SITE INFORMATION AND CORRESPONDENCE

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INFORMATION SHEET -2- STOCKTON COGENERATION COMPANY SAN JOAQUIN COUNTY Effluent/Receiving Water Limitations The following is a discussion of the significant changes from the previous permit: Total Dissolved Solids - The TDS secondary drinking water MCL is the basis for the effluent limitations of 500 mg/l as a monthly average and 1000 mg/l as a daily maximum in Order No. 95-148. These limitations will be unchanged in this version of the permit. For protection of the agricultural supply beneficial use based on crop tolerance, the TDS water quality criteria is 450 mg/l as an annual average. It is unknown if the Discharger is currently capable of meeting this effluent limitation. This Order provides for a time schedule for meeting the effluent limitation. The Board considers the application of a water quality objective that has never been applied to North Littlejohns Creek as a new interpretation of the Basin Plan. Chlorine Residual—The chlorine residual effluent limitations have been modified from the previous permit. A chlorine residual monthly average limit of 0.01 mg/l and a daily maximum effluent limit of 0.02 mg/l have been added to the permit in order to protect freshwater aquatic life. It is unknown if the Discharger is currently capable of meeting this effluent limitations. This Order provides for a time schedule for meeting the effluent limitations. The Board considers the application of a water quality objective that has never been applied to North Littlejohns Creek as a new interpretation of the Basin Plan. Copper- Based on a worst-case effluent hardness of 95 mg/l as CaCO3, the CTR hardness dependent copper water quality criteria for protection of aquatic life is 8.6 µg/l as a four day average and 13 µg/l as a 1 hour average. Of seven samples collected biannually between January 1997 and January 2000 the two samples collected in 1998 were both above detection limits at 30 and 50 pg/l. These higher copper concentrations were attributed to process changes in the Discharger's cooling tower disinfection system implemented that year. Once the impact of these process changes on effluent copper concentrations was discovered, the disinfection system was adjusted to reduce copper concentrations below detection limits again. Because of the process changes made to prevent these increased copper concentrations, the Board does not find these to be indicative of reasonable potential. However, the other five samples were below detection limits with four of the five having detection limits ranging between 10 and 30 µg/l. These levels are above the most stringent copper water quality criteria. The fifth of the non-detect samples was at a 4 pg/l detection level. Based on the fact that all but one of the samples had detection limits higher than the most stringent CTR criteria, more data with appropriate detection limits will be required per SIP section 2.2.2.A. The Discharger is required to develop a workplan and conduct sampling with detection limits low enough to allow for evaluation with the CTR criteria. Arsenic -Arsenic is an inorganic priority pollutant that produces human health effects. The Basin Plan water quality objective for arsenic is a maximum concentration of 10 µg/l dissolved, the U.S. EPA Primary MCL for arsenic is 10 µg/l total, and the California Proposition 65 Regulatory Drinking Water Level (10-6 cancer risk) is 5 µg/l total. There are no CTR human health criteria for arsenic. The U.S. EPA recommended translator for converting total arsenic concentrations to equivalent dissolved