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RESPONSE TO WRITTEN COMMENTS -5- City Of Stockton-Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting £ Fact Sheet @ 14—Staff calculated a harmonic mean of 848 cfs and an arithmetic mean effluent flow of 41.8 cfs. Together these yield a harmonic mean dilution of 20.3:1, per the SIP. g. Fact Sheet @ 15 —The lowest_30-day net flow average had a value of 233 cfs. The 30-day average effluent discharge flow over this same time period was 35.7 cfs. This translates into an available chronic dilution of about 6:1. h. Fact Sheet @ 20—Figure 10-1 illustrates effluent ammonia concentration and instream ammonia concentration at Station R-2. The maximum observed instream concentration, 5.9 mg/1, occurred when the effluent concentration was 25 mg/l. This corresponds to a dilution of 4.2;1. RWQCB Response: Responses to the above comments are shown below: a. The modeling conducted in 1993, which is the basis for the determination that there is an initial dilution of 7:1, is only valid for pulse discharges such as chlorine, and is only applicable when there is a strong tidal current, not at slack tide. Finding 32 and the Fact Sheet @ 3 refers to chlorine residual. b. Table 8-1 represents data that were collected during above average wet years. c. Figures 8-3 and 8-3a represent data that were collected during above average wet years. d. The evaluation outlined in the Fact Sheet @ 13 represents data that were collected during above average wet years. e. The evaluation outlined in the Fact Sheet @ 14 represents data that were collected during above average wet years. . f. Harmonic dilution has been granted in the tentative permit. g. The evaluation outlined in the Fact Sheet @ 15 represents data that were collected during above average wet years. h. Figure 10-1 represents data that were collected during above average wet years. Stockton Comment No. I.0 (2): In addition to these Findings, all of which document available dilution, staff acknowledged (Finding 1) that the discharger submitted supplemental information on 11 April 2001, including an "Evaluation of Chronic Dilution in the San Joaquin River Under Drought Flow" (Hall & Associates). This report evaluated effluent and receiving water ammonia concentrations (i.e., real-time data) observed during the period from January 1990 through December 1999. This period includes the drought that occurred in 1991, which is identified as a critically dry year for the San Joao,, n River(Fact Sheet 1 5).