TMDL Case Study
Delaware - EPA Region 3
TMDL for Nutrients and Dissolved Oxygen under Low-Flow Conditions in the Christina River Basin
Christina River Basin
• Study area
basin covers 565 square miles spans 3 states (PA, DE, MD) 39 stream segments on 303(d) list
• Major watersheds
Brandywine Creek Red Clay Creek White Clay Creek Christina River
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Christina River Basin
• Agencies involved:
EPA Region 3 Delaware River Basin Commission Pennsylvania DEP Delaware DNREC Maryland Dept. of Environment New Castle County WRA USGS Brandywine Valley Association
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Christina River Basin Problem Statement
• 24 stream segments included on Pennsylvania’s 1998 303(d) list • 15 stream segments included on Delaware’s 1998 303(d) list • Impaired due to nutrients and dissolved oxygen not meeting water quality criteria • Exceedances of dissolved oxygen criteria occur primarily during low-flow, summer conditions • Most probable sources during low-flow periods are point source discharges
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Impairment Analysis
• In-stream water quality data inventory • Impairment confirmation
Magnitude of impairment Frequency
• Seasonal and daily patterns • Background conditions • Identification of data sets to support modeling • Additional sampling to support modeling
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Christina River Basin TMDL
• Step 1 - TMDL under low-flow conditions
focus on point sources critical summer conditions TMDL was issued January 19, 2001
• Step 2 - TMDL under variable-flow conditions
considers both point and nonpoint sources considers both high and low-flow conditions TMDL due December 2004
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Water Quality Model
• Point sources
122 NPDES facilities 0.0005 mgd to 7.0 mgd
• Nonpoint sources
39 subwatersheds Agriculture (31%) Forest (28%) Urban (34%) Open land, water, other (7%)
• 28 water withdrawal facilities • Free flowing streams • Tidal rivers and streams
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Christina River Basin Modeling Effort
Watershed Loading Model
Monitoring Program
Receiving Water Model TMDL Development
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Christina River Basin Monitoring Program
• PA, DE, DRBC, and EPA reached agreement in 1993 to initiate a cooperative and coordinated monitoring program for the Christina River Basin TMDLs • Beginning in 1995, PDEP and DNREC sampled monthly or semi-monthly at 23 stations • The USGS monitored flow and/or quality at 17 stations • In August 1997, an intensive survey was conducted by PDEP and DNREC at 12 stations to characterize daily variations in DO and nutrients • In 1999, an effluent monitoring study of the 14 largest WWTPs in the basin was conducted to determine organic carbon and CBOD characteristics • Historical data were used from STORET, PCS, DMRs, and state archives VIII-9
Location of STORET monitoring stations
Christina River Basin Water Quality Model
• OBJECTIVES • Provide a calibrated and validated hydraulic and water quality model of Christina River basin representative of critical low-flow conditions • Provide a model capable of addressing variable flow conditions for high-flow phase of the TMDL • Provide training on the model use • Develop TMDL allocation scenarios • Scientific credibility documented in technical report
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Christina River Basin Water Quality Model
EFDC Model
Hydrodynamics
Water Quality
Sediment Transport
Toxics
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EFDC Water Quality Model
• 16 stream reaches
Christina River and West Branch Christina River (tidal and nontidal) Brandywine Creek (tidal and nontidal) East and West Branch Brandywine Creek Buck Run White Clay Creek and East Branch White Clay Creek (tidal and nontidal) Red Clay Creek and West Branch Red Clay Creek Little Mill Creek Mill Creek Pike Creek Muddy Run Burroughs Run Delaware River (tidal)
• Designed to be linked to watershed runoff model for seasonal analysis VIII-13
Low-head Dam on Brandywine Creek
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Flow Structures
• 32 flow structures included in the model • 8 tidal inlets to connect peripheral streams to Christina River • 24 overflow structures representing lowhead dams, submerged weirs, bridge culverts, fall lines, stream confluences • Rating curve representing a free overfall was used for the overfall structures
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Water Quality Model Calibration
• MODELED PARAMETERS
Chlorophyll-a Nitrogen (ammonia, nitrate, total) Phosphorus (orthophosphate, total) Organic carbon (dissolved, total) Total suspended solids Chloride concentrations Daily average dissolved oxygen Diel dissolved oxygen (daily minimum, maximum)
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Water Quality Model Calibration
• Model calibration period: May 1 to Sep 21, 1997 • Model validation period: May 1 to Sep 21, 1995 • Nonpoint source loads from peripheral tributaries were computed using estimated low-flow rates and a characteristic concentration for each parameter • Model-data statistics were computed and summarized in report
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Calibration Results
• Results presented as longitudinal transect plots for the August period (low-flow condition) • Results were also presented as timeseries plots at a single monitoring station on the major tributaries • Model statistics compare well with other similar studies
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Low-flow TMDL Analysis
• Nonpoint source loads computed based on estimated low-flow rates • Point sources set to their existing permit limits for both flow and concentration of CBOD, NH3-N, TP, and DO • Stream conditions set to low-flow rates • Stream temperature set to 75th percentile summer conditions
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Low-flow Data Set
• Low-flows estimated for each watershed based on nearby stream gages • Nonpoint source loads were estimated for each of the 39 subwatersheds and distributed to the EFDC grid cells within those watersheds • Water withdrawals were set to either the safe yield or 75% of the peak withdrawal rate
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TMDL Endpoints
Parameter Daily Average DO, freshwater, Pennsylvania Daily Average DO, freshwater, Delaware Daily Average DO, tidal waters, Delaware Target Limit 5.0 mg/L 5.5 mg/L 5.5 mg/L Reference Pennsylvania Water Quality Standards Delaware Water Quality Standards Delaware Water Quality Standards Maryland Water Quality Standards Pennsylvania and Delaware Water Quality Standards Pennsylvania Water Quality Standards
DO at any time, freshwater, 5.0 mg/L Maryland Minimum DO Nitrate Nitrogen, Pennsylvania Ammonia Nitrogen, Pennsylvania 4.0 mg/L 10.0 mg/L as N
function of Temp, pH
Pennsylvania Water Quality Standards
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Stream Use Designations
Baseline Conditions
• NPDES point sources set to existing permit limits. • Tributary loads based on low-flow concentrations for the subwatersheds from the period 1988-1998 (STORET) • Reasonable agreement with Omernik (1977) study of watersheds in the eastern U.S.
