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Modelling of Seawater Intrusion

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					Modelling of Seawater Intrusion

          C. P. KUMAR




     National Institute of Hydrology
             Roorkee (India)

               17 June, 2005
Seawater Intrusion

• A natural process that occurs in virtually all coastal
  aquifers.

• Defined as movement of seawater inland into fresh
  groundwater aquifers, as a result of

          higher seawater density than freshwater

          groundwater withdrawal in coastal areas
                 Densities
•   Freshwater: 1000 kg m-3
•   Seawater: 1025 kg m-3
•   Freshwater: 0 mg L-1
•   Seawater: 35,000 mg L-1

    dρ 1025 − 1000 kg m       −3
       =           −3
                                   = 0.714
    dC     35 kg m
             Salt Water Intrusion




Pumping causes a cone of depression and...
                    …draws the salt water upwards into the well.
PROPER MANAGEMENT WILL PREVENT
SALINIZATION OF WELLS!


 Not PREVENTING sea water intrusion,
 but CONTROLING sea water intrusion.
Presence of salinity in coastal aquifers can be
detected by

(a)   Geophysical methods
      - Resistivity method

(b)   Geochemical investigations
      - Chemical composition of groundwater
      - Isotope studies (age of water to identify the
      source of salinity)
Field surveys (geophysical and geochemical studies)
can only reveal the present state of seawater intrusion
but can not make impact assessment and prediction
into the future.

Mathematical models are needed for these purposes.

Ghyben-Herzberg relation is a highly simplified
model.

Dynamic movement of groundwater flow and solute
transport needs to be considered.

A density-dependent solute transport model including
advection and dispersion is needed for the modelling.
                       Solute Transport Model




 Flow Equation                          Advection-Dispersion Equation




Distribution of Head




 Velocity Field




                               Concentration distribution in time and space
Most popular models for seawater intrusion

o   SUTRA
o   SEAWAT
o   HST3D
o   FEFLOW

    Recently released Visual MODFLOW Pro 4.1
    now integrates SEAWAT-2000 to solve variable
    density flow problems, such as seawater intrusion
    modeling projects.
                                   USGS
•   HST3D
    – Three-dimensional flow, heat, and solute transport model

•   MOCDENSE
    – Fluid density and viscosity are assumed to be a linear function of the first
      specified solute.

•   SEAWAT
    – A computer program for simulation of three-dimensional variable-density ground
      water flow

•   SHARP
    – A quasi-three-dimensional, numerical finite-difference model to simulate
      freshwater and saltwater flow separated by a sharp interface in layered coastal
      aquifer systems

•   SUTRA
    – 2D, 3D, variable-density, variably-saturated flow, solute or energy transport
                                    Others
•   3DFEMFAT
     – 3-D finite-element flow and transport through saturated-unsaturated media.
       Combined sequential flow and transport, or coupled density-dependent flow and
       transport. Completely eliminates numerical oscillation due to advection terms,
       can be applied to mesh Peclet numbers ranging from 0 to infinity, can use a very
       large time step size to greatly reduce numerical diffusion.

•   FEFLOW
     – FEFLOW (Finite Element subsurface FLOW system) saturated and unsaturated
       conditions. FEFLOW is a finite element simulation system which includes
       interactive graphics, a GIS interface, data regionalization and visualization tools.
       FEFLOW provides tools for building the finite element mesh, assigning model
       properties and boundary conditions, running the simulation, and visualizing the
       results.

•   FEMWATER
      - 3D finite element, saturated / unsaturated, density driven flow and
         transport model.
Numerical Dispersion
•   Numerical approximations of the derivatives of the non-linear
    solute transport equation may introduce truncation errors and
    oscillation errors.
•   The truncation error has the appearance of an additional
    dispersion-like term, called numerical dispersion, which may
    dominate the numerical accuracy of the solution.
•   Oscillations may occur in the solution of the solute transport
    equation as a result of over and undershooting of the solute
    concentration values.
•   If the oscillation reaches unacceptable values, the solution
    may even become unstable.
  The complex density-dependent ground water flow and mass transport
  models provide stable and accurate results when employed with proper
  spatial and temporal discretization.
  The grid Peclet Number (ratio of the spatial discretization and the
  dispersion length) and the Courant Number (ratio of the advective
  distance during one time step to the spatial discretization) should match
  the following constraints:

