# Presentation: 704793

## TMDL Case Study

**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**

## 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 **

## 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**

## 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**

## 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**

## 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**

## Christina River Basin Modeling Effort

- Loading
- Model

- Monitoring
- Program

- Receiving Water
- Model

- TMDL Development

## 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**

## Location of STORET

**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**

## Christina River Basin Water Quality Model

- Christina River Basin Water Quality Model

## 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**

## Low-head Dam on Brandywine Creek

## Flow Structures

**32 flow structures included in the model **
**8 tidal inlets to connect peripheral streams to Christina River**
**24 overflow structures representing low-head dams, submerged weirs, bridge culverts, fall lines, stream confluences**
**Rating curve representing a free overfall was used for the overfall structures**

## 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)**

## 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**

## Calibration Results

**Results presented as longitudinal transect plots for the August period (low-flow condition)**
**Results were also presented as time-series plots at a single monitoring station on the major tributaries**
**Model statistics compare well with other similar studies**

## 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**

## 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**

## TMDL Endpoints

## Stream Use Designations

**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.**

## Baseline conditions

**Baseline conditions**
**Daily Average DO**

## Baseline conditions

**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**

## EMPR Level I

**EMPR Level I**
**Daily Average DO**

## EMPR Level I

**EMPR Level I**
**Minimum DO**

## EMPR Level 1 Allocations

## EMPR Level 2 Allocations

## 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

## 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