ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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Near and Far Field Dilution Analysis of Manteca Discharge October 2006 Model Simulations Near field dilution analysis simulations were performed for three San Joaquin River flow conditions, three treatment plant discharge rates and two discharge methods. The three river flow conditions corresponding to SIP Table 3 are: Acute—"1Q10 river flow," or 564 cfs; Chronic—"7Q 10 river flow," or 615 cfs; and Human health—Harmonic mean river flow, or 2015 cfs. For each of the above river flows, average daily treatment plant discharge rates of 9.87 MGD, 17.5 MGD and 27 MGD were simulated. Each of these nine flow-discharge combinations was simulated with a side bank discharge and with a diffuser. For the side bank discharge scenarios, the treatment plant discharge was input to two elements as shown in Figure 7. For the diffuser scenarios, the treatment plant discharge was distributed among 16 elements in a 140' long diffuser as shown in Figure 8. Flows into each diffuser element varied based on channel depth, with greater flows into the elements in the deeper area of the channel. This method resulted in a relatively even effluent concentration distribution across the width of the diffuser. The treatment plant discharge was tagged with a tracer concentration of 100 so that effluent dilution could be computed. The upstream boundary concentration was set to zero. Because the effluent does reach the downstream boundary at some low concentration and, therefore, returns on the incoming tide, for the acute and chronic simulations the downstream boundary concentration is set to a rough average of the concentration that occurs on the outgoing tide. This concentration is applied on the incoming tide only. Sensitivity tests were done and this number has no effect on the results within 10,000 feet of the discharge. Further downstream, this has the conservative effect of slightly reducing dilution near the downstream boundary. Overall, the effect is minimal. For the human health simulations, only one brief flow reversal occurs, so the downstream concentration boundary condition is not an issue. Acute Simulations For the Acute simulations, the San Joaquin River flow was set constant at 564 cfs. The downstream tide was set using observed stage data at the CDEC station at Old River at Head for July 1992, a time when San Joaquin River flows were similar to the acute river flow conditions. The Old River at Head stage was smoothed using a 3 point centered moving average. Under the low flow conditions of the Acute simulation, timed treatment plant discharge is required. The discharge schedule defined in the thermal exemption request is used to determine allowable discharge. The following points summarize the method used to determine the times to start and stop the discharge. When, under an ebb (falling) tide, San Joaquin River flow at Mossdale Bridge exceeds 800 cfs in the downstream direction, discharge from the outfall restarts. 10