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WASTE DISCHARGE REQUIREMENTS 6_ CITY OF STOCKTON REGIONAL WATER QUALITY CONTROL PLANT SAN JOAQUIN COUNTY excursion above a narrative or numerical water quality standard. The City of Stockton has been collecting water quality samples of the effluent and receiving water monthly since July 1993. The constituents evaluated included those which were regulated by the Inland Surface Waters Plan. To date, results of twelve sample events have been received. Of the results reviewed, all constituents appear significantly below potentially toxic concentrations for those chemicals cited in the literature, taking into account dilution and receiving water concentrations, with the exception of mercury. The published toxicity information for mercury includes an acute 1-hour average of 2.4 µg/l to protect freshwater aquatic life, and a 30-day average of 0.012 µg/1 to protect human health (due to bioaccumulation of mercury in fish tissue). Mercury was found at concentrations greater than 0.012 µg/1 at every sampling station. This Order contains provisions that : a. require the Discharger to use 'clean technique' for sample collection, handling, and analysis, in order to provide information as to whether the levels of mercury in the discharge causes or contributes to an in-stream excursion above a water quality objective; b. if the discharge causes or contributes to an in-stream toxicity caused by mercury, requires the Discharger to submit information to calculate effluent limitations for those constituents; and c. allows the Board to reopen this Order and include effluent limitations for mercury. 24. The permitted discharge is consistent with the antidegradation provisions of 40 CFR 131.12 and State Water Resources Control Board Resolution 68-16. The impact on existing water quality will be insignificant. Pretreatment 25. The Discharger is required to protect the environment to the greatest degree possible (Public Resources Code, Section 21000, et seq.). A Pretreatment Program is needed to protect the receiving waters, the ground water underlying the reclamation areas, the sludge from being hazardous or otherwise interfering with the Discharger's reuse or disposal plans, and the treatment plant from upsets. 26. The Discharger developed a pretreatment program in conformance with 40 CFR Part 403 which was initially approved in 1982. Additional amendments to the program have been approved since that time. Storm Water 27. Federal Regulations for storm water discharges were promulgated by the EPA on 16 November 1990 (40 CFR Parts 122, 123, and 124). The regulations require specific categories of facilities,