –150
                                                                                                                                                                                                                                                       l
                                                                                                                                                                                                                                                                                                                                                        l




                                                                                                                                                                                                                                                                                                                                                            l
                                 0         25                   50 KILOMETERS
                                                                                                                                                                                                                                                       l




                                                                                                                                                                                                                                                                                                                                                                    l
                                                                                                                                                                                                                                                           –300
                                                                                                                                                                                                                                                    l




                                                                                                                                                                                                                                                                                                                               –1
                                                                                                                                                                                                                                                           l




                                                                                                                                                                                                                                                                                                                                                                    l
                                                                                                                                                                                                                                                                                                                                    00
                                                                                                                                                                                                                                                               l
                               Base from U.S. Geological Survey                                                                                                                                                                                                        l




                                                                                                                                                                                                                                                                                                                                                                        l
                                                                                                                                                                                                                                                       –3                      l
                                                                                                                                                                                                                                                                                                                                                                                        l
                                                                                                                                                                                                           50
                                                                                                                                                                                                                                                                                       l
                                                                                                                                                                                                                                                               0




                                                                                                                                                                                                                                                                                                                                                                    l
                                                                                                                                                                                                                                                                                               l
                               1:100,000-scale digital data                                                                                                                                            2




                                                                                                                                                                                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                                                                                   l




                                                                                                                                                                                                                                                                                                                                                                                    l
                                                                                                                                                                                                                                                                                                  – 50




                                                                                                                                                                                                                                                                                                                                                                    l
                                                                                                                                                                                                                                                                                                                                                            l




                                                                                                                                                                                                                                                                                                                                                                                l
                                                                                                                                                                                                                                                                                                                                   l            l
                                                                                                                                                                                                                                                   0                                                                   l




                                                                                                                                                                                                                                                                                                                                                                                l
                                                                                                                                                                                                                                                                                               – 40




                                                                                                                                                                                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                             20                                            l l                                                                          l
                                                                                                                                                                                                                                                                       –1




                                                                                                                                                                                                                                                                                                                                                                                l
                                                                                                                                                                                                                                                                                                       l                                                        l
                                                                                                                                                                                                                            0




                                                                                                                                                                                                                                                                                                                                                l




                                                                                                                                                                                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                                                               0                           l                       l
                                                                                                                                                                                                                           10




                                                                                                                                                                                                                                                                                                                   l




                                                                                                                                                                                                                                                                                                                                                                            l
                                                                                                                                                                                                                                                                   10                                                                                                   l
                                                                                                                                                                                                                                                                                                                                                                        0
                                                                                                                                                                                                                                                                                                                                                            –2
                                                                                                                                                                                                                                                                                                                                                            l




                                                                                                                                                                                                                                                                                                                                                                                        l
   C. Lower Floridan aquifer (unit 7)                                                                                                                                                                                                                                                                                                               l




                                                                                                                                                                                                                                                                                                                                                                                    l
                                                                                                                                                                                                                                                                                                                                        l
                                                                                                                                                                                                                           50     40                                                                                           l



                                                                                                                                                                                                                                                                                                                                                                                l
                                      350                                                                                                                                                                                                                                                                                               l                   l               l
                                                                                                                     200
                                                                                                                                                                                                                                              30




                                                                                                                         150
                                                                                                                                                                                                                                50




                                 300
                                                                                                                                                                     0
                                                                                                                                                                 10




                                     250
                                                                                                                                                                                               50                                                                                                                                  –20                                                  0
                                                                                                                                                                                                 30
                                                                                                                                                                                                  20
                                                                                                                                                                                                                                                                                                                   –10




                                                                                                                                         40
                                                                                                                                                                                                                                                                                                       –20
             250                                                                       l
                                                                                                                                                                     0                                                                        50
                        0
                   20




                                                                     l         l
                                                                                           l




                                                                l                                                                l
                                                        l                                       l       l    l       l
                                                                                                                                                                                                                                                                                                                                                    20
                                                                                                                                             l
                                                                                   –100
                                                                                                                                                 l
                                                   l




                                                                                                                         –150                                l
                                                   l




                                                                                                                                                                     l
                                               l




                                                                                                                                                                         l




                                                       –300
                                                                                                                                                                             l
                                               l




                                                                                                                                         –10


                                                                                                                                                                                 l
                                                   l




                                                       l
                                                                l
                                                                                                                                            0


                                                                                                                                                                                 l




                                                                    l
                   150




                                                       –3l             l           l
                                                                                                                                                                                                   l
       250                                 l                         l
                                                                                                                                                                                 l




                                                                                           l
                                                         0                 l                    l
                                                                                                                                                                                                                                                                                                    SOUTH
                                                                                                                                                                                                       l
                                                                                                                                                                             l




                                                   l                                                    –l50
                                                                                                                                                                                          l




                                                                                                                                                                             l
                                                                                                                                                                                 l




                                                                                                                                                                 l
                                                                                                                                                                                     l




                                                                                                                                         l           l
                                               0                                                                                 l                                                                                                                     GEORGIA                                     CAROLINA
                                                                    l
                                                                                   l




                                                            l
                                                                                                                                                                                                       l
                                                                                                                                                                             l


                                                                                                                                                                                         l




                                                                                               –l40l
                                                                    l




                                                                                                                                                                                 l




                                                                                                                                                                         l
                                                                         l




                                                                                                                 l
                                      20                         –1 l l
                                                                                                                             l               l           l                       l
                                                                     l
                      0




                                                                                                                                                                                         l

                                                                                                                                                                                                       l




                                                                                                    l                                                                l
                                                                    l
                   10




