ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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Near and Far Field Dilution Analysis of Manteca Discharge October 2006 Far Field Dilution Analysis Far field dilution analysis simulations were performed to investigate the incremental impact of Manteca effluent at various key locations in the Delta. Two one-year periods were simulated: October 1991 through September 1992, a critically dry year, and January 2002 through December 2002, a normal dry year. For each simulation, a two month spin up period was simulated to develop a reasonable initial condition tracer concentration for the year long simulation. Model Description RMA's existing two-dimensional depth averaged model of the Sacramento-San Joaquin Delta was used for the Manteca far field dilution analysis simulations. This model has been developed, calibrated and refined during numerous modeling projects in the Bay— Delta system (RMA, 2005, 2004, 2002a, 2000, 2000a, 1998, 1997). The RMA staff has successfully applied these models in previous studies to evaluate the incremental water quality responses of treated wastewater discharges to San Francisco Bay and the Delta (RMA, 2004, 2002, 2002a, 2001, 2001a, 2000, 1998, 1997, 1997a, 1996). The model extends from Martinez at the downstream end,up the Sacramento River to the American River confluence, and up the San Joaquin River to Vernalis. The finite element mesh is shown in Figure 34. Boundary conditions for the model include historical 15-minute tide at Martinez, historical daily inflows for the Sacramento River, San Joaquin River, Cosumnes River, Mokelumne River, Yolo Bypass and Miscellaneous east side flows (includes Calaveras River, French Camp Slough and others), historical daily average exports at the Delta Mendota Canal, Contra Costa at Old River(2002 only), Contra Costa at Rock Slough and North Bay Aqueduct, historical hourly exports at Clifton Court, and estimated historical monthly average Delta Islands consumptive use inflows and withdrawals. Sacramento and San Joaquin River flows for the two simulation periods are shown in Figure 35 Figure 36. Historical operations of operable gates and temporary barriers were included in the model as well. For each of the simulation years, average daily treatment plant discharge rates of 9.87 MGD, 17.5 MGD and 27 MGD were simulated. During the majority of the 1991-1992 simulation and much of the 2002 simulation, timed treatment plant discharge is required. 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. • If the approaching flood(rising) tide encompasses a slack tide or flow reversal (San Joaquin River flow less than 400 cfs in the downstream direction), then the discharge stops at least 30 minutes before the minimum low tide. 41