ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Copper(Dissolved) 4.9-10 Concentrations in Receiving Waters. The project would be designed and operated to comply with the NPDES permit effluent limits for dissolved copper and would result in downstream dissolved copper concentrations that are substantially below adopted water quality objectives. Therefore, the project's near- field copper impacts would be less than significant. Manteca WQCF NPDES self-monitoring data from the San Joaquin River at monitoring location R-1 (just upstream of the WQCF discharge) for dry/below normal water years were used to calculate an estimated impact of WQCF effluent dissolved Copper in the San Joaquin River under critical(600 cfs)and dry/below normal (1,250 cfs)river flows at the existing permitted discharge of 9.87 mgd and at proposed discharges of 17.5 mgd and 27 mgd. For the purpose of the near-field analysis, and to be conservative, all arsenic present in WQCF treated effluent is assumed to be in the dissolved form for the same reasons as described above for arsenic. The CTR freshwater chronic objective for dissolved copper is hardness dependent. It is 14.24µg/l when using a river hardness value of 172 mgl measured during the low fall flows of October 2002 in the San Joaquin River at monitoring location R-1. To be conservative the more stringent Basin Plan dissolved copper objective of 10 ug/ (CVRWQCB 2006a)was used for assessing the impact of increased WQCF discharge on dissolved copper levels in the San Joaquin River. The incremental change in near-field(measured at R-3, see Exhibit 4.9-1) dissolved copper concentrations in the San Joaquin River resulting from an increase in WQCF effluent discharged from the current permitted rate(9.87 mgd)to the proposed rate (27 mgd) is projected to range from 0.10µg/l to 0.20µg/l during dry/below normal and critical water years respectively(LWA 2007). Total downstream(measured at R-3) dissolved copper concentrations would range from 2.07 to 2.24µg/1 during dry/below normal and critical water years,respectively(LWA 2007). A significant level of copper removal would be accomplished through implementation of filtration at the WQCF. Historical removal efficiencies at similar advanced treatment plants are on the order of 25 percent(Nolte 2007). Assuming 25 percent removal through filtration,mean and maximum effluent concentrations at the point of discharge would be 7µg/l and 10µg/l are projected for copper. When mixed with the San Joaquin River median dissolved copper concentrations in the San Joaquin River would be substantially below the Basin Plan objective of 10µg/l(i.e., 2.07 to 2.24µg/1 measured at R-3). Therefore,the project's near-field copper impacts would be less than significant. IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Cyanide(Total) 4.9-11 Concentrations in Receiving Waters. The project would be designed and operated to comply with the NPDES permit effluent limits for total cyanide and would result in downstream cyanide concentrations that are substantially below adopted water quality objectives. Therefore, the project's near-field cyanide impacts would be less than significant. Manteca WQCF NPDES self-monitoring data from the San Joaquin River at monitoring location R-1 (just upstream of the WQCF discharge) for dry/below normal water years were used to calculate an estimated impact of WQCF effluent total cyanide in the San Joaquin River under critical(600 cfs) and dry/below normal(1,250 cfs)river flows at the existing permitted discharge of 9.87 mgd and at proposed discharges of 17.5 mgd and 27 mgd. The proposed ultraviolet disinfection process is projected to produce treated effluent at full buildout(27 mgd)having an average total cyanide concentration of 1 µg/1 at the point of discharge (LWA 2007). The CTR freshwater chronic objective for total cyanide as it applies to the San Joaquin River is 5.2µg/1. The incremental change in near-field(measured at R-3, see Exhibit 4.9-1)cyanide concentrations in the San Joaquin River resulting from an increase in WQCF effluent discharged from the current permitted rate(9.87 mgd)to the proposed rate(27 mgd) is projected to range from 0.01 to 0.03µg/1 during dry/below normal and critical water years,respectively,which is below the magnitude of change that could be reliably measured in the field. Total cyanide concentrations would range from 0.30 gg/1 to 0.33 gg/1 during dry/below normal and critical water years, EDAW DER Hydrology and Water Quality 4.9-40 City of Manteca