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*./ RESPONSE TO WRITTEN COMMENTS -17- City Of Stockton-Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting October through March. As the only potential time frame for elevated nitrate levels was the winter months and nitrification should not be required during that period, there is no factual basis to support the Board's req,.test for further study of this issue. Based upon these comments, the requirement to evaluate nitrate discharges should be deleted from the order. RWQCB Response: Provision G.14 of the tentative order requires the City to study existing and future nitrate concentrations and allows the Regional Board to reopen the order to include nitrate effluent limitations if necessary. This requirement will remain in the tentative order, as the City's comments do not justify the removal of the provision. In response to the City's specific comments regarding this requirement: • A discharge of 10 mg/1 of nitrate may exceed the primary MCL. The lowest 30-day average dilution credit estimated by the Board is 2.7:1 during above average wet years. The Board has not granted chronic dilution so the discharge must be regulated at the end-of-pipe. • The receiving water has the designated beneficial use of MUN. In addition, Resolution 88-63, which was incorporated into the Basin Plan pursuant to Regional Board Resolution 89-056 (Incorporation of Sources of Drinking Water Policy into the Water Quality Control Plans) states that all surface and ground waters of the State are considered to be suitable, or potentially suitable, for municipal or domestic water supply and should be so designated by the Regional Board, with certain exceptions. The Regional Board is required to follow this Policy and consider the San Joaquin River in the vicinity of the discharge as MUN. To exclude a MLN use, the Regional Board must amend its Basin Plan. No change to the permit language was made as a result of this comment. • The City's Report of Waste Discharge indicates that nitrates as nitrogen (NO3-N) has been measured on 48 occasions from 1995 through 1999. The results of the limited monitoring show that a monthly high NO3-N of 9.33 mg/1 has been measured. It is reasonable to expect that the RWCF's discharge could have an increase in NO3-N with the requirement to nitrify. Stockton Comment No. IV.F (pH Requirements): The prior permit allowed effluent pH to range from 6.0— 8.5,,,, allowing a small mixing zone to achieve compliance with the receiving water pH objectives (Finding 52). The proposed permit disallows the mixing zone and raises the minimum pH up to 6.55„ based upon the conclusion that mixing is not available, and applies the limit as one instantaneous requirement. Moreover, the pH provisions of the Basin Plan have been determined to apply as a 30-day average, and improperly use the 30-day average criteria to set an instantaneous permit limit. RWQCB Response: The comment provides that the proposed 30-day average pH receiving water lic w is correct, m: ' � - . instantaneous effluc 'imitation improper. However, the opposi trL acid is obvious fio e iscussion regarding the decision to not grant a mixing zone. The: _ur;.