                                                                                                                                                     l
                                                                   0                                        l                            l                                      l0
                                30




                                                                                                                         l
                                                                10    l                                                                                                      l –2
                                                                                                                                                                                                l
                                                                                           l




                                                                                                                                                             l
                                                                                                    l




                                                                                                                                                                                               l




                          40                                                                                                                 l
                   50                                                                                                                                                                      l
                                                                                               l
                                                                                                            l




                                                                                                                                                                                                                                                                                                                                          ry
                                                                                                                                                                                                                                                                                                                                   bou odel




                                                                                                                                                                                     l
                                                                                                                                                                                                                                                                                                                                      nda
                                                                                                                 l
                                                                                                    l




                                                                                                                     l               l           l                   l
                                                                                                                                                                                                                                                                                                                                     M




                                                                                                                                                                                                                                                               F L OR
                                                                                                                                                                                                                                                                                   ID
                                                                                                                                                                                                                                                                                     A
                        50




                                                                                                                                                                                                   0                                   Figure	.. Simulated.potentiometric.surfaces.for.
                                                                                                                                                                                                                                       Scenario.D2.(County.Comprehensive.Water-Supply.
                                                                                                                             –10




                                                                                                        –20
                                                                                                                                                                                                                                       Plans.projection;.24-county.coastal.area.shaded).
                                      50
                                                                                                                                                                                                                                       during.2035.of.the.(A).Brunswick.aquifer.system.
                                                                                                                                                             20
                                                                                           –40 –30                                                                                                                                     (unit.3),.(B).Upper.Floridan.aquifer.(unit.5),.and.
                                                                                                                                                                                                                                       (C).Lower.Floridan.aquifer.(unit.7).
                                                                                 Simulation	of	Ground-Water	Management	Scenarios	 	 

A. Brunswick aquifer system (unit 3)                                                                            EXPLANATION
                                                                                                Change in simulated head between
                                         SC                                                      Scenario D2 during 2035 and
                                       GA
                                                                                                 Base Case (2000), in feet
                                                                                                          0




                                                                       Mo
                                                                          d
                                                                                                          – 20




                                                                          el
                                                                            bo
                                                                                                                  – 40




                                                                              un
                                                                                da
                                                                                                                  – 60




                                                                                  ry
                                                                                                                  – 80     Decrease in
                                                                  Approximate                                     – 100    water level
                                                                   limit of Brunswick
                                                                   aquifer system                                 – 150
                                                                                                                  – 200
                                                                                                                  – 250




                                                                an
                                                                                                                  – 400




                                                            Oce
                                                          ntic
                                                         Atla
                                                                     B. Upper Floridan aquifer (unit 5)


                            GA
                 N          FL




                        0         25   50 MILES

                        0    25    50 KILOMETERS

                      Base from U.S. Geological Survey
                      1:100,000-scale digital data




C. Lower Floridan aquifer (unit 7)




                                                                                                                 SOUTH
                                                                                               GEORGIA          CAROLINA




                                                                                                 F L OR
                                                                                                          ID
                                                                                                            A


                                                                                        Figure	.. Change.in.simulated.head.between.
                                                                                        Scenario.D2.(County.Comprehensive.Water-Supply.
                                                                                        Plans.projection;.24-county.coastal.area.outlines.
                                                                                        in.magenta).during.2035.and.the.Base.Case.(2000).
                                                                                        for.the.(A).Brunswick.aquifer.system.(unit.3),.
                                                                                        (B).Upper.Floridan.aquifer.(unit.5),.and.(C).Lower.
                                                                                        Floridan.aquifer.(unit.7).
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

The extent of this cone of depression is limited by the Gulf                    recharge and discharge during 2000. The maximum recharge
Trough hydraulic-conductivity zone—outside of this zone, the                    rate of almost 5 inches per year (in/yr) for any given grid cell
assigned hydraulic conductivity is higher and pumping is less                   for Scenario D2 during 2035 is at the low end of calculated
concentrated. For example, in Brantley County, Ga., Scenario                    baseflow rates in the model area (from about 4.5 to 10 in/yr
D2 ground-water use increases by 187 Mgal/d between 2000                        [Priest, 2004]), and within the range of acceptable per-cell
and the year 2035 (table 1). In this area, the assigned hydraulic               recharge rates (Payne and others, 2005). The recharge rates
conductivity for the Upper Floridan aquifer is three orders of                  are poorly constrained, however, and simulated recharge in
magnitude higher than in the Gulf Trough area; thus, draw-                      some areas may exceed actual recharge. Substantial increases
down is less and deep cones of depression do not develop.                       in recharge and decreases in discharge, as a response to large
     Simulated pumpage for Scenario D2 during 2035, about                       increases in pumpage, could affect the unconfined ground-
1,373 Mgal/d, is substantially larger than that during 2000                     water system or surface-water bodies. Because the boundary
(tables 2 and 10). Correspondingly, for Scenario D2, simulated                  conditions used in the model provide an unlimited source
inflows increase and outflows decrease during 2035 relative                     of water to the system, the effects cannot be evaluated for
to those during 2000 (fig. 47). Simulated inflow from the gen-                  unconfined aquifers or surface-water bodies. The increase in
eral-head boundary is about 139 Mgal/d (37 percent) higher                      inflow from the general-head boundary in the offshore area
for Scenario D2 during 2035 than during 2000, and simulated                     also indicates an increase in potential to recharge saltwater
inflow from the southern specified-head boundary is about                       for Scenario D2. Within the model boundaries, the change in
293 Mgal/d (40 percent) higher than during 2000. Outflow                        stresses also resulted in a notably larger landward flux during
to the general-head boundary in the onshore area is about                       2035 relative to the 2000 Base Case (fig. 19). The substan-
24 Mgal/d (17 percent) lower for Scenario D2 during 2035                        tial simulated increases in inflow at the model boundaries,
than during 2000. Figure 48 shows that simulated per-cell                       however, may be an indication that the Scenario D2 pumpage
recharge to the system increases in magnitude and extent and                    is not realistic.
discharge decreases for Scenario D2 during 2035 relative to




