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RESPONSE TO WRITTEN COMMENTS City Of Stockton-Regional Wastewater Control Facility 6 NPDES No. CA0079138 26 April 2002 Board Meeting The chronic dilution report calculated "actual dilution"using the effluent and receiving water data, in a manner similar to the calculations provided by staff in Table 8-1. During this critically dry period, the maximum instream ammonia concentration observed was about 6.2 mg/1, with a maximum 30-day average ammonia concentration of 5.55 mg/I(see Fact Sheet @ 25).4 Over this period,the effluent ammonia concentration averaged 19 mg/l. Together these data demonstrate available dilution on the order of 3.4:1. RWQCB Response: The data referred to in the above comment are from receiving water monitoring stations R-2 and R-3, which are significant distances from the discharge. In addition, the monitoring is sporadic with only monthly monitoring during the winter months when the RWCF has the propensity to discharge high levels of ammonia. Frequent monitoring closer to the outfall is needed to determine the actual dilution. Stockton Comment No. I.D (1)• UVM data are reliable for predicting critical low flow. RWQCB Response: The Regional Board does not agree with the findings of the Jones & Stokes flow model. Detailed comments are provided in Attachment A. Stockton Comment No. I.D (2)• Box Model accurately predicts dilution. RWQCB Response: The report describing the Box Model is void of information regarding the calibration and validation of the model. Therefore, it is premature to state that the Box Model accurately predicts dilution. The Regional Board has many concerns regarding the Jones & Stokes Box Model. Detailed comments are provided in Attachment B. Stockton Comment No. LD (3) and (4)• Acute and chronic dilution are available. Also, the UVM gauge cannot be used on a day-to-day basis to estimate actual dilution flow. RWQCB Response: During above average wet years the chronic dilution was estimated to be as low as 5:1 (Fact Sheet @ 13). This is based on net SJR flows and the actual RWCF flow. Using the wet year 7-day average SJR flow (102 cfs) and the maximum consecutive 4-day average flow from the RWCF (95.5 cfs), as called for by the SIP, the estimated dilution credit is only 1.07:1. This suggests that during a wet year there could be minimal dilution. As discussed above in greater detail in the response to Stockton Comment No. LC (1), the 7-day average SJR flow at Stockton could be much less during a critically dry year. Therefore, it is reasonable to assume that during a critically dry year, there would be minimal to no dilution. The City proposes that there is chronic dilution based on the Hall &Associates chronic dilution model and the Jones & Stokes Box Model. Detailed comments regarding the models are contained in Attachments B and C. With regards to the acute dilution, we agree that the UVM data cannot be used on a day-to-day basis to estimate dilution. Dilution credits for acute dilution have not been granted, in part, because there is little or no chronic dilution based on minimal weekly and monthly average flows,. Since the acute averaging period is much shorter than the chronic averaging period, there cannot be any acute dilution either.