ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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Near and Far Field Dilution Analysis of Manteca Discharge October 2006 In Figure 17, a plot of plume centerline dilution profiles for the 9.87 MGD cases with the side bank outfall and with the diffuser shows that the profiles only differ significantly within about 5,000 feet of the discharge point. Plan view color contour plots of minimum effluent dilution are shown in Figure 18 through Figure 20 for each of the discharge rates. The lowest dilution values at 450' down the plume centerline from the point of discharge are summarized in Table 1. For the 9.87 MGD simulation, the dilution credit is 2.4:1, which is slightly less than the 3:1 dilution credit in the permit. This difference is due to the expanded model and refined critical conditions. Human Health Simulation Results For the Human Health simulations, the San Joaquin River flow was set constant at 2,015 cfs. The downstream tide was set using observed stage data at the CDEC station at Old River at Head for November 20 through December 5, 2001, a time when San Joaquin River flows averaged around 2,015 cfs. The Old River at Head stage was smoothed using a 3 point centered moving average. Under the higher flow conditions of the Human Health simulation, constant treatment plant discharge is allowed. Tidally averaged effluent dilution was computed for each scenario and the times of lowest dilution were selected for plotting. Figure 21 shows the lowest tidally averaged dilution profiles along the centerline of the effluent plume for the three side bank discharge scenarios. At this time, the lowest dilution occurs at the outfall where it is nearly pure effluent for all flow scenarios. At 10,000 ft downstream of the discharge, dilution is approximately 134:1 for the 9.87 MGD simulation, 75:1 for the 17.5 MGD simulation, and 47:1 for the 27 MGD. Figure 22 shows the lowest hourly average dilution profiles along the centerline of the effluent plume for the three diffuser scenarios (the lowest dilution for the diffuser case does not occur at the same time as for the side bank case). At this time, the lowest dilution occurs at the diffuser. For the 9.87 MGD simulation, the lowest dilution is approximately 69:1, for the 17.5 MGD simulation 3 8:1 and for the 27 MGD simulation 25:1. At 10,000 ft downstream of the discharge, dilution is approximately 126:1 for the 9.87 MGD simulation, 71:1 for the 17.5 MGD simulation, and 46:1 for the 27 MGD, similar to the side bank scenario. A plot of plume centerline dilution profiles for the 9.87 MGD cases with the side bank outfall and with the diffuser is shown in Figure 23 at the time of lowest dilution for the side bank discharge case. This plot shows that the dilutions differ for the two cases down to the location where the model transitions to 1-D (approximately 5,400 ft), at which point the concentrations become fully mixed across the channel cross-section. Plan view color contour plots of minimum effluent dilution are shown in Figure 24 through Figure 26 for each of the discharge rates. The lowest dilution values at 5280' down the plume centerline from the point of discharge are summarized in Table 1. 13