ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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respectively and would not be substantially different from cyanide concentrations projected for current permitted discharge rate of 9.87 mgd(i.e., 0.29 to 0.30 gg/l dry/below normal and critical water years,respectively) (LWA 2007). Projected,median total cyanide concentrations in the San Joaquin River are well below the CTR freshwater chronic objective of 5.2 pg/l. Therefore,the project's near-field cyanide impacts would be less than significant. IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Iron(Dissolved) 4.9.12 Concentrations in Receiving Waters.Dissolved iron mass loading to the San Joaquin River is projected to be slight relative to existing in-stream loads. Projected dissolved iron concentrations in the San Joaquin River would be substantially below the Basin Plan objective of 300,ug/I and mass loadings would be minor compared to measured upstream dissolved iron concentration. The project would be designed and operated to comply with the NPDES permit effluent limits for dissolved iron would result in downstream dissolved iron concentrations that are substantially below adopted water quality objectives. Therefore, the project's near-field iron 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)corresponding to a wet water year were used to calculate an estimated impact of WQCF effluent dissolved iron 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 and 27 mgd. Ideally, a dissolved iron data set corresponding to dry/below normal water years would be used for the near-field analysis,but was not available. The proposed project is projected to produce treated effluent with an average total iron concentration of 50 gg/l at the point of discharge. For the purpose of the current analysis,and to be conservative,all iron present in WQCF treated effluent is assumed to be in the dissolved form for the same reasons as described above for arsenic. The Basin Plan objective for dissolved iron as it applies to the San Joaquin River is 300 gg/l expressed as the dissolved fraction. The incremental change in near-field(measured at R-3, see Exhibit 4.9-1)dissolved iron 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-0.03 to 0.6 gg/l during dry/below normal and critical water years,respectively. Total dissolved iron concentrations would range from 61.1 µg/1 to 60.6µg/1 during dry/below normal and critical water years,respectively and would not be substantially different from cyanide concentrations projected for current permitted discharge rate of 9.87 mgd (i.e., 61.4 to 61.2 pg/1 dry/below normal and critical water years,respectively) (LWA 2007). Projected,median total cyanide concentrations in the San Joaquin River are well below the Basin Plan objective of 300µg/1. Therefore,the project's near-field dissolved iron impacts would be less than significant. IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Manganese(Dissolved) 4.9-13 Concentrations in Receiving Waters. Dissolved manganese mass loading to the San Joaquin River is projected to be slight relative to existing in-stream loads. Additionally, projected dissolved iron concentrations in the San Joaquin River are substantially below adopted Basin Plan objective. The project would be designed and operated to comply with the NPDES permit effluent limits for dissolved manganese would result in downstream dissolved iron concentrations that are substantially below adopted water quality objectives. Therefore, the project's near-field manganese 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 a wet water year were used to calculate an estimated impact of WQCF effluent dissolved manganese 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 and 27 mgd. Ideally, a dissolved manganese data set corresponding to dry/below normal water years would be used for the near-field analysis,but was not available. The proposed project is projected to produce treated effluent with an average total manganese concentration of 10 gg/11 at the point of discharge. For the purpose of the current DER EDAW City of Manteca 4.9-41 Hydrology and Water Quality