ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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the current permitted rate(9.87 mgd)to the proposed rate (27 mgd)would be minor. The projected effluent concentration under the proposed project would be 7 mg/1,which is below the current 10 mg/l adopted effluent limitation for BOD as described in the City's existing NPDES permit. Because the project is projected to reduce BOD effluent concentrations compared to the existing WQCF discharge concentrations and projected concentrations would be below the City's existing adopted NPDES effluent limitation,which has been established by the RWQCB to be protective of water quality,this impact is considered less than significant. IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Total Suspended Solids 4.9-5 (TSS)Concentrations in Receiving Waters. The proposed WQCF upgrades would be designed and operated to comply with the NPDES permit effluent limits for TSS, which are, based on the lowest criteria determined to be protective of beneficial uses. Projected TSS concentration in the effluent would be less than TSS concentration in the San Joaquin River,the WQCF effluent would have a diluting effect and would result in a slight decrease in the TSS concentration in the San Joaquin River. Thus, the project-related effluent discharges to the San Joaquin River would not adversely affect beneficial uses. This impact is considered less than significant. There is no numerical water quality objective for TSS in the San Joaquin River. The Basin Plan narrative objective for suspended material is as follows: "Waters shall not contain suspended material in concentrations that cause or adversely affect beneficial uses."Manteca WQCF NPDES self-monitoring data from the San Joaquin River were analyzed from monitoring location R-1 just upstream of the WQCF discharge(see Exhibit 4.9-1). Data corresponding to a wet water year were used to calculate the effect of WQCF effluent TSS 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 use of wet water year TSS data (dry/below normal water year data were not available)may have imparted a slightly high bias to the calculation of downstream, estimated TSS concentrations in the San Joaquin River because of increased TSS in runoff to the San Joaquin River. As such,the projections for future TSS concentrations measured during dry/below normal water years in the San Joaquin River might be slightly lower,relative concentrations than presently estimated. Projected ambient TSS effluent concentrations at a 9.87 mgd discharge rate would be approximately 25.3 mg/l under critical(600 cfs) flow conditions. The proposed project would implement tertiary filtration,which would result in the more efficient removal of TSS from the wastewater stream. TSS concentrations discharged with full buildout of the WQCF (27 mgd) are projected to be 10 mg/l at the point of discharge. Because the projected WQCF effluent TSS concentration(10 mg/1)is lower than the ambient TSS concentration in the San Joaquin River(25.7 mg/l Station R-1 50th percentile value), increasing WQCF effluent discharge will have a diluting effect on receiving water concentrations. Existing TSS mass loading to the river is 83,114 pounds per day (lbs/day)under critical (600 cfs)conditions and 173,154 lbs/day under dry/below normal(1,250 cfs)conditions. The proposed project is projected to result in in-stream TSS loads of 24.7 lbs/day and 25.2 lbs/day under critical and dry/below normal conditions,respectively,which is less than 1 percent of existing TSS loads to the river. Because projected TSS loadings from the effluent would be less than TSS concentrations in the San Joaquin River,the WQCF effluent would have a diluting effect and would result in a slight decrease in the TSS concentration in the San Joaquin River. The project would assist in reducing the concentration of suspended material in the river and complying with the Basin Plan narrative water quality objective. Furthermore,NPDES permit effluent limitations will be sufficiently stringent to provide the intended level of protection to the beneficial uses of the San Joaquin River. Therefore this impact is considered to be less than significant. EDAW DER Hydrology and Water Quality 4.9-34 City of Manteca