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WASTE DISCHARGE REQU MENTS • -5- STOCKTON COGENERATIO7COMPANY SAN JOAQUIN COUNTY demonstrated to the satisfaction of the Regional Water Board that the mixing zone will not adversely impact beneficial uses... In determining the size of such mixing zones, the Regional Water Board will consider the applicable procedures and guidelines in EPA's Water Quality Standards Handbook and the Technical Support Document for Water Quality-based Toxics Control..." The Board finds that, based on water quality evidence and the applicable procedures guidelines currently available, provisions for a mixing zone by allowing compliance with water quality objectives to be determined at French Camp Slough should not be continued. Because available dilution is negligible, the Board will not designate any mixing zone within which Basin Plan water quality objectives will not apply. The elimination of the mixing zone within North Littlejohns Creek requires that this permit apply Basin Plan water quality objectives, which have never been applied to this specific area of the water body for this Discharger. Receiving water limitations for temperature and effluent limitations for TDS and chlorine residual have been included in this permit, as explained below, based on the application of the Basin Plan water quality objectives within North Littlejohns Creek. It is unknown if the Discharger is currently capable of complying with these limitations. This Order provides for a time schedule for meeting the receiving water limitations for temperature and dissolved oxygen and the effluent limitations for arsenic, TDS and chlorine residual. The time schedule is authorized to be included in this Order based on 40 CFR §122.47. The Board considers the elimination of the mixing zone in North Littlejohns Creek for compliance with the Basin Plan water quality objectives as anew interpretation of the Basin Plan. 20. 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/1 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 µg/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/1. These levels are above the most stringent copper water quality criteria. The fifth of the non-detect samples was at a 4 µg/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. See Provision G.2 for the terms and compliance schedule of the required study. 21. 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/1 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/1 total. There are no CTR human health criteria for arsenic. The U.S. EPA recommended translator for converting total arsenic concentrations to equivalent dissolved concentrations is 1.0. This means all arsenic measured as total recoverable is to be