ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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analysis, and to be conservative, all manganese present in WQCF treated effluent is assumed to be in the dissolved form for the same reasons as described above for arsenic. Dissolved manganese mass loading to the San Joaquin River is projected to be slight relative to existing in-stream loads and would only increase by 1.5 lbs/day with buildout of the WQCF. The highest estimated downstream monitoring location R-3 river concentration of 14.2 gg/l(under existing and proposed buildout scenarios)would be less than the current ambient concentration of 14.3 gg/l measured at monitoring location R-1. Because the projected effluent manganese concentration at the point of discharge would be 10 gg/1,which is less than ambient river concentrations,the project would have a diluting effect and downstream San Joaquin River concentrations are anticipated to decrease with implementation of the project. Further,projected manganese concentrations for the San Joaquin River would be substantially below the adopted Basin Plan objective of 50 gg/1. Therefore,the project's near-field manganese impacts would be less than significant. IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Methylene Blue Active 4.9.14 Substances(MBAS)Concentrations in Receiving Waters. The project would be designed and operated to comply with the NPDES permit effluent limits for MBAS, which are, in turn, based on the lowest criteria determined to be protective of beneficial uses. The proposed project is projected to produce treated effluent having an average total manganese concentration of 160,ug/I, which is well below the adopted Basin Plan objective. Therefore, the project's near-field MBAS 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 was used to calculate an estimated impact of WQCF effluent Methylene Blue Active Substances(MBAS) 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. Because a limited MBAS receiving water data set featuring all nondetected monitoring location R-1 data was available for the near-field analysis,an estimated MBAS concentration at monitoring location R-1 was used. The ambient MBAS concentration employed in the near-field analysis was estimated using the median method detection limit(MDL)associated with the nondetected data(LWA 2006a). The Basin Plan adopted water quality objective for MBAS is 500 pg/l. The median MDL for MBAS from the City's NPDES self-monitoring data is 20 gg/1. The proposed project is projected to produce treated effluent having an average total manganese concentration of 160 pg/l,which is well below the adopted Basin Plan objective. Therefore,the project's near-field MBAS impacts would be less than significant. IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Nitrate (as Nitrogen) 4.9.15 Concentrations in Receiving Waters. The project would be designed and operated to comply with the NPDES permit effluent limits for nitrate, which are, in turn, based on the lowest criteria determined to be protective of beneficial uses. Projected nitrate concentrations would be well below the adopted Basin Plan objective of 10 mg/I. Therefore, the project's near-field nitrate 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 nitrate(as nitrogen)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 project is projected to produce treated effluent having an average nitrate concentration of 5 mg/l. The Basin Plan utilizes the adopted Title 22 Primary MCL water quality criterion for nitrate plus nitrite (sum as nitrogen)of 10 mg/1. Projected average total concentrations of nitrate in WQCF effluent at the point of discharge would be 5 mg/1,which is substantially below the adopted Basin Plan objective. Projected median nitrate concentrations in the San Joaquin River downstream of the WQCF outfall(measured at R-3,Exhibit 4.9-1)would range from 2.63 to 2.71 mg/l during dry/below normal and critical water years,respectively. Therefore,the project's near-field nitrate impacts would be less than significant. EDAW DER Hydrology and Water Quality 4.9-42 City of Manteca