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INFORMATION SHEET 5_ CITY OF STOCKTON REGIONAL WATER QUALITY CONTROL PLANT SAN JOAQUIN COUNTY Reasonable Potential to Exceed Federal regulations require effluent limitations for all pollutants that are or may be discharged at a level that will cause or have the reasonable potential to cause, or contribute to an in-stream 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 nine sample events have been received. Of the results reviewed, all constituents appear to meet applicable water quality objectives, taking into account dilution and receiving water concentrations, with the exception of mercury. The water quality objectives for mercury include an acute 1-hour average of 2.4 µg/1 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/I at every sampling station. Results make it appear that the mercury problem is ubiquitous throughout the receiving water during the winter period. However, the accuracy of mercury analyses at the low regulatory levels can be questionable without implementing 'clean technique' for sample collection, handling, and analysis. Therefore, this Order requires additional study to assess whether the levels of mercury in the discharge causes or contributes to an in-stream excursion above a water quality objective. If the discharge causes or contributes to an in-stream excursion above a water quality objective, then the City must submit information to calculate an effluent limit. Finally, the permit can be reopened to include the effluent limit. Chronic Toxicity Requirements The Basin Plan requires that there be no toxics in toxic amounts. This Order implements the Basin Plan by prohibiting toxic pollutants to be present in the water column, sediments, or biota in concentrations that adversely affect beneficial uses; that produce detrimental response in human, plant, animal, or aquatic life; or that bioaccumulate in aquatic resources at levels which are harmful to human health. The Order requires effluent chronic toxicity testing to be conducted as specified in the Monitoring and Reporting Program, according to standard EPA protocol. If the testing indicates that the discharge causes, has the reasonable potential to cause, or contributes to an in-stream excursion above a water quality objective for toxicity, the Discharger must conduct a Toxicity Reduction Evaluation (TRE). The Order can be reopened and a chronic toxicity limitation or specific chemical toxicant included. Additionally, if a chronic toxicity water quality objective is adopted by the State Water Resources Control Board, this Order may be reopened and a limitation based on that objective included.