Table	10.. Flow-budget.components.for.Scenario.D2,.2035..
[Results from MODFLOW model; in million gallons per day; —, not applicable]

                                              Inflow                                                              Outflow

      Model		         From		           From		            From		                        To		            To		            To		
       unit        general-head	   general-head	       specified-	                general-head	   general-head	     specified-	   Discharge		
                                                                       Total	                                                                    Total	
                    boundary,		     boundary,	           head		                    boundary,		     boundary,		        head		       to	wells
                     onshore         	offshore         boundary                     onshore         offshore        boundary

         1             350.21          36.57              —           386.78        102.57           12.12             —             —           114.69
         2              78.27           —                 —            78.27           1.57            —               —             —              1.57
         3              —               —                 —             —              —               —               —              0.24          0.24
         4              —               —                 —             —              —               —               —             —             —
         5             184.95           —               999.91       1,184.86        13.56             —              167.33      1,231.67      1,412.56
         6              —               —                  0.00          0.00          —               —                0.00         —              0.00
         7              —               —                21.28         21.28           —               —                0.72        141.08       141.79
 Total all units       613.43          36.57           1,021.19      1,671.20       117.71           12.12            168.04      1,372.99      1,670.86

 Percent flow           36.7            2.2              61.1         100.0            7.0               0.7           10.1          82.2        100.0
                                                                                                Simulation	of	Ground-Water	Management	Scenarios	 	 




                           600
                                                                                                                                 562.24


                           500




                           400




                           300
                                                                293.33
MILLION GALLONS PER DAY




                           200

                                     139.25


                           100



                                                   12.81                                                                                   12.41
                                                                                                                        0
                             0
                                                                                             –3.40
                                                                               – 23.86


                          –100
                                                                                                         – 102.05



                          – 200
                                    Inflow from  Inflow from   Inflow from    Outflow to   Outflow to    Outflow to   Pumpage   Pumpage   Pumpage
                                   general-head general-head    specified-   general-head general-head   specified-    unit 3    unit 5     unit 7
                                     boundary,    boundary,        head       boundary,    boundary,       head
                                      onshore      offshore     boundary       onshore      offshore     boundary

                                                                                         FLOW COMPONENT


                                  Figure	.. Simulated.flow.budget.for.Scenario.D2.(County.Comprehensive.Water-Supply.Plans.
                                  projection).during.2035,.relative.to.the.Base.Case.(2000).
0	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	


   A. Base Case (2000)
                                                                                                                       EXPLANATION
                                             SC
                                           GA                                                                      Simulated recharge or
                                                                                                                     discharge per model cell,




                                                                          Mo
                                                                                                                     in inches per year




                                                                            de
                                                                              lb
                                                                                                                            4 to 5




                                                                                ou
                                                                                  nd
                                                                                    ar
                                                                                                                            3 to 4




                                                                                       y
                                                                                                                            2 to 3   Recharge
                                                                      Approximate
                                                                       limit of Brunswick                                   1 to 2
                                                                       aquifer system
                                                                                                                            0 to 1

                                                                                                                            0 to 2   Discharge




                                                                    an
                                                                Oce
                                                              ntic
                                                             Atla
                                                                              B. Scenario D1, 2035


                                GA
                   N            FL




                            0         25   50 MILES

                            0    25    50 KILOMETERS

                          Base from U.S. Geological Survey
                          1:100,000-scale digital data




   C. Scenario D2, 2035




                                                                                                                        SOUTH
                                                                                                      GEORGIA          CAROLINA




                                                                                                        F L OR
                                                                                                                 ID
                                                                                                                   A



                                                                                            Figure	.. Simulated.recharge.or.discharge.(flow.
                                                                                            from.or.to.general-head.boundary).for.(A).Base.Case.
                                                                                            (2000);.(B).Scenario.D1.(projection.based.on.Regional.
                                                                                            Economic.Models,.Inc.),.2035;.and.(C).Scenario.D2.
                                                                                            (County.Comprehensive.Water-Supply.Plans..
                                                                                            projection),.2035.
                                                                                                          Model	Limitations	 	 1

