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

nutrients are not explicitly considered in the allocations of either analysis. Attaining
adequate dissolved oxygen levels is the major goal of both TMDL’s.
The Christina Low flow TMDL was one of the first to be completed in the Lower
Delaware River basin. The target of this TMDL is to attain and maintain applicable DO
levels at low flow conditions (Table 3-4). Because of the complex bio-physical
interactions that influence DO levels (Figure 3-5), the low flow TMDL specifically
addresses PS load allocations for total phosphorus, total nitrogen, ammonia-nitrogen, and
Carbonaceous Biochemical Oxygen Demand (CBOD). In the analysis, the 10 th percentile
concentrations during the 7Q10 low-flow period were assumed to represent low flow
NPS contributions (EPA 2004a). No shading of the stream due to vegetation canopy or
cloudiness was incorporated into model. Thus the model is based on maximum
photosynthetic activity. Stream water temps were also set to historic highs and the month
of August was used in developing allocation scenarios. This time period was selected as
the most representative of when critical water quality conditions are expected to occur.
Figure 3-5. Interrelationships of major kinetic processes that influences dissolved oxygen
in surface water bodies. EPA 2004a
56