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WASTE DISCHARGE REQUIREMENTS -3- CITY OF STOCKTON REGIONAL WATER QUALITY CONTROL PLANT SAN JOAQUIN COUNTY 10. The SWRCB, on 16 May 1974, adopted Resolution No. 74-43 titled "Water Quality Control Policy for the Enclosed Bays and Estuaries of California". These requirements are consistent with the Policy. 11. The SWRCB Water Quality Control Plan for Control of Temperatures in Coastal and Interstate Waters and Enclosed Bays and Estuaries of California (the Thermal Plan) is applicable to this discharge. For purposes of the Thermal Plan, the Discharger is considered to be an existing Discharger of Elevated Temperature Waste. 12. The dissolved oxygen standards applicable to this reach of the San Joaquin River are as follows: a. The Basin Plan prescribes a minimum dissolved oxygen concentration in the San Joaquin River of 5.0 mg/I. This standard is applicable throughout the year. b. The Salinity Plan prescribes a minimum dissolved oxygen concentration of 6.0 mg/l in the San Joaquin River inside of the reach from Turner Cut to Stockton during the period 1 September through 30 November. This higher dissolved oxygen concentration was imposed to enhance aquatic conditions during critical migration periods for salmon, and supersedes the 5.0 mg/l standard prescribed by the Regional Board where it is applicable. 13. The San Joaquin River in the vicinity of the Stockton Waste Water Treatment Plant discharge seasonally fails to meet the 5.0 and 6.0 mg/l dissolved oxygen standards. In 1978 the Board adopted Waste Discharge Requirements Order No. 78-105 which prescribed tighter effluent BOD and Total Suspended Solids effluent limitations for the Discharger with the intent of reducing or eliminating the low dissolved oxygen conditions in the San Joaquin River in the vicinity of Stockton. The City constructed the necessary additional treatment facilities and has complied with the tighter effluent limitations. Despite the City's efforts, the low dissolved oxygen conditions persist. 14. The City recently completed a river model to assess the impact of the discharge on the receiving water. Water quality parameters considered include TDS, BOD, ammonia, nitrate, and dissolved oxygen. The model provided the following information: (1) Initial dilution, not taking into account recirculation of effluent, is 10:1; (2) Long-term average dilution, taking into account recirculation of effluent, is 4:1; (3) Critical water quality conditions occur in the fall and spring, due to a high mass loading of BOD and ammonia; (4) The current discharge contributes up to 43% of the oxygen demand to the river during critical low river dissolved oxygen periods, under current Delta flow conditions, as simulated in the model over the calibration period; (5) Addition of activated sludge/nitrification units to provide a CBOD (Carbonaceous Biochemical Oxygen Demand) of 5 mg/l and ammonia of 0.5 mg/l will significantly improve water quality by raising the dissolved oxygen level during critical periods from 2.5 to 3.0 mg/l, and increasing the dissolved oxygen in the summer by an additional 1.0