ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Nitrite(as Nitrogen) 4.9-16 Concentrations in Receiving Waters. The project would be designed and operated to comply with the NPDES permit effluent limits for nitrite, which are, in turn, based on the lowest criteria determined to be protective of beneficial uses. Projected nitrite concentrations at the point of discharge and downstream of the WQCF outfall would be equal to or below adopted Basin Plan objectives. Therefore, the project's near- field nitrite 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 nitrite(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 nitrite concentration of 1 mg/1. The Basin Plan uses the Title 22 Primary MCL criterion for nitrite(as nitrogen)of 1 mg/1 as the adopted water quality standard. Because the data set used in the near-field analysis included 83.3%nondetected data, an estimation of an ambient river concentration was required. The monitoring location R-1 ambient nitrite concentration employed in the near-field analysis was estimated by setting the ambient level at the method detection limit(MDL)of 0.002 mg/l associated with the City's nondetected data. The MDL is from the City's NPDES self-monitoring data. Projected median nitrite concentrations in the San Joaquin River at buildout of the WQCF(27 mgd)under critical (600 cfs)river flows would be 0.03 mg/1,which is well below the Basin Plan objective(1)mg/1. Further,the WQCF effluent at the point of discharge is projected to have an average total nitrite concentration of 1 mg/1, which is equal to the adopted Basin Plan objective. Therefore,the project's near-field nitrite impacts would be less than significant. IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Mercury(Total) 4.9-17 Concentrations in Receiving Waters. The incremental change in total mercury concentration in the San Joaquin River due to an increase in proposed Project effluent would be slight and below the magnitude of change that could be reliably measured in the field.Additionally,projected, median total mercury concentrations in the San Joaquin River are well below the CTR Human Health criterion of 0.051,ug/L. The proposed project WWTP upgrades would be designed and operated to comply with the NPDES permit effluent limits for mercury, which are, in turn, based on the lowest criteria determined to be protective of beneficial uses. This impact is considered 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 mercury 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. The project is projected to produce treated effluent with an average total mercury concentration of 0.01 ug/µg/11 at the point of discharge. Projected median total mercury concentrations in the San Joaquin River with buildout of the WQCF (27 mgd)under critical(600 cfs)river flows would be 0.006µg/1,which is well below the California Toxics Rule (CTR)Human Health criterion of 0.051 gg/1. Regarding methylmercury, it is anticipated based on an analysis of the data generated from the 2004-2005 Central Valley Clean Water Agency Mercury Study that future methylmercury levels in WQCF tertiary treated effluent should not exceed,on average,the 0.06 ng/1 methylmercury implementation goal set forth in the draft Sacramento–San Joaquin Delta Methylmercury TMDL (CVRWQCB,2006a). Mercury is a 303(d)-listed constituent for Delta waters,with resource extraction(i.e.,mining activity) cited as its key source. Sources of total mercury in the Delta include tributary inflows from upstream watersheds, atmospheric deposition,urban runoff, and municipal and industrial wastewater,with more than 96%of identified DER EDAW City of Manteca 4.9-43 Hydrology and Water Quality