Solute.Transport.                                                   properties of a hydrologic system, and as such are subject to
                                                                    the limitations described by Payne and others (2005) and
      The simulated chloride distributions for Scenario D2
                                                                    Provost and others (2006). These limitations include: (1) error
show an increase in extent during 2000–2035. As for
                                                                    and uncertainty in field measurements of water level and chlo-
Scenario D1, plumes emanating from Colleton River and
                                                                    ride concentration and in estimates of pumpage; (2) limitations
Pinckney Island areas show a greater increase in extent
                                                                    of the conceptual models; approximations made in represent-
than that at the northern end of Hilton Head Island by 2035
                                                                    ing the physical properties of the flow system and errors inher-
(figs. 12 and 49). When Scenario D2 pumpage for 2035 is
                                                                    ent in estimating the spatial distribution of these properties;
maintained until 2100, the plumes continue to increase in           (3) approximations made in the formulation and application
extent, particularly those emanating from Colleton River and        of model boundary and initial conditions; (4) errors associated
Pinckney Island (fig. 49D). When compared with plumes               with numerical approximation and solution of the mathemati-
generated by maintaining 2000 pumpage conditions until              cal model of the flow system; (5) uncertainty in interpretation
2100, the plumes originating at Colleton River and Pinckney         of Base Case results, for example, the hydraulic intercon-
Island generated under Scenario D2 conditions are larger in         nection between the Upper and Lower Floridan aquifers; and
extent, by at most about 2 mi during 2100. The plume origi-         (6) assumptions made in using the models to predict the future
nating at the northern end of Hilton Head Island, however, is       behavior of the flow system. The discussion below, which
similar in extent during 2100 for simulations for which 2000        addresses additional limitations associated with the model
pumpage and Scenario D2 2035 pumpage are applied until              scenarios presented in this report, is intended as a supplement
2100. This indicates that the projected 574-Mgal/d increase         to the discussions of model limitations in Payne and other
in pumpage relative to 2000 pumpage (table 1) would have a          (2005) and Provost and others (2006).
limited effect on chloride-plume development in the Hilton                The interpretation of scenario results is influenced by
Head Island area. Plumes that develop away from the areas           the assumptions and approximations made in the implemen-
where calibration data are available or where present-day           tation of the scenarios, and how far the scenario conditions
chloride concentrations are elevated (for example, at Broad         are removed from model calibration conditions. Generally,
Creek, at May River, and offshore of Hilton Head Island) are        the further removed from calibration conditions the scenario
more speculative.                                                   conditions are, the less reliable are the model results. The con-
      Despite a large difference in projected pumpage increases     ditions that define the scenarios may be inherently uncertain,
for Scenario D1 during 2035 and Scenario D2 during 2035             for example the uncertainty in future pumpage. Additionally,
(477 Mgal/d), the simulated chloride distributions for              conditions that are far removed from calibration conditions
Scenario D2 show only a slightly larger extent by 2035 and          may induce unrealistic response from the model, if model
2100 (fig. 49C, D) than those for Scenario D1 (43C, D).             assumptions are violated.
The three nearest counties to the Hilton Head Island area                 For Scenario B, the elimination of pumping at the
for which pumpage was projected are Bryan, Chatham, and             industrial site in Camden County was intended to represent
Effingham Counties. Pumpage increases in these counties             an actual permanent shutdown of wells that occurred during
likely have a larger effect on the plume movement than pump-        2002. The nearest regional pumpage data, however, repre-
age in the rest of the model area. The projected pumpage for        sented annual daily conditions for 2000, and may not accu-
Chatham County is about 13 Mgal/d higher for Scenario D1            rately represent conditions during the period for which simula-
than for Scenario D2, and the projected pumpage for Bryan           tion results were compared with actual observation data. For
and Effingham Counties is higher for Scenario D2 than Sce-          example, Scenario B does not account for the possible causes
narioD1 by a total of about 34 Mgal/d (table 1). These results      of an observed regional rise in water levels after 2000, mak-
indicate that the effect of relatively high stresses far from the   ing uncertain the ability of the model to simulate accurately
Hilton Head Island area on plume development probably are           the effects of the shutdown. The physical construction of the
dissipated by distance, and that the effect of the greater pump-    model also may limit the reliability of the Scenario B results.
age increases for Scenario D2 relative to Scenario D1 at Bryan      For example, the scale and discretization of the MODFLOW
and Effingham Counties is somewhat offset by the lesser             model may be too coarse to represent accurately such a local-
pumpage increases at Chatham County.                                ized phenomenon. In addition, the proximity of the southern
                                                                    specified-head boundary to the simulated shutdown area may
                                                                    dissipate the simulated recovery toward the southern boundary
Model	Limitations                                                   and underestimate the response to the shutdown. The known
                                                                    error in the pumpage distribution of the calibrated model at
      Model results must be interpreted in light of uncertain-      this site probably affected the local calibrated hydraulic prop-
ties and approximations inherent in the formulation of the          erties. Pumpage for Scenario B, however, is reduced by the
model and the several scenarios that the model simulates. The       actual observed amount, and the simulated recovery is likely
ground-water flow and solute-transport models used in this          similar to that, had Scenario B been simulated using a cor-
study are simplified representations of natural processes and       rected and recalibrated model.
2	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

  A. Scenario D2, 2010                                                                       B. Scenario D2, 2020

                                                                Parris Island




                                                      Po
           ol                                              rt
          C




                let                                             Ro
                      on                                             ya
                           R                                              lS
                                                                               ou
                                                                                    nd
                                           I
                                        ney
                                          ck
                                      Pin




                                               Hilton Head Island
                                                                                         n
                                                                                    ea
                                                                                Oc
                                                                           tic




                                                                                                                                                                        N
                                                                      la n
                                                                 At




 Base from U.S. Geological Survey                                                                                                0       1       2       3   4    5 MILES
 1:100,000 and 1:250,000-scale data
                                                                                                                                 0   1       2   3   4   5 KILOMETERS

                                                                                                              SOUTH
  C. Scenario D2, 2035                                                                           GEORGIA     CAROLINA


                                                                                                                        Map
                                                                                                                        area
                                                                                                        Study
                                                                                                        area



                                                                                                             Model
                                                                                                           boundary


                                                                                                                 EXPLANATION
                                                                                                 Simulated chloride concentration, in milligrams
                                                                                                   per liter (mg/L), in ground water in the middle
                                                                                                   of the Upper Floridan aquifer for Scenario D2

