ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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These incremental increases in river ammonia concentrations and mass loadings observed when evaluating the June through September(associated with the EPA chronic ammonia criterion)and October through May (associated with the EPA acute ammonia criterion)time periods are shown in Exhibits 4.9-3 and 4.9-4, respectively. Because the consumptive oxygen demands of ammonia nitrification are most strongly expressed downstream of the City's wastewater outfall,the impact of WQCF effluent ammonia levels on dissolved oxygen(DO)levels in the San Joaquin River are evaluated in the far-field analysis in a discussion of far-field DO impacts(see Impact 4.9-19 below). The near-field analysis shows that projected,median ammonia concentrations in the San Joaquin River would range from 0.17 to 0.22 mg/1 for dry/below normal and critical water years,respectively,which is substantially below the more stringent chronic criterion of 0.62 mg/l for the June through September time period. Further, ammonia concentrations would be substantially lower than the acute criterion of 5.62 mg/1 during the October through May time period with concentrations ranging from 0.13 to 0.17 mg/l for dry/below normal and critical water years,respectively. Because the project would result in ammonia concentrations that are below adopted water quality criterion,the project's near-field ammonia impacts would be less than significant. IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Arsenic(Dissolved) 4.9-9 Concentrations in Receiving Waters. The project would be designed and operated to comply with the NPDES permit effluent limits for dissolved arsenic and projected dissolved arsenic concentrations would be substantially below adopted water quality objectives. Therefore, the project's near-field arsenic concentrations 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 ammonia 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 average total arsenic concentration after the WQCF buildout(27 mgd)is projected to be 8 ug/1. Although the projected effluent concentration is presented as total Arsenic, 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. The RWQCB is required to assign an effluent limit to the total fraction of a metal; limits are not assigned to the dissolved fraction. The project's effluent quality for metals is the projected concentration of the total fraction of a metal. However, tertiary treated effluent has all of its suspended particles removed by the very fine tertiary filtration process, leaving only dissolved constituents in the effluent, and the Basin Plan water quality objective for arsenic specifies that compliance is determined through comparison of the objective with dissolved arsenic concentrations. Because the Basin Plan water quality objective for arsenic specifies that compliance is determined through comparison of the objective with dissolved arsenic concentrations, it is appropriate to state that all arsenic in WQCF tertiary effluent will be considered to be in the dissolved form. The incremental change in near-field(measured at R-3, see Exhibit 4.9-1)dissolved arsenic 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 0.13 ug/1 to 0.25 ug/1 during dry/below normal and critical water years,respectively. Total downstream(measured at R-3)dissolved arsenic concentrations would range from 2.12 to 2.32 ug/1 during dry/below normal and critical water years,respectively(LWA 2007). The Basin Plan objective for dissolved arsenic in the Sacramento–San Joaquin Delta is 10 ug/L, expressed as the dissolved fraction(CVRWQCB 2006b). Projected,median dissolved arsenic concentrations at full buildout(27 mgd) are well below the Basin Plan objective. Because the project would result in dissolved arsenic concentrations that are below adopted water quality objectives,the project's near-field arsenic impacts would be less than significant. DER EDAW City of Manteca 4.9-39 Hydrology and Water Quality