Combined Sewer System Long Term Control Plan - DC Water

completion of separation as required is recommended for CSO 059.
Selection of Draft LTCP
9.6.1.3 Amphipod Location Near National Zoo
The discharge frequency and volume in the average year, for Scenario C3 (Phase I Controls and
Pump Station Rehabilitation) for CSOs located upstream of the reported amphipod location at the
Zoo are listed in Table 9-10.
Table 9-10
Predicted CSO Overflow Volume and Frequency for CSOs Upstream of Amphipod at Zoo
(Average Year - Average of 1988-1990)
CSO NPDES No. Description No. Overflows /yr CSO Overflow Volume (mg/yr)
041 Ontario Rd 0 0.00
042 Quarry Rd 0 0.00
043 Irving St. 1 0.15
044 Kenyon St. 0 0.00
045 Lamont St. 2 0.03
046 Park Road 2 0.01
047 Ingleside Terr. 3 0.25
048 Oak St-Mt Pleasant 2 0.08
049 Piney Branch 25 39.73
Total 40.25
Piney Branch is the only CSO with a discharge frequency more than four times per year and with a
discharge volume of more than 0.3 million gallons per year. In accordance with the CSO Policy, the
following alternatives are considered:
Separation – Separation of CSOs 041 through 049 would entail separating approximately
2,662 acres of combined sewer area. The estimated capital cost is $515 million (Year 2001,
ENR = 6383). Section 12 addresses WASA's financial capability. Considering the
conclusion of that section and the relatively small overflow volume, separation is not
economically achievable or practical.
Relocation – The only feasible places to relocate the CSOs are to Rock Creek downstream of
the reported amphipod location, or all the way to the Potomac River. Both Rock Creek and
the Potomac are designated as Class A waters with primary contact recreation. Thus
relocation of the outfalls would shift the problem away from the amphipods but cause
additional problems due to potential human contact. In addition, preliminary sizing indicates
a 25’ to 30’ diameter conveyance system would be needed to convey the 15 year storm
downstream. Conveyance facilities of this size would consist of a tunnel. Because of its
depth, the tunnel would not dewater by gravity and would require a pump station. The
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