                                                                                                  250 to 2,000
                                                                                                  2,000 to 6,000
                                                                                                  6,000 to 10,000
                                                                                                  Greater than 10,000

                                                                                                 Chloride concentration, in mg/L, in ground water
                                                                                                   in the middle of the Upper Floridan aquifer
                                                                                                   for Base Case (2000)
                                                                                                           250                 10,000
                                                                                                 Chloride concentration, in mg/L, in ground water
                                                                                                   in the middle of the Upper Floridan aquifer for
                                                                                                   2000 pumpage during (B) 2020 and (C) 2035
                                                                                                           250                 10,000
                                                                                                 Observation well
 Figure	.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper.Floridan.aquifer.in.the.
 Hilton.Head.Island,.South.Carolina,.area.for.Scenario.D2.(County.Comprehensive.Water-Supply.Plans.projection).
 during.(A).2010,.(B).2020,.(C).2035,.and.(D).2100.(larger.area).
                                                                                                                                        Model	Limitations	 	 


D. Scenario D2, 2100
                                                                                                                                     Parris
                                                                                                                                     Island

                                                             ol




                                                          C
                                                               let
                                                                  on R                                                          Por
                                                                                                                                   tR
                                                                                                                                     oy
                                                                                                                                       al




                                                                                                                                              So
           JASPER




                                                                                                                                                un
                                                                                                                                                  d
                                                                                                               I
                                                                                                            ey
                                                                                                         ckn
                                    BEAUFORT




                                                                                                      Pin
                                                        Bluffton

                                                                         May
                                                                             R   ive
                                                                                    r
                                                                                                                                ee




                                                                                                                            k
                                                                           Bull                                            Cr
                                                                                                                   Broad
                                                                          Island




                                                                                             d
                                                                                           un
                                                                                         So
                                                                                        e
           Center of                                                                             Hilton




                                                                                    gu
                                                                                  bo
           cone of                                                                             Head




                                                                               li
                                                                            Ca
           depression                                                                     Island
                                                          Daufuskie
                                                            Island




                                                                                                          an
                                        SC




                                                                                                          ce
                                                                                                          O
                                                                                                     ic
                                   GA                                                             nt
 N   Savannah                                                                                la

                                                                                         t
             CHATHAM                                                                    A

Base from U.S. Geological Survey 1:100,000 and 1:250,000-scale data                                           0            2            4             6 MILES
Cone of depression from Peck and others, 1999

                                                                                                              0      2           4     6 KILOMETERS
                                                                 EXPLANATION
                                 Simulated chloride concentration,                      Chloride concentration, in mg/L, in
                                   in milligrams per liter (mg/L), in                     ground water in the middle of the Upper
               SOUTH
 GEORGIA      CAROLINA
                                   ground water in the middle of                          Floridan aquifer for Base Case (2000)
                                   the Upper Floridan aquifer                                             250                        10,000
                                   for Scenario D2
                         Map
                         area                                                           Chloride concentration, in mg/L, in
       Study                                  250 to 2,000
       area                                                                               ground water in the middle of the
                                              2,000 to 6,000                              Upper Floridan aquifer for 2000
                                                                                          pumpage during (D) 2100
              Model
                                              6,000 to 10,000
            boundary                                                                                      250                        10,000
                                              Greater than 10,000
                                                                                                   Observation well


Figure	.. Simulated.chloride.concentration.in.ground.water.in.the.middle.of.the.Upper.Floridan.aquifer.in.the.
Hilton.Head.Island,.South.Carolina,.area.for.Scenario.D2.(County.Comprehensive.Water-Supply.Plans.projection).
during.(A).2010,.(B).2020,.(C).2035,.and.(D).2100.(larger.area)—continued.
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	

