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INFORMATION SHEET -6- CITY OF STOCKTON REGIONAL WATER QUALITY CONTROL PLANT SAN JOAQUIN COUNTY recommended that wastewater discharged to receiving waters used for water contact recreation be sufficiently disinfected so that swimming would result in no more than 20 intestinal virus infections per 10,000 recreationist-days. It also provided a preliminary health risk assessment for contact recreation in the San Joaquin River in the vicinity of the discharge. The assessment indicated there may be a significant health risk due, in part, to minimal dilution flows. In order to better quantify the health risk, and extent of contact recreation in the river in the vicinity of the discharge, this Order requires a comprehensive health risk assessment be completed, and includes a reopener for addition of more stringent effluent limitations for disinfection, if needed. During the interim, the effluent limitations for total coliform remain unchanged from the previous permit. Ammonia- The river model contained information that indicates there is a potential for ammonia toxicity in the receiving water during the winter due to the discharge. This Order requires the City to monitor ammonia in both the effluent and receiving water, as well as conduct effluent chronic toxicity bioassays. The permit may be reopened and the effluent limitations modified as necessary to address toxicity issues. Temperature- The permit contains effluent and receiving water temperature limitations which are contained in the Thermal Plan. Because the City believes it is unclear how natural receiving water temperature (not already affected by other dischargers) is determined and the City does not want to be held liable for temperature violations if it is due to other dischargers, the permit requires the City to evaluate how to best monitor compliance with the limits. pH- EPA regulations (40 CFR 401.17) state that allowable pH effluent limitations under continuous monitoring may allow for excursions from the pH range given that: (1) the total time during which the pH values are outside the required range of pH values shall not exceed 7 hours and 26 minutes in any calendar month; and (2) no individual excursion from the range of pH values shall exceed 60 minutes. This regulation essentially allows pH limitations to be exceeded 1 percent of the time. Effluent from the plant is chlorinated to meet the coliform limitation, and then the plant strategy is to dechlorinate with sulfur dioxide to eliminate all chlorine, and leave a residual of dechlorination chemical. However, overdosing with dechlorination chemicals can occasionally depress the effluent pH below the minimum 6.0. Caustic can be added to bring the effluent pH back into compliance. The effluent is discharged to the San Joaquin River through two siphon pipes which run from the chlorine contact channel over the levee to the river. Control systems