ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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(27 mgd)is approximately 5.1 to 10.1 umbos/cm during the months of April through August(agricultural season) and 0.8 to 1.5 umhos/cm during the months of September through March(nonagricultural season) during critical and dry/below normal water years,respectively. Total EC measurements would range from 583 to 591.2 umhos/cm from April through August and 787.2 to 788.4 umhos/cm during September through March,which are substantially below established EC water quality objectives. Additionally,the City's potable water supply is expected to reduce EC concentrations compared to pre-August 2005 conditions as a result of blending surface water from the South County Water Supply Program(which has low EC values)with the City's groundwater. The blending of surface water with groundwater for the potable water supply has significantly decreased the EC measured in WQCF effluent when comparing pre- and post-August 2005 plant effluent measurements(LWA 2006b). Because the project would result in minor increases in EC in the San Joaquin River at full buildout(27 mgd) and effluent concentrations would be below established EC water quality objectives during agricultural and nonagricultural seasons,the project would not result in significant EC water quality impacts. the project's near- field EC impacts would be less than significant. IMPACT Hydrology and Water Quality—Effects of Proposed Project Discharges on Ammonia(as Nitrogen) 4.9-8 Concentrations in Receiving Waters. The near-field modeling analysis indicates that projected, median ammonia concentrations in the San Joaquin River would be below the more stringent U.S. EPA ambient water quality criteria for ammonia. Concentrations would be lower than the chronic criterion of 0.62 mg/I during the June through September time period and substantially lower than the acute criterion of 5.62 mg/1 during the October through May time period. Therefore, the project's near-field ammonia 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) corresponding to 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 proposed treatment processes are projected to produce treated effluent having an average ammonia concentration of 1.5 mg/1 at the outfall. Due to the seasonal variation of U.S. EPA ambient water quality criteria for ammonia,two data sets,June through September and an October through May,were used for the near-field analysis. The acute and chronic criteria for ammonia as promulgated by U.S. EPA 1999 Update of Ambient Water Quality Criteria for Ammonia are used by the RWQCB to interpret the Basin Plan's narrative toxicity objective. Using pH and temperature values previously used by the RWQCB in WDR Order No. R5-2004-0028, a chronic criterion of 0.62 mg/1 ammonia was calculated and applied to the San Joaquin River during June through September based on an ambient pH of 8.4 standard units and an ambient temperature of 26°C. Similarly, an acute criterion of 5.62 mg/l ammonia, nontemperature dependent,was calculated and applied to the San Joaquin River during October through May based on an ambient pH of 8.0 standard units. In an analysis of compliance with chronic and acute seasonal objectives (Exhibits 4.9-3 and 4.9-4),projected median ammonia(as nitrogen) concentrations in the San Joaquin River remain below the more stringent chronic ammonia objective(0.62 mg/1)calculated for the river on a year round basis. During critical and dry/below normal San Joaquin River flow conditions, an increase in WQCF effluent discharge would produce a moderate increase in ammonia levels in the San Joaquin River downstream of the discharge during June through September relative to its seasonal objective of 0.62 mg/1, and would produce a slight increase in ammonia levels in the river during October through May relative to its seasonal objective of 5.62 mg/l. The more distinct increase in ammonia levels in the river during the June through September time period is due to the higher ambient levels of ammonia measured in the San Joaquin River at monitoring location R-1 during this time period, in conjunction with the more stringent chronic ammonia objective in effect from June through September. DER EDAW City of Manteca 4.9-37 Hydrology and Water Quality