      Uncertainty in model results increases for simulations         head or pressure in an adjacent confined aquifer. If the con-
at increasingly distant future times because of unpredictable        fined aquifer is highly stressed, these boundary conditions can
changes in future conditions. The two estimates of projected         allow water to flow into the simulated system to an unlim-
water use for Scenarios D1 and D2 were estimated using               ited extent, as illustrated by the increase in area and rate of
different methods, and the differences between these esti-           calculated recharge for Scenarios D1 and D2 (figs. 47 and 48).
mates indicates substantial uncertainty in projected water use       This may result in an underestimation of drawdown. Specifi-
and the future response of the ground-water flow system. In          cally, a source-sink boundary condition may be inappropriate
particular, the CWSP projection (Scenario D2) was based on           to represent recharge in the onshore area because this type
each county’s estimate of future growth, and in some cases           of boundary condition behaves as an unlimited source of
may have been unrealistic. Furthermore, neither projection           water to the system. In the offshore area, on the other hand,
considered available water supply. Simulations are contin-           the ocean may be an unlimited source. If the models allow
ued to 2100 for the 2000 pumpage distribution, and for the           more freshwater than is realistic to enter the system from the
2035 pumpage distribution for Scenarios D1 and D2. There             onshore area, then they also may underestimate the amount of
is considerable uncertainty that these pumpage values would          saltwater entering the system from the offshore area.
be maintained until 2100, although they may represent a                    Under the high stresses imposed in Scenarios D1 and
maximum range of conditions. Thus, results of these simula-          D2, simulated heads for the Upper Floridan aquifer are below
tions provide insight into general, long-term response to a          the top elevation of that unit in some areas (fig. 50). North
range of stresses, rather than specific conditions during 2100.      and west of the Gulf Trough, and at a few locations along the
Also, future pumping is assumed to occur at increased rates          southwestern boundary, these areas coincide with or are near
at currently existing pumping sites; possible redistribution         cells where the controlling head of the boundary condition is
of pumpage to other aquifers, or by the introduction of new          below the top of the Upper Floridan aquifer. In the Savannah
wells or retirement of existing wells is not considered. In          area, at the center of the potentiometric cone of depression,
addition to uncertainty in pumping conditions and distribu-          the predictive scenarios simulate heads for unit 5 that are
tion, potential changes in sea level, climate, and recharge          below the top of the unit, by a maximum of 16 ft and 33 ft
rates could affect the ground-water flow system, and are not         for Scenarios D1 and D2, respectively, during 2035. In this
accounted for by any of the scenarios.                               case, the drawdown is a result of high localized stresses and
      In applying predictive scenarios to ground-water flow          not proximal boundary conditions. For such highly stressed
models, it is important to examine where the model may be            conditions, dewatering of the aquifer may occur, the models
inappropriate to address stresses that exceed those for calibra-     may not accurately represent the hydraulic properties for
tion conditions. The pumpage for Scenarios D1 and D2 is              unsaturated conditions, and the assumed rates of withdrawal
higher by about 12 percent and 71 percent, respectively, than        may not be sustainable. The steady-state response assumption
the highest pumpage for which the models were designed               of the MODFLOW model may be violated under such high
(815 Mgal/d during 2000). An unintended consequence of               stress conditions.
this higher pumpage may be that larger inflow rates than                   In interpreting the results of the solute-transport simula-
realistically occur are induced from the source-sink-type            tions, the conditions for which the SUTRA solute-transport
boundary conditions (including head-dependent flux,                  model was calibrated must be taken into account. Results for
specified-head, and specified-pressure boundary conditions).         conditions very different from the calibration conditions and
Source-sink-type boundary conditions can be used to simu-            in areas devoid of calibration data are speculative; for exam-
late recharge to the system that is buffered by an unconfined        ple, results of Scenarios C1, C2, D1, and D2 are uncertain,
aquifer with a consistently saturated thickness, or a confined       and at most indicate general trends and approximate rates
aquifer that is far from the area of interest. This type of bound-   of chloride-plume movement in response to time-varying
ary condition allows flow into the model where the controlling       stresses in different locations. Furthermore, the area is limited
head or pressure is higher than the simulated head or pressure       where data exist to calibrate the solute-transport model; thus,
in an adjacent confined aquifer, and out of the model where          simulation results showing plume expansion beyond this area
the controlling head or pressure is lower than the simulated         are subject to considerable uncertainty.
                                                                                                             Model	Limitations	 	 


                        SOUTH
      GEORGIA          CAROLINA




        F L OR
                 ID
                   A




                                                                                                                  Mo
                                                                                                                     d
                                                                                                                    el
                                                                          SC




                                                                                                                     bo
                                                                         GA




                                                                                                                         un
                                                                                                                         da
A. Model area




                                                                                                                             ry
                                                                                                               an




                                                                                                          ce
                                                                                                         O
                                                                                                       ic
                                                                                                     nt
                                                                                                    la
                                                                                                  At
                                                                                                EXPLANATION
                                                                                           Simulated head below top of
                                                                                              Upper Floridan aquifer (unit 5)
                                                                                            Base Case, calibrated model in 2000
                                                                                            Scenario D1, 2035
                                                                                            Scenario D2, 2035
B. Scenarios D1 and D2
                                                                                           Boundary-condition head
                                                                                            Below top of Upper Floridan
                                                                                             aquifer (unit 5)
                                              GA
                                                                                            Above top of Upper Floridan
                                              FL
             CHATHAM                                                                         aquifer (unit 5)
              COUNTY

                                                                                                                         N

                                         0          25          50 MILES
                                                                               Base from U.S. Geological Survey
                                         0     25        50 KILOMETERS         1:100,000-scale digital data




  Figure	0.. (A).Simulated.head.below.the.top.the.Upper.Floridan.aquifer.for.Base.Case.(2000),.Scenarios.D1.(projection.
  based.on.Regional.Economic.Models,.Inc.).and.D2.(County.Comprehensive.Water-Supply.Plans.projection).during.2035,.and.
  boundary-condition.head.above.and.below.the.Upper.Floridan.aquifer,.and.(B).Chatham.County,.Georgia,.enlarged.view.
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models	