               Δx                    Δy                   Δz
       Px=             ≤ 2,   P y=           ≤ 2,   Pz=           ≤ 2
               α   L                 α   Τ                α   Τ




              V x Δt            V y Δt           V Δt
       C x=          ≤ 1, C y =        ≤ 1, C z = z   ≤1
               Δx                Δy               Δz



where Px, Py and Pz are the Peclet Numbers; Cx, Cy and Cz are the Courant
Numbers; Δx, Δy and Δz are the grid spacings; αL and αT are the
longitudinal and transverse dispersivity, respectively; and Δt is the time step.
Expertise and Studies at NIH
  • Modelling of Seawater Intrusion
  Dr. Anupma Sharma
  Dr. S. V. N. Rao
  Mr. C. P. Kumar
  Dr. Vijay Kumar
  Mr. P. K. Majumdar
  Dr. M. K. Jose (on deputation)

  • Nuclear Hydrology Group
  • Kakinada Regional Centre
UNDP Training:
Two scientists were trained under UNDP Project (Vijay Kumar,
1997 & C. P. Kumar, 1998) - Application of SUTRA model.

Ph.D. Thesis:
 Numerical Modelling of Seawater Transport in Coastal
Aquifers (Anupma Sharma, University of Roorkee, 1996)
 Planning Models for Water Resources Management in
Coastal and Deltaic Systems (S. V. N. Rao, IIT Madras, 2003)

Research Project:
Freshwater-Salinewater Interrelationships in Multi-Aquifer
System of Krishna Delta, Coastal Andhra Pradesh
(Hydrology project in collaboration with Ground Water
Department, Andhra Pradesh)
Recent Publications (excluding national conferences)

Simulation of Sea Water Intrusion and Tidal Influence
C. P. Kumar
ISH Journal of Hydraulic Engineering, March 2001.

New MOC Model of Seawater Transport in Coastal Aquifers
Anupma Sharma, Deepak Kashyap and G. L. Asawa
Journal of Hydrologic Engineering, September/October 2001.

Numerical Simulation Models for Seawater Intrusion
C. P. Kumar
Journal of Indian Water Resources Society, July 2002.

Simulation of Seawater Intrusion in Ernakulam Coast
Dipanjali D. Bhosale and C. P. Kumar
International Conference on "Hydrology and Watershed Management", 18-20
December 2002, Hyderabad.
Modelling Strategies to Simulate Miscible Transport of
Seawater in Coastal Aquifers
Anupma Sharma, Deepak Kashyap and G.L. Asawa
Hydrology Journal of IAH, March-June 2003.

SUTRA and HST3D Modeling and Management of Saltwater
Intrusion from Brackish Canals in Southeast Florida
Manfred Koch and Anupma Sharma
The Second International Conference and Workshop on Saltwater Intrusion and
Coastal Aquifers Monitoring, Modeling, and Management (SWICA-M3), March
31-April 2, 2003, Mexico.

Effect of Various Parameters on the Size of Fresh Water Lens
in Home Island
Vijay Kumar and John L. Luick
AHI Journal of Applied Hydrology, 2004.
Constraints in the Numerical Modelling of Salt Water
Intrusion
C. P. Kumar
Journal of Soil and Water Conservation, December 2004.

Aquifer Restoration from Seawater Intrusion: A Field Scale
Study of Minjur Aquifer System, North Chennai, Tamilnadu,
India.
S. V. N. Rao
18th Seawater Intrusion meeting in Cartagena, Spain

Few other papers on groundwater development and
management in coastal aquifers by Dr. S. V. N. Rao
SIMULATION OF SEA WATER INTRUSION AND
TIDAL INFLUENCE

Objective: Simulation of sea water intrusion in Nauru
Island and examine the effect of tidal forcing on the fresh
water resources.

• Nauru Island is a coral island in the central Pacific
Ocean, very near the equator and occupies a land area of
22 km2.

• The Nauru aquifer was simulated in two-dimensions
using vertical section with SUTRA.
• The water table is at an average elevation of 0.3 m
  above sea level and ground water flows radially
  outward to the sea.

• The island is underlain by a lens of fresh water as
  much as 7 m thick with average thickness of 4.7 m.
  Fresh water overlies a thick mixing zone which in
  turn overlies sea water.

• The unusually thick mixing zone of brackish water is
  due to the high hydraulic conductivity of the
  limestone.
•   Quantitative estimates of hydraulic conductivity have not been
    undertaken in Nauru Island, but by analogy with similar raised
    limestone islands elsewhere, hydraulic conductivity is
    estimated to be about 800 - 1,000 m/d.

