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RESPONSE TO WRITTEN COMMENTS -3- City Of Stockton- Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting chronic (4-day and 7-day), Section 8.7 pathogen/disinfection considerations (7-day and 30- day), and Section 8.9 ammonia (1-day and 30-day)) • The available UVM data is from above average wet years (Fact Sheet, Section 8.2) • Tidal influences permit the receiving water to be dosed with effluent multiple times (Fact Sheet, Section 8.5) • The existing water quality models are inadequate to determine dilution (Finding 44 and Fact Sheet, Section 8.2). Detailed comments provided in Attachments A, B, and C. • The SJR is an impaired water body that is 303(d) listed for several contaminants and is inhabited by several threatened and endangered species (Findings 19 and 44, Fact Sheet @ 8and 13) • Minimal dilution may exist for certain averaging periods, but due to the severely impaired condition of the receiving water and the presence of threatened and endangered species, dilution has not been proposed in the tentative order. Stockton Comment No. LC (1): Contrary to the advice provided by experts in estuarine dynamics (e.g., Jones & Stokes reports), staff determined the available dilution solely as a ratio between the net river flow and the effluent flow. This approach is not applicable to tidal river systems as noted by EPA in the 1991 TSD. RWQCB Response: The models presented by the Discharger are inadequate and cannot be the basis for Regional Board decisions (detailed comments are presented in Attachments A, B, and Q. Even though using the net river flow to determine dilution may not be precisely accurate, there is enough actual data to make a decision on whether to grant dilution. Stockton Comment No. LC (1): The staff s conclusion regarding dilution and mixing during slack '.:des (i.e., flow stagnation equates to little or no dilution; Fact Sheet @ 12) is similarly misplaced. As noted by EPA, the available dilution occurring under the short slack tide event is governed by the receiving water volume and plume mixing dynamics, not the river velocity. The Findings did not cite any consideration of these factors to assess dilution and mixing near slack tide even though available studies confirmed that initial mixing followed this pattern. Prior studies submitted to the Board, which were confirmed by recent studies, showed at least 7:1 dilution from initial mixing at slack conditions (see Fact Sheet @ 7). Thus, the conclusion that very little dilution occurs during tide reversal is unsupported by fact, and is directly refuted by the available information. RWQCB Response: The 1993 dye study referred to in the City's comment was only performed during specific tidal conditions; ebb, low, and flood. Dye dosing was not continuous during the test; the dosing stopped for approxic two hours betwee: "des. The dosing connentration was increased prr ively frons i' condition to the ne .; to minimize possib. :.terference of dye