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RESPONSE TO WRITTEN COMMENTS 4 ' tS, City Of Stockton-Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting introduced earlier. These modifications make it difficult to identify the actual mixing and dilution over multiple tidal cycles, as would be necessary to assess a continuous discharge. The dye study should only be used to estimate mixing of short duration pulse discharges, such as chlorine, and is only applicable when there is a strong tidal current, not at slack tide. Stockton Comment No. I.0 (1)• Finally, the statement that dilution would not exist during a critically dry year is arbitrary in that no data are provided to support its conclusion. The statement does not reflect the understanding that the San Joaquin River is a regulated river, nor does it attempt to identify a critically dry flow condition that would produce zero flow. RWQCB Response: Dilution is not granted because based on available information it is reasonable to assume that there would be little or no dilution during a critically dry year. The minimum weekly SJR flow measured by the Stockton UVM was 102 cfs, which occurred from 4 December through 10 December 1999. The weekly average flow at Vemalis during this time period was 1,780 cfs. The minimum weekly average SJR flow at Vemalis during the winter months of 1991 and 1992 (i.e. during critically dry years)was only 589 cfs from 20 February through 27 February 1992. Therefore, it is reasonable to assume that the flow at Stockton during a critically dry year could be 1/3 of the measured flow during an above average wet year, or approximately 34 cfs. With a permitted discharge of 55 mgd (85 cfs), there would likely be minimal dilution during a critically dry year. Depending on the pumping rates of the Central Valley Project and the State Water Project, the SJR flow at Stockton could potentially be less. This information and the fact that the receiving water is severely impaired with the presence of threatened and endangered species, provides the bases for not granting dilution in the tentative order. Stockton Comment No. I.0 (2): Real-time data confirm dilution always available. The following examples are cited: a. Finding 32/Fact Sheet @ 3 —jet mixing of the outfall produces 7:1 initial dilution. b. Fact Sheet @ 8 —Table 8-1 presents instream ammonia concentrations and effluent ammonia concentration. The Fact Sheet defines "actual dilution" as the ratio of the effluent ammonia to the instream ammonia. The analysis presented in Table 8-1 demonstrates dilution ranging from 5.3:1 to 38.4:1. c. Fact Sheet @ 12—Figures 8-3 and 8-3a illustrate substantial dilution when net river flow is compared with daily effluent flow. This simplistic evaluation under-estimates the actual dilution available. d. Fact Sheet @ 13 —Real time data confirmed dilution was available and show a low of about 5:1 for chronic conditions. e. Finding 45/Fact Sheet @ 14—The available dilution for pathogen/disinfection considerations range from 1.2:1 to 2.7:1 applying the permitted flow, higher dilution is acknowledged for existing flows.