Total Nitrogen (mg/L) Subwatershed Baseline Main stem and East Branch Brandywine Creek West Branch Brandywine Creek Red Clay Creek White Clay Creek Christina River 1.56 Omernik (1977) (67% range) 0.33 - 6.64 Baseline 0.01 Omernik (1977) (67% range) 0.008 - 0.251 Total Phosphorus (mg/L)
2.44 2.65 2.31 1.08
0.33 - 6.64 0.33 - 6.64 0.33 - 6.64 0.33 - 6.64
0.03 0.05 0.02 0.02
0.008 - 0.251 0.008 - 0.251 0.008 - 0.251 0.008 - 0.251 VIII-23
Baseline conditions Daily Average DO
Baseline conditions Minimum DO
EMPR Allocation Strategy
• Equal Marginal Percent Removal • Level 1
Analyze each NPDES discharge individually to determine if water quality standards are met Set other tributaries and point sources not being considered to a baseline (background) condition If an individual point source does not meet water quality standards (WQS), then reduce the CBOD, TN, TP load until WQS are achieved
• Level 2
Add other discharges to the analysis one at a time based on the size of the mass load of CBOD If WQS are not met, reduce CBOD, TN, TP load for all point sources in the analysis by equal percent
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EMPR Level I Daily Average DO
EMPR Level I Minimum DO
EMPR Level 1 Allocations
Existin g Perm it Lim its NPDES Facility Flow (mgd) CBOD5 (mg/L) NH3-N (m g/L) TP (mg/L) Level 1 Allocation Lim its CBOD5 (mg/L) NH3-N (mg/L) TP (mg/L) Level 1 Reduction CBOD5 NH3-N TP
East Branch Brandywine Creek PA0026531 7.0 10 2.0 2.0 10 2.0 1.6 0% 0% 20%
West Branch Brandyw ine Creek PA0026859 3.85 15 2.0 2.0 10.5 2.0 1.05 30% 0% 48%
West Branch Red Clay Creek PA0024058 1.1 25 3.0 7.5* 17.5 2.1 1.35 30% 30% 82%
West Branch Christina River MD0022641 0.70 22** 6.45* 1.0 22** 2.0 1.0 0% 69% 0%
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EMPR Level 2 Allocations
Level 1 Allocation Lim its NPDES Facility Flow (mgd) CBOD5 (mg/L) NH3-N (mg/L) TP (mg/L) Level 2 Allocation L imits CBOD5 (mg/L) NH3-N (mg/L) TP (mg/L) Level 1 + Level 2 Red uction CBOD5 NH3-N TP
East Branch Brandywine Creek PA0043982 PA0012815 PA0026531 0.4 3.0 7.0 25 34 10 2.0* 6.0 2.0 2.0 1.0 1.6 14.38 19.55 5.75 2.0 3.45 1.15 1.15 0.58 0.91 42% 42% 42% 0% 42% 42% 42% 42% 54%
West Branch Brandyw ine Creek PA0026859 3.85 10.5 2.0 1.05 10.5 2.0 1.05 30% 30% 48%
West Branch Red Clay Creek PA0024058 PA0057720-001 1.1 0.05 17.5 10 2.1 2.0 1.35 2.0* 16.62 9.50 1.99 1.90 1.28 1.90 34% 5% 34% 5% 83% 5%
West Branch Christina River MD0022641 0.70 22** 2.0 1.0 22** 2.0 1.0 0% 69% 0%
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Results of EMPR Level 2 TMDL Allocations, East Branch Brandywine Creek
Daily Average DO
Downingtown WWTP discharges 7.0 mgd at RM103.7
Sonoco discharges 3.0 mgd at RM 104.1 Existing Permits: DO sag = 3.23 mg/L After TMDL: DO sag = 5.92 mg/L
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Results of EMPR Level 2 TMDL Allocations, East Branch Brandywine Creek
Diel Minimum DO
Downingtown WWTP discharges 7.0 mgd at RM103.7
Sonoco discharges 3.0 mgd at RM 104.1 Existing Permits: diel min DO = 0.63 mg/L After TMDL: diel min DO = 4.02 mg/L
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