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

discharges are added one-at-a-time based on the size of their CBOD load. This modeling
indicated that WQS for DO were not met in the East Branch of the Brandywine creek,
West Branch of the Brandywine, West Branch Red Clay Creek, and in the tidal portion of
the river. The allocation procedure was then implemented by running the water quality
model in an iterative fashion where CBOD, NH3-N, and TP were reduced in 5% intervals
for all the NPDES dischargers upstream of the farthest downstream DO violations. In the
final level of analysis, dischargers outside the basin that influence the tidal portions of the
Christina were included in the model and the allocations were fine tuned in 1%
increments until no violations were found. Summaries of the resulting allocations by
watershed are included in Table 3-5.
Table 3-5: Summary of proposed allocations and reductions by subwatershed of the
Christina that are recommended by the 2004 Low Flow TMDL of the Christina Basin
(EPA 2004a).
Watershed
CBOD5
allociation
CBOD6
reduction
NH3-N
allociation
NH3-N
reduction
TP
allociation
TP
reduction
Units lb/day lb/day lb/day Lb/day lb/day lb/day
Brandywine 1710 436 299 59 198 65
Christina 76 0 14 26 6 0
Red Clay 271 77 24 9 88 57
White Clay 143 0 19 0 16 0
Total 2200 513 356 94 308 122
Units lb/day lb/day lb/day lb/day lb/day lb/day
Overall, the 2004 low-flow TMDL recommended that eight facilities, seven in
Pennsylvania and one in Maryland, have their NPDES permits modified to reduce the
quantity of pollutants they can discharge. Three of the eight facilities were private
companies. The remaining permits were discharging municipal waste. These 8 PS were
distributed in 4 of the basins subwatersheds (Figure 3-2). Within each of these
watersheds there are other NPDES permit holders whose permits were not recommended
for modification. In some subbasins the combined discharge from theses permit holders
is less than the reduction recommended from the others. In other instances the combined
discharges from excluded dischargers is greater than the needed reduction. For example,
in the West Branch of the Brandywine, the combined total phosphorus allocation of the
16 NPDES permit holders who are not recommended for permit modifications (12.91
lb/day) is less than the total TP reduction (17.88 lb/day) that is recommended from the
other two dischargers. However, the combined CBOD5 allocation (176.83 lb/day) of the
16 NPDES permit holders who were not recommended for a reduction is greater than the
recommended reduction from the two dischargers (117.46 lb/day) who were
recommended for a reduction. Because these 16 permits are not recommended for
modification, they have little incentive to develop or trade reductions.
The allocations for the Christina High Flow TMDL were also made using
successive model runs in which land uses, meteorological conditions, and kinematic
parameters remained the same. Unfortunately, the modeling report does not explicitly
state how the “loads from various sources were adjusted” when allocating bacterial loads
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