Summary                                                                the pumping center, the model generally predicts a smaller
                                                                       recovery than observed by approximately 10 feet (ft) near
      Increased ground-water pumpage in the coastal area of            the center of pumping. This discrepancy may be attributed
Georgia, South Carolina, and Florida has resulted in substantial       to (1) an observed regional water-level rise that began dur-
water-level decline near Savannah, Georgia, and saltwater intru-       ing 2000, which is not accounted for in the model; (2) the
sion at the northern end of Hilton Head Island, South Carolina,        use of 2000 pumpage for the rest of the model area, instead
and at Brunswick, Georgia. To develop a strategy to address            of 2002 pumpage; or (3) proximity of the model boundary. A
these problems and manage projected future coastal water-              simulated recovery of 1–2 ft extends as far north as southern
resource needs, the Georgia Environmental Protection Division          Beaufort County and as far inland as the Gulf Trough.
(GaEPD) has implemented the Georgia Coastal Sound Science                    Scenarios C1 and C2 are used to illustrate the relative
Initiative (CSSI), a series of scientific and feasibility investiga-   effects of pumping in Chatham County, Ga., and southern
tions designed to assess coastal-area ground-water resources and       Beaufort County, S.C., on ground-water levels and saltwater
address issues of saltwater intrusion and resource sustainability.     distribution and movement in that area. Scenario C1 simulates
As part of this initiative, the U.S. Geological Survey (USGS)          a hypothetical pumping history to 2000, assuming pump-
synthesized available and new data into new digital models that        ing never occurred in Chatham County, Ga. Compared with
describe the ground-water flow system and saltwater transport.         the Base Case, the large potentiometric cone of depression
The GaEPD will use results of model simulations to help design         centered at Savannah, Ga., disappears for Scenario C1, with a
a coastal-area, ground-water management strategy.                      simulated recovery at the Savannah pumping center of about
      This report describes results of simulations using a             140 ft. This simulated recovery extends at least as far north as
regional MODFLOW ground-water flow model of coastal                    the northern part of Hilton Head Island, where the maximum
Georgia, and adjacent parts of Florida and South Carolina,             water-level increase was 2 ft. The resulting decrease in flow
and a more locally focused SUTRA solute-transport model of             from the offshore general-head boundary indicates a reduced
the Savannah–Hilton Head Island area to evaluate the effects           potential for saltwater intrusion. The plumes originating at
of current and hypothetical ground-water withdrawal, and the           Colleton River and Pinckney Island are of a lesser extent for
relative effects of pumping in specific areas on ground-water          Scenario C1 than for the year 2000 Base Case, and the plume
flow and saltwater transport. The models used in this study            at the northern end of Hilton Head Island is approximately the
are designed to be as consistent as possible in framework,             same shape and of only a slightly lesser extent overall. This
hydraulic properties, pumpage distribution, and boundary               indicates that pumping in Chatham County has a greater influ-
conditions. The discretization of the models differs because           ence on chloride transport from the two western source areas,
they are designed to address different processes at different          and less influence on chloride transport from the source area
scales. The regional MODFLOW model assumes steady-                     on the northern end of Hilton Head Island.
state ground-water flow and is calibrated to 1980 and 2000                   Scenario C2 simulates a hypothetical pumping history to
pumping conditions. The SUTRA model is run as a transient              2000, assuming pumping never occurred in southern Beau-
simulation from a predevelopment (1885) steady-state flow              fort County, S.C. The simulated water-level rise in the Upper
field to 2004, and calibrated to water levels during September         Floridan aquifer is greatest at the northern end of Hilton Head
1998 and estimated chloride values during 2000, 2002, 2003,            Island (about 11 ft), and extends as far southwest as Savannah,
and 2004. Simulation results for future pumpage scenarios              where the maximum water-level increase is about 2 ft. The
were compared with those during 2000 (the Base Case), and to           resulting decrease in flow from the offshore general-head
a scenario for which 2000 pumpage is projected until 2100.             boundary indicates a reduced potential for saltwater intrusion.
      Scenario A represents the flow system during 1997, when                Comparison of results of Scenarios C1 and C2 indi-
the GaEPD implemented the “Interim Strategy for Managing               cates that, in general, pumping in southern Beaufort County
Saltwater Intrusion in the Upper Floridan Aquifer of Southeast         has a smaller effect on plume development than pumping in
Georgia.” During 1997–2000, simulated water levels showed              Chatham County for the plumes west of Hilton Head Island.
a combination of rises and declines in response to changing            At the northern end of Hilton Head Island, the effect of both
pumping patterns. Simulated potentiometric surfaces are simi-          scenarios on plume development is small. Local pumping,
lar to 2000 simulated potentiometric surfaces. The simulated           however, in southern Beaufort County appears to have a
chloride distribution for Scenario A shows a slightly smaller          slightly greater effect on the plume at the north end of Hilton
extent than that for the Base Case; saltwater plumes continued         Head Island, than does pumping in Chatham County. Com-
to expand during 1997–2000 because of persistent drawdown              parison of water-level profiles for Scenarios C1 and C2 shows
of heads in the Upper Floridan aquifer.                                that eliminating pumping in Chatham County would result in
      Scenario B was designed to simulate the effect of the            a greater water-level recovery at the southern end of Hilton
Durango Paper Company well shutdown during 2002 by                     Head Island than eliminating pumping in southern Beaufort
eliminating about 36 million gallons per day (Mgal/d) of               County, and that eliminating pumping in southern Beaufort
pumpage. Although the maximum simulated recovery in                    County would result in a greater water-level recovery at the
the Upper Floridan aquifer is about 29 ft, and the maximum             northern end of Hilton Head Island than eliminating pumping
observed recovery is about 20 ft, located about 1 mile from            in Chatham County.
                                                                                                         Selected	References	 	 

