ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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discharged into San Joaquin River would not be substantial in relation to the total flow rate of the San Joaquin River. Therefore, the project would not cause flooding downstream of the WQCF along the San Joaquin River. This is considered a less-than-significant impact. Implementation of the project would result in the discharge of up to an additional 17.13 million gallons of treated effluent into the San Joaquin River per day. The amount of water flowing in the San Joaquin River near the project site is estimated to be 1,899,000 acre-feet per year. The increased amount of treated effluent that would be discharged into the San Joaquin River from implementation of the project would account for approximately 1.0% of the total water flowing in San Joaquin River(1.899 million acre-feet per year= 1,695 million gallons per day; 17.13 mgd/ 1,695 mgd= 1.0%). Because the increased flows in the San Joaquin River from the treated effluent discharged from the project are minor in comparison to the overall volume of water in the river, lands downstream of the project site are not expected to be exposed to increased flooding risk. Therefore,the project would result in a less-than-significant impact related to off-site flooding risks. Surface Water Quality Impacts The results of the near-field and far-field water quality analyses described above are presented in this section. For a number of parameters(total suspended solids (TSS),turbidity,coliform organisms, etc.)the impact of the discharge of effluent from the Manteca WQCF into the San Joaquin River(San Joaquin River)would result in a localized improvement in water quality,because their low levels in the discharge compared to higher levels in the San Joaquin River would have a localized diluting effect within the mixing zone. For example,the 50th percentile TSS level in the San Joaquin River at sampling station R-1 (just upstream of the WQCF discharge)is 26.7 mg/1. The existing effluent level is 10 mg/1 and the projected effluent concentration level after Phase III completion would be 10 mg/l. This subsection provides impact assessments for constituents of concern for which quantitative water quality modeling was performed. For a detailed description of the water quality modeling approach, see Appendix D. Near-field, zone-of-initial-mixing water quality concerns center upon: (1)the potential for discharge-driven exceedances of applicable water quality standards; and(2)plumes of elevated contaminant concentrations and temperature that could adversely impact fish and/or other aquatic biota and thus certain aquatic life uses.No municipal/domestic, industrial, or irrigation diversions occur from the San Joaquin River within the zone of initial mixing;therefore,potential project-specific water quality impacts to these beneficial uses were based primarily on the far-field(i.e., fully mixed)modeling results. Hence,the potential for project-specific water quality impacts to recreational uses were assessed using the far-field(i.e., fully mixed)modeling results. Finally,project-specific changes to near-field and far-field water quality would have no effect on the navigation beneficial use. Near-Field Impacts IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharge on Biochemical Oxygen 4.9-4 Demand(BOD)Concentrations in Receiving Waters. The project is projected to reduce effluent BOD 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. Therefore, this impact is considered less than significant. The consumptive oxygen demand of BOD reduces ambient dissolved oxygen(DO) levels in the river,and therefore a connection exists between BOD and DO concentrations. The influence of the WQCF BOD input is most strongly expressed as an oxygen demand downstream of the City's wastewater outfall. Therefore the impact of WQCF effluent on DO levels in the San Joaquin River is addressed in the Far-Field Impacts Analysis,which incorporates the near-field BOD analysis. See Impact 4.9-19 below. There is no adopted water quality objective for BOD in the San Joaquin River. The incremental change in BOD concentration and mass loading in the San Joaquin River due to an increase in WQCF effluent discharged from DER EDAW City of Manteca 4.9-33 Hydrology and Water Quality