•   Tidal fluctuations may also have some effect on the
    distribution of salinity in the mixing zone, particularly in areas
    near the coastline.

•   Oceanic tides have an amplitude of 0.8 m.

•   Mean annual rainfall is 1,994 mm and annual rainfall has a
    high degree of variability.

•   For this study, a uniform recharge rate of 540 mm/year was
    assumed.
Discretization

• A vertical section of the aquifer along the line C-C’ -
  6400 m long and 120 m deep, was discretized to 832
  rectangular elements and 891 (27 x 33) nodes.

• The horizontal spacing was kept constant as 200 m.
  The vertical spacing was made variable, being 2, 3, 5
  and 10 m from top of the aquifer to depths of 20, 35,
  60 and 120 m, respectively, below mean sea level
  (MSL).
Boundary Conditions

•   A no-flow boundary condition is specified along the bottom
    of the mesh at a depth of 120 m where the limestone is
    considered to be impervious.

•   A recharge boundary due to rainfall is specified at the top of
    the aquifer.

•   Along the left and right vertical boundaries, a hydrostatic
    pressure defined by p = ρs g d was imposed. Here, p is the
    hydrostatic pressure, ρs is the density of sea water, g is the
    acceleration due to gravity, and d is the depth.

•   Any inflow, occurring through the specified pressure
    boundaries, has a sea water concentration of 35,700 mg/L
    TDS (i.e. C* = 0.0357 kg TDS/kg fluid).
Model Parameters

• The Nauru aquifer is reported to be not under any
  major stress such as pumping, it was therefore
  assumed to be in a steady state condition.

• Only one set of salinity data, measured during 1987,
  was available.

• No measurement of hydraulic parameters has been
  undertaken in the island and therefore estimated by
  trial and error using relevant information from similar
  cases.
Values of Hydraulic Parameters for Nauru Island
for Simulation with SUTRA

Hydraulic Parameter                     Value

Horizontal hydraulic conductivity, Kh   900 m/d
Anisotropy, Kh/Kv                       50
Recharge rate                           540 mm/year
Porosity                                0.30
Longitudinal dispersivity, αL           65 m
Transverse dispersivity, αT             0.15 m
Molecular diffusivity                   1.0x10-10 m2/s
The following fixed values were used in the computations:

   Fresh water density ρ = 1,000 kg/m3

   Sea water density ρs = 1,025 kg/m3

   Fluid viscosity μ = 10-3 kg/m/s

   Coefficient of fluid density change with
   concentration ∂ρ/∂C = 700 kg/m3
Simulation of Ground Water Salinity

•   The 1997 version of SUTRA (2D) was used for the simulation.

•   To obtain a steady state solution, the simulation run was
    divided into 1,000 time steps of 15 days each, which
    corresponds to a total simulation period of about 41 years.

•   Figure 4 presents the measured salinity concentrations along
    section C-C’ and figure 5 presents the ground water salinity
    obtained in the present study.

•   The ground water salinity contours for the concentrations
    0.005, 0.01, 0.02 and 0.03 in figure 5 are found to compare
    well with measured.
• The results indicate that the model represents the
  behaviour of the aquifer quite well under the existing
  conditions.

• The model is very sensitive with respect to changes
  in hydraulic conductivity and recharge. Higher values
  of hydraulic conductivity facilitate intrusion of sea
  water, whereas increased recharge has the opposite
  effect, diluting saline water within the aquifer.

• The model is also sensitive to changes in porosity,
  anisotropy and dispersivity but less sensitive to
  changes in molecular diffusivity.
Tidal Influence

•   The tidal signal is manifested as a pressure wave that
    propagates inside from the coastal boundaries towards the
    centre of the model area.

•   Sinusoidally varying pressures were applied at the boundaries
    to simulate tidal forcing.

•   The amplitude of sine wave function (assumed for sea water
    tides) was taken as 0.80 m with frequency of two cycles per
    day.

•   The tidal influence on sea water intrusion has been shown in
    figure 6 which can be compared with figure 5 (without tidal
    forcing).
• A dramatic reduction of the fresh water lens was
  observed when tidal influence is also considered.

• The area of fresh water (concentration less than
  0.0005 i.e. 500 mg/L TDS) was reduced by
  approximately one half in figure 6 (with tidal
  forcing).

