ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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Near-Field Analysis The near-field water quality impacts assessment evaluates the effects of increasing WQCF discharge, from the existing permitted 9.87 mgd to a proposed 27 mgd effluent flow rate(LWA 2007). Because the construction of WQCF improvements is scheduled to occur in two phases,with the completion of the first phase resulting in a discharge capacity of 17.5 mgd,water quality impacts were estimated for this incremental discharge stage.Near- field effects on San Joaquin River water quality will occur between the point of discharge and WQCF monitoring location R-3 (approximately 1-mile downstream of the City's discharge)where advanced treatment effluent and ambient river water is well-mixed as determined by recent hydrodynamic modeling conducted by the City(RMA 2006).Near-field water quality impacts were estimated using the following parameters,which characterize WQCF effluent and San Joaquin River water quality: ► projected WQCF effluent quality after Phase III expansion; ► ambient river concentrations calculated from dry/below normal water years,where possible; ► current permitted and proposed future WQCF effluent flowrates; and ► average late summer/early fall San Joaquin River flows observed during historic critical and dry water years. The estimated near-field water quality impacts were calculated using the following mass balance equation: _ ((Cupstream)(Qupstream))+((Ceff)(Qeff X 1'55)) Cdownstream Qupstream +(Qeff X 1'55) Where Cdownstream= San Joaquin River concentration, downstream of discharge at R-3 Cupstream=San Joaquin River concentration,upstream of discharge at R-1 Ceff=WQCF effluent concentration Qupstream= San Joaquin River flow(cfs),upstream of discharge at Vernalis Qeff=WQCF effluent flow(mgd) Projected average effluent concentrations resulting from proposed Phase III improvements to the WQCF (improvements that would result in a capacity of 9.87 mgd)were used to estimate future impacts of the WQCF discharge on San Joaquin River water quality(Table 4.9-12). In other words,the modeled effluent constituent concentrations mixed with San Joaquin River that would occur with buildout of the WQCF (27 mgd)were compared to projected effluent concentrations(mixed with the San Joaquin River)that would occur at a discharge rate of 9.87 mgd because this is the current permitted discharge rate authorized by the RWQCB. Where possible, median(50th percentile)receiving water concentrations were calculated using data collected at WQCF Station R-1 (see Exhibit 4.9-1). The median concentration is used for the receiving water because the analysis is strictly based on evaluating the change in receiving water quality. Ambient R-1 concentrations serve as a basis for comparing the magnitude of future change in receiving water quality due to the project. The analysis and results for each constituent specified in Table 4.9-11 selected for near-field analysis are summarized in Appendix D. Constituents were evaluated under both critical and dry/below normal flow conditions corresponding to critical and dry/below normal water years,respectively. For the critical water year, a San Joaquin River flowrate of 600 cubic feet per second(cfs)was chosen as representative of extremely low flow conditions based on historic monthly average flows calculated for the San Joaquin River at Vernalis during fall 1991 and 1992;both years classified as critical by the DWR hydrologic classification scheme (LWA 2006a). Similarly, a flow of 1,250 cfs was chosen as representative of dry/below normal water year flow conditions based on San Joaquin River at Vernalis flows measured during fall 2002; a year classified as a dry water year(LWA 2006a). Due to agreements currently in place to provide water for migration of anadromous fish species in the Delta(1995 Bay-Delta Plan; SWRCB, 1995; and the Revised Water Right Decision 1641; SWRCB, 2000), as well as future San Joaquin River flow objectives that may be set forth in a future Bay-Delta Plan, future San Joaquin River flows should be similar to or greater than those observed during historic dry water years during future rainfall-limited years. Evaluating changes in San Joaquin River water quality due to an increase in WQCF effluent discharge under critical water DER EDAW City of Manteca 4.9-27 Hydrology and Water Quality