Water Quality Trading in the Lower Delaware River Basin: A ...

Table 3-4. Christina TMDL target limits for Dissolved Oxygen that are recommended by
the 2004 Low Flow TMDL of the Christina Basin (EPA 2004a).
Parameter Target Limit Reference
Daily Average DO, freshwater, 5.0 mg/l Pennsylvania Water Quality
Pennsylvania
Standards
Daily Average DO, freshwater, 5.5 mg/l Delaware Water Quality
Delaware
Standards
DO at any time, Maryland 5.0 mg/l Maryland
Standards
Water Quality
Minimum DO, freshwater 4.0 mg/l Pennsylvania and Delaware Water
Quality Standards
The Christina high flow TMDL is focused on sediment and bacteria from point
and NPS, including storm water and storm sewage systems. The TMDL’s target
endpoint for bacteria was selected to maintain recreational contact use during both
swimming and non-swimming seasons. In this analysis the main PS were municipal
separate storm sewer systems (MS4s). Within the basin, there are 40 MS4-NPDES
permits. Nonpoint sources of sediment and bacteria that were considered include septic
systems, agriculture, wildlife, and domestic pets. The sediment endpoint was determined
by using the reference watershed method and ranges between 0.09 to 1.04 tons/acre/yr.
Both the high-flow TMDL and the low-flow TMDL’s for the Christina Watershed
used the EFDC model to simulate conditions in the waterway. However, each TMDLs
used a different watershed loading model. Inputs for the low-flow TMDL were estimated
from the product of the flow rate for each subwatershed and a characteristic concentration
for the particular contaminant. These concentrations were based on a combination of
STORET data and field observations at monitoring stations. The high-flow TMDL used
a combination of HSPF and SWMM to develop the input loads (EPA 2005). HSPF was
used to develop the loads from point and nonpoint sources. SWMM was used to identify
loads from combined sewer overflows (CSOs) from the city of Wilmington. All of these
modeling efforts were based on data characterized as “extensive and high-quality”.
Model calibration and validation was confirmed by EPA by showing that the model was
able to simulate the locations of the impaired stream.
The allocation of TMDL based reductions
The EMPR allocation strategy was used in allocating the reductions required in the low
flow TMDL analysis. In this strategy all point-source dischargers are reduced by equal
percentages in reiterative model runs until WQS are met. Thus the marginal rate of
removal for all affected dischargers is the same but the absolute quantity removed maybe
different. In the Christina basin the EMPR strategy was applied in multiple levels of
analysis (EPA 2004a).
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