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WASTE DISCHARGE REAEMENTS ORDER NO. 5-01-119 (AISED) -5- STOCKTON COGEN COMPANY SAN JOAQUIN COUNTY 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 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. 19. Based on a worst-case effluent hardness of 95 mg/1 as CaC031 the CTR hardness dependent copper water quality criteria for protection of aquatic life is 8.6 9g/1 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 gg/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 gg/1 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 F.2 for the terms and compliance schedule of the required study. 20. Arsenic is an inorganic priority pollutant that produces human health effects. Of seven samples collected and analyzed for arsenic biannually between January 1997 and January 2000, all were above the State of California Proposition 65 Level of 5 µg/l for arsenic. Effluent concentrations ranged between 20 and 26 gg/l. Data indicate that the source of the arsenic is the groundwater that is the supply for the majority of the cooling tower feedwater. There is no arsenic added in any part of the SCC operation. The Basin Plan requires the Board to consider information submitted by the discharger and others and numerical criteria and guidelines for toxic substances developed by other agencies in determining what numeric effluent limitation will properly implement the narrative toxicity objective. Numerical criteria include, for example, USEPA's 106 incremental cancer risk estimate for drinking water of 0.02 µg/l, the California Office of Environmental Health Hazard Assessment's 10`risk level of 0.023 µg/l, USEPA's recommended ambient water quality criterion for protection of human health in waters considered to be potential sources of drinking water of 0.018 gg/L at the 10`risk level, the Proposition 65 limit of 5 µg/l, and the California MCL of 50 µg/l. The Board finds that it is more appropriate to use the MCL of 50 µg/1 in this situation than the cancer risk levels. Therefore, there is no reasonable potential for the effluent to exceed 50 µg/1, and an effluent limit for arsenic is not included in this Order. 21. The water quality goal for the protection of agricultural use for molybdenum is 10 gg/1 as an annual average. Of seven samples collected and analyzed for molybdenum biannually between January 1997 and January 2000, four samples were at or below 5.0 µg/1, one was 9.0 µg/l, one was 15 µg/1, and one was non-detect with a detection limit of 20 µg/l. The data indicate that there is a