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2 <br /> RESPONSE TO WRITTEN COMMENTS -7- <br /> City Of Stockton-Regional Wastewater Control Facility <br /> NPDES No. CA0079138 <br /> 26 April 2002 Board Meeting <br /> Stockton Comment No. I.D (5): Dilution for pathogens available. <br /> RWQCB Response: The City suggests that there is dilution based on the real-time UVM data and the <br /> ones & Stokes Box Model. The UVM data are from above average wet years and the Box Model is .. <br /> inadequate. There is minimal dilution during above average wet years; therefore, it is reasonable to <br /> assume that there will be less dilution, if any, during critically dry conditions. <br /> The 7-day median coliform limitation averaging period is tied to the statistics of the test method and <br /> adequate plant performance to meet those limits. The 20:1 dilution recommendation is not tied to the <br /> same longer term averaging period, but is based on short term, relatively instantaneous conditions such <br /> as might be encountered during contact recreation. Therefore, the 20:1 dilution cannot be determined <br /> over a 7-day or 30-day averaging period. <br /> Stockton Comment No. LD (6): Dilution for human carcinogens incorrectly calculated. <br /> RWQCB Response: The SIP states that the harmonic dilution is the harmonic mean river flow <br /> divided by the long-term average arithmetic mean effluent flow. The arithmetic mean effluent flow is <br /> the permitted flow(55 mgd), since the permit allows flows up to this amount. The harmonic mean <br /> river flow was calculated using the available UVM data, which occurred during above average wet <br /> years. The harmonic dilution was estimated to be 10:1. <br /> Stockton Comment No. I.D ('n: Dilution credits for ammonia should be allowed. An evaluation of <br /> the instream pH at stations R-2 and R-3 demonstrates that the mixed pH is typically less than 7.1 when <br /> the effluent concentration in the river is high (e.g., as indicated by elevated ammonia concentration). <br /> With less dilution, the mixed pH would be lower still. This information was provided to the staff as <br /> part of the City's submissions and should have been used to select the appropriate pH condition for <br /> applying EPA's ammonia criteria. <br /> RWQCB Response: Section 10.4.1 of the Fact Sheet (specifically page 23) details the method staff <br /> used to determine the critical pH, in Method 4 of the reasonable potential analysis. The evaluation <br /> used data from receiving water monitoring stations R-2 and R-3 during critically dry years. The <br /> appropriate pH condition has been used in applying EPA's ammonia criteria. Additionally, the <br /> evaluation has been reviewed and approved by US EPA Region IX, as stated in their comment letter <br /> dated 22 March 2002. <br /> Stockton Comment No. I.D ('n(a): Board staff evaluated acute dilution for ammonia by comparison <br /> of the design effluent flow (85 cfs)with the minimum UVM daily flow of-6 cfs, concluding that no <br /> dilution was available. We have already noted that the daily UVM data cannot be used to determine <br /> dilution credits due to the ability of the estuary to store flow, thus this Finding is without technical <br /> support. The proper analysis provided by Dr. Brown demonstrated that a minimum dilution of 1:1, <br /> within 125 feet from the discharge, is available at critically dry flow condirinns even if the effluent <br /> were * lischarge at the design flow over an exte i- i period of tin e. \o , i iution during glact rd <br />