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RESPONSE TO WRITTEN COMMENTS -16- City Of Stockton-Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting provide a means for offsetting these loads. This banking provision allows for growth in the RWCF service area. The tentative permit issued on 9 April 2001 originally allowed the City to offset Group A Pesticides in- --- the same manner as is allowed for mercury in the proposed permit. The offsetting of Group A Pesticides was removed from the tentative permit issued on 15 February 2002 because of its inherent conflict with the Basin Plan non-detect water quality objective. However, the performance-based interim yearly mass limitations were still based on current levels and had the potential to restrict growth. Therefore, the interim effluent limitations for Group A Pesticides have been recalculated and a growth factor has been applied. This change was necessary to clarify our intention that the interim limits would not preclude growth. The City is still required to conduct pollution prevention and construct treatment facilities to meet the final effluent limitations during the permit term. The interim performance-based effluent limitations for the Group A pesticides are yearly mass limitations. The effect of one sampling event is minimized as the limitation is averaged over a year. The SIP's statistical approach for daily maximum concentration-based interim limitations would provide excessive yearly mass limitations. The interim effluent limitations for the Group A pesticides were calculated using an uncertainty factor of 1.2 and an additional 10 percent was added to account for growth in the RWCF service area. Based on historical effluent data, the City should be able to meet the proposed interim limits while allowing for projected growth. Table 11-6 of the tentative permit has been modified to reflect the above change. The changes to the table are not shown in redline format. The State Water Resources Control Board Office of Chief Counsel has prepared a Memorandum that concludes that offset programs are appropriate in certain circumstances and those circumstances are present in this situation. See Memorandum to Arthur G. Baggett, Jr., Chair, State Water Resources Control Board, from Craig. M. Wilson, Chief Counsel (October 16, 2001) Stockton Comment No. IV.E (Nitrate Requirements): The proposed order includes a requirement for a nitrate study due to the potential for the discharge to cause elevated levels of nitrate in the receiving waters. (Finding 34) These elevated levels are predicted to exist once the facility nitrifies on a year round basis. The City objects to this requirement on the following grounds: • There is ample dilution available to ensure that nitrate levels in the receiving water do not exceed 10 mgl1. The Board estimated that the dry weather chronic dilution factor is 2.7:1. This should allow a 37 mg/1 nitrate discharge. The effluent could not possibly go that high. • There are no potable water intakes in the vicinity of Stockton and nitrate is a non-conservative substance. There is no reasonable basis to believe that any nitrate discharged by the City would actually impair any drinking water uses. • Current nitrate levels in the summer months are low when the ponds promote ammonia reduction: Winter ammonia removal, when properly established, will not dictate the need to nitrify from