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RESPONSE TO WRITTEN COMMENTS 2 City Of Stockton-Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting ammonia criteria have become less stringent and the narrative toxicity objective has not changed. Therefore, a compliance time schedule for meeting the ammonia limitation cannot be allowed within the Order and must be pursued through a Cease and Desist Order. Comments on Tentative Waste Discharge Requirements I. Dilution Factors Stockton Comment No. I.A: A report submitted to the Board in April 2001 fully describes the available flow records at Vernalis and at the Stockton UVM station ("Evaluation of San Joaquin River Flows at Stockton,"Jones & Stokes 2001). RWQCB Response: Detailed comments regarding the above report are provided in Attachment A. Stockton Comment No. LB: The City has submitted several reports that describe the tidal dilution that occurs in the vicinity of the RWCF discharge. The most complete was prepared by Dr. Russ Brown of Jones & Stokes ("Tidal Dilution of the Stockton Regional Wastewater Control Facility Discharge into the San Joaquin River," April 2001). RWQCB Response: Detailed comments regarding the above report are provided in Attachment B. Stockton Comment No. I.C: The proposed Findings conclude that no dilution is available in the San Joaquin River. The Findings cite only three reasons for this conclusion: • Real-time data on flow from the UVM, corresponding to "wet flow conditions," indicates that the minimum daily flow is—6 cfs (Finding 18; Fact Sheet @ 12). • Because the direction of flow reverses with the tide, it results in river flow stagnation(i.e., slack tides) with very little dilution of effluent (Finding 45; Fact Sheet @ 12). • It is reasonable to assume that during a critically dry year, there may be no net flow over averaging periods of seven to thirty consecutive days when no chronic dilution would exist (Fact Sheet @ 13). RWQCB Response: The Findings conclude that based on the available data there is little or no dilution. The Discharger's comment above sites three reasons for the determination of dilution. This is an incorrect representation of the basis for the decision to not allow dilution. Section 8.0 of the Fact Sheet thoroughly documents the basis for the granting of dilution. The decision to not grant dilution for all constituents except human health constituents was based on the following: • Real-time flow data from the UVM shows that dilution has been minimal during above average wet years for all conditions (Fact Sheet, Section 8.5 acute (1-day), Section 8.6