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Attachment C Comments on Evaluation of Chronic Dilution in the San Joaquin River Under Drought Flow 11 April 2001 The Hall &Associates report,Evaluation of Chronic Dilution in the San Joaquin River Under Drought Floio, dated 11 April 2001 (Report)was prepared for the City of Stockton. The Report describes a simple modelused ,)-estimate the_i inimum monthly flow for the Regional Wastewater Control Facility (RWCF) during drought flow conditions. Using RWCF effluent ammonia data and receiving water ammonia data, the Report attempts to estimate the minimum flow during drought conditions,which presumably occurred in the early 1990's. The Report uses a mathematical model to evaluate the flow based on the dilution of ammonia in the San Joaquin River. Specific comments regarding the Report are listed below: 1. Due to multiple dosing effects, caused by tidal reversals, it is not accurate to estimate river flows using ammonia data. The expected hydraulic dilution is not consistent with actual dilution. Therefore, the standard mixing equation (Equation [1] on page 2) is not applicable to the RWCF discharge. This is demonstrated in Table 8-1 of the tentative permit. Since the validity of the standard mixing equation is the basis for this model, the accuracy of the model is questionable. 2. The Report recognizes the difficulty in determining dilution using mathematical equations as stated on page 10, "it is clear...that ammonia concentration in the river is in a continuous state of flux, and many factors must be considered in evaluating dilution using equations." The Report discusses how the model was optimized to minimize error. However, there is no mention of how much error can be expected in the results. 3. Using Ultrasonic Velocity Meter(UVM) data during "wet years"has shown a 30- day low flow of 233 cfs from 18 December 1999 to 16 January 2000. Based on the permitted flow of 55 mgd (-85 cfs), the 30-day dilution is 2.7:1, and based on actual RWCF flow during the 30-day period, the dilution was 6.4:1. Based on the approach presented in this report, the"drought" 30-day dilution of 2.2:1 seems high in comparison. 4. Receiving water temperatures in February and March were not always less than 10°C. Therefore, ammonia decay is probably leading to an overestimation of the dilution ratio. 5. Sampling locations R2 and R3 are a significant distance from discharge point. Sampling points nearer the discharge point will yield lower dilution credits, since higher ammonia concentrations are expected. This is shown in the difference in the calculated SJR flows using r 73, and R4 data (see Table 5 of the Report).