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a u CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD '. CENTRAL VALLEY REGION ORDER NO. REQUIRING THE CITY OF MANTECA AND CITY OF LATHROP WASTEWATER QUALITY CONTROL FACILITY TO CEASE AND DESIST FROM DISCHARGING CONTRARY TO REQUIREMENTS _ The California Regional Water Quality Control Board, Central Valley Region, (hereafter Board)finds: 1. The City of Manteca wastewater treatment plant treats the flows from the City of Manteca and certain areas of the City of Lathrop. The City of Manteca is solely responsible for the wastewater treatment facility. 2. The Board on 20 June 1997, adopted Order No. 97-115 (NPDES NO. CA 0081558)which prescribes requirements for an average discharge of 5.3 million gallