SITE INFORMATION AND CORRESPONDENCE_CASE 2

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Response to Written Comments -12- 24 April 2007 City of Tracy WWTP San Joaquin County the US EPA recommends the use of a maximum daily effluent limitation in lieu of average weekly effluent limitations for two reasons. "First, the basis for the 7-day average for POTWs derives from the secondary treatment requirements. This basis is not related to the need for assuring achievement of water quality standards. Second, a 7-day average, which could comprise up to seven or more daily samples, could average out peak toxic concentrations and therefore the discharge's potential for causing acute toxic effects would be missed." (TSD, pg. 96) The proposed Order utilizes maximum daily effluent limitations in lieu of average weekly effluent limitations for ammonia as recommended by the TSD for the achievement of water quality standards and for the protection of the beneficial uses of the receiving stream. Additionally, mass-based effluent limitations have been included for ammonia, nitrate, and nitrite because they are oxygen-demanding substances and the receiving water is impaired for dissolved oxygen deficiencies. CITY OF TRACY - NPDES COMMENT #19: Section IV.A.2.b., IV.3.b., and IVA.b. Flow Restrictions. The City suggests that the there is no justification for flow requirements and that they be removed or set as an average over three consecutive dry weather months each year. Further, the City is concerned that its current discharge flow limit of 9 mgd (ADWF) might be exceeded before 2008. Request: Revise IV.A.2.b. to read: "The Average Dry Weather Discharge Flow shall not exceed 9.0 million gallons per day. The average dry weather flow shall be determined over three consecutive dry weather months each year" RESPONSE: It is appropriate to limit the flow of the discharge and also include mass limitations. Additional language is proposed to be added to the Order as a late revision to clarify how compliance with the flow limit will be determined (see response to CITY OF TRACY — NPDES COMMENT #27. CITY OF TRACY - NPDES COMMENT #20: Section IV.A.2.d. Mercury Mass Limits. The City questions the determination of reasonable potential for mercury and contends that this action is more stringent than required by federal law and that an analysis under Water Code section 13263 must be performed. The City also requests that if the mercury mass limit is maintained, that it be changed back to the previously suggested performance-based annual mass limit of 0.51 pounds per year to better reflect the long term concerns with mercury mass loadings and to provide a bit more regulatory flexibility. Request: Replace the proposed monthly limit with the previously suggested annual mass limit of 0.51 pounds per year because the monthly limit is impracticable, or remove the mass limit in Provision W.A.2.d. because compliance is Regional Water Quality Control Board, Central Valley Region Board Meeting —3/4 May 2007 Item#17