      The pumpage distribution for Scenario D1 is based on the       there is considerable uncertainty in the assumed pumpage val-
estimated change in pumpage in the 24-county coastal area in         ues and distribution, and considerable difference in pumpage
Georgia as a function of population and employment. Maxi-            between the two scenarios. In addition, the substantial draw-
mum simulated water-level declines for Scenario D1 occur             down resulting from these scenarios may indicate that these
in the Savannah–Chatham County, and Statesboro–Bulloch               scenarios exceed the models’ abilities to simulate accurately
County, Ga., areas. The water budget for Scenario D1 shows           such stresses. Finally, the limited spatial and temporal condi-
increasing recharge at the general-head and specified-head           tions for which the SUTRA model was calibrated make model
boundaries during 2035 relative to 2000, and decreasing              results uncertain beyond the areas of the known plumes and
discharge to the these boundaries in response to the increased       for increasing time into the future.
pumpage. The simulated chloride distributions during 2000–
2035 are similar to those simulated for 2000 pumpage, with
slightly greater extent of plumes originating at Colleton River
and Pinckney Island for Scenario D1 pumping conditions.              Selected	References
By 2100, the differences in plume extent are greater.
      The pumpage distribution for Scenario D2 is based on           Applied Coastal Research Laboratory, Georgia Southern Uni-
estimated change in pumpage for the 24 coastal counties in             versity, 2002, Gulf Trough and Satilla Line data analysis:
Georgia as a function of each county’s water-supply plan. For          Georgia Geologic Survey Project Report 48, 15 p.
Scenario D2, the simulated potentiometric surface shows an
                                                                     Camp Dresser and McKee, Inc., 2001, Southeast Georgia
extensive lowering of the potentiometric surface, and the devel-
                                                                       24-county area water supply services, alternative sources to
opment of small but deep cones of depression in inland coun-
                                                                       groundwater supply report, CD–ROM-2.
ties where pumpage is increased and the assigned hydraulic
conductivity is low. The simulated water-level declines during       Clarke, J.S., 2003, The surficial and Brunswick aquifer sys-
2000–2035 exceed 20 ft across most of the 24-county coastal            tems—alternative ground-water resources for coastal Geor-
area in Georgia, and maximum declines exceed 250 ft in the             gia: in Proceedings of the 2003 Georgia Water Resources
Upper and Lower Floridan aquifers. In response to the increase         Conference, April 23–24, 2003, The University of Georgia,
in pumpage, recharge at the general-head and specified-head            K.J. Hatcher, ed., Institute of Ecology, the University of
boundaries increases substantially during 2035 relative to 2000,       Georgia, Athens, Georgia, CD–ROM, accessed January 15,
and discharge to the specified-head boundary decreases.                2005, at http://ga.water.usgs.gov/pubs/other/gwrc2003/pdf/
      The simulated chloride distributions for Scenario D2 show        Clarke-GWRC2003.pdf
a larger extent during 2000–2100 than for the 2000 pumpage
applied during the same years, particularly for plumes emanat-       Falls, W.F., Harrelson, L.G., Conlon, K.J., and Petkewich,
ing from the Colleton River and Pinckney Island areas. The             M.D., 2005a, Hydrogeology, water quality, and water-sup-
250-milligram-per-liter chloride contour in the Upper Floridan         ply potential of the Lower Floridan aquifer, coastal Georgia
aquifer during 2100 for Scenario D2, however, is less than             1999–2002: U.S. Geological Survey Scientific Investiga-
2 miles farther southwest than that for the 2000 pumpage con-          tions Report 2005-5124, 98 p., accessed July 28, 2005, at
ditions maintained until 2100. As is the case for Scenario D1,         http://pubs.usgs.gov/sir/2005/5124/
the plume originating at the northern end of Hilton Head Island
                                                                     Falls, W.F., Ransom, Camille, III, Landmeyer, J.E., Reuber, E.J.,
shows little difference in extent relative to the plume generated
                                                                       and Edwards, L.E., 2005b, Hydrogeology, water quality,
for 2000 pumping conditions for 2010, 2020, 2035, and 2100.
                                                                       and saltwater intrusion of the Upper Floridan aquifer in the
      Despite large differences in pumpage between 2000
                                                                       offshore area near Hilton Head Island, South Carolina, and
conditions, Scenario D1 conditions for 2035, and Scenario D2
                                                                       Tybee Island, Georgia, 1999–2002: U.S. Geological Survey
conditions for 2035, the simulated chloride distributions for
                                                                       Scientific Investigations Report 2005-5134, 48 p.
2100 are similar. This apparent lack of sensitivity of the plume
growth to large differences in pumpage is because distance           Fanning, J.L., 2003, Water use in Georgia by county for 2000
and hydrologic features reduce the effects of these increases          and water-use trends for 1980–2000: Georgia Geologic
on the flow system in the Hilton Head Island area.                     Survey Information Circular 106, 180 p.
      The ground-water flow and solute-transport models used
in this study are subject to the limitations of the models used      Foyle, A.M., Henry, V.J., and Alexander, C.R., 2001, The Mio-
and scenario conditions. Generally, the further removed from           cene aquitard and the Floridan aquifer of the Georgia/South
calibration conditions the scenario conditions are, the less reli-     Carolina coast: Geophysical mapping of potential seawater
able are the model results. The ability of the model to simulate       intrusion sites: Georgia Geologic Survey Bulletin 132, 61 p.
accurately the effects of the well shutdown in Scenario B is
                                                                     Georgia Environmental Protection Division, 1997, Interim
limited by the use of 2000 instead of 2002 pumping condi-
                                                                       strategy for managing saltwater intrusion in the Upper Flori-
tions, not accounting for the regional water-level rise, the scale
                                                                       dan aquifer of southeast Georgia, April 23, 1997: Atlanta,
and discretization of the model, and the proximity of the area
                                                                       Ga., Georgia Environmental Protection Division, 19 p.
of concern to a model boundary. For Scenarios D1 and D2,
	 	 Application	of	Ground-Water	Flow	and	Solute-Transport	Models

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Manuscript.approved.for.publication,.March.21,.2006
Prepared.by.USGS.Georgia.Water.Science.Center
Edited.by.Patricia.L..Nobles
Graphics.by.Caryl.J..Wipperfurth
For.more.information.concerning.the.research.in.this.report,.contact.
   USGS.Georgia.Water.Science.Center,.Atlanta,..
   telephone:.770-903-9100

				
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