• This result highlights the importance of including
  tidal forcing in numerical studies of coastal and
  island aquifers.
Location of the earthquakes /
          tsunami
Tsunami Animation
The 26-12-04 tsunami has affected groundwater systems in the low-
lying coastal zones of the stricken areas.
Schematic representation of the possible effects of the 26-12-04 tsunami on coastal
groundwater systems:




    * Upconing of brackish groundwater under abstraction wells,
    * Intrusion of brackish or saline water from ponds,
    * Fingering of brackish water from pools,
    * Reduction in freshwater volume due to shoreline retreat, etc.
    There are three primary modes through which the saltwater may
    enter the underlying aquifers.
    The first is direct contamination of wells, both large-diameter dug
    wells and small-diameter tubewells that were either open at the top or
    damaged during the flooding.
    The second contamination pathway is widespread infiltration of
    seawater into the aquifer from the land surface through the
    unsaturated zone, the quantity controlled by the permeability of the
    surface sediments and the depth to the water table.
    Following drainage to the sea, some seawater may remain inland as
    surface-water bodies in local low-lying areas. It acts as long-term
    point sources of saltwater to the groundwater system.


•   Numerical models can be used to analyse the impact
    of tsunami on groundwater resources.
Potential Remediation Approaches
  Widespread infiltration of a dense non-reactive contaminant is
  difficult to remediate.
  Removal of bodies of standing saline water and purging of
  wells.
  Allow natural recharge to flush salt from the aquifer.
  If the seawater is isolated in a particular aquifer horizon, it may
  be pumped out of the aquifer and discarded. However,
  application of this method near the coast may induce classical
  seawater intrusion.
  If saltwater contamination is contained in shallow aquifers
  which are isolated from deep aquifers by confining units, the
  deep confined aquifers may become an alternative source of
  fresh water through installation of deeper tubewells.
Future Action
  Data collection and long-term monitoring is necessary
  to assess and manage the impact of the tsunami-
  induced saltwater contamination.
  Measurements of well salinity levels over time as well
  as salinity profiles with depth at selected locations
  should be obtained.
  Generic cross-sectional or three-dimensional
  numerical groundwater models of variable-density
  flow and solute transport can be constructed to better
  understand contamination mechanisms and the
  effectiveness of different remediation strategies.
saltwater intrusion and submarine ground water discharge
investigators

http://users.coastal.ufl.edu/~jnking/SGD/investigators.htm


Saltwater intrusion and submarine ground water discharge are foci of research
on every continent of the world. The following list contains links to investigators
each with interesting insight into these phenomena:


NM Abboud (United States: University of Connecticut)
I Acworth (Australia: University of New South Wales)
WP Anderson (United States: Radford University)
B Ataie-Ashtiani (Iran: Sharif University of Technology)
D Bartlett (United States: University of Maryland)
J Bear (Israel: TECHNION - Israel Institute of Technology)
WC Burnett (United States: Florida State University)
M Al Farajat (Jordan)
AD Cheng (United States: University of Mississippi)
HW Chang (Korea: Seoul National University)
G Dagan (Israel: Tel Aviv University)
GO Essink (The Netherlands: Free University Amsterdam)
A Habbar (Germany: Hannover University)
I Holman (United Kingdom: Cranfield University Silsoe)
KWF Howard (Canada: University of Toronto at Scarborough)
H Klock (Germany: University of Wurzburg)
M Koch (Germany: University of Kassel)
CP Kumar (India: Ministry of Water Resources)
CD Langevin (United States: United States Geological Survey)
JA Liggett Cornell University
PLF Liu Cornell University
L Motz (United States: University of Florida)
H Mahjoub (Spain: University of Barcelona)
AM Mushtaha (Palestine)
Y Ozorovich (Russia: Space Research Institute)
S Oswald (United Kingdom: University of Sheffield)
HW Park (Korea: Korea Institute of Geoscience and Mineral Resources)
P Renard (Switzerland: University of Neuchatel)
Y Ren (United States: University of Virginia)
N Riad (United States: University of Texas)
O Scholze (Germany: Technical University of Hamburg)
YP Sheng (United States: University of Florida)
L Simon (Switzerland: ETH Zurich)
M Stewart (United States: University of South Florida)
M Taniguchi (Japan: Nara University of Education)

NDl Tiruneh (United States: University of Florida)

DS Ward (United States)

C Zheng (United States: University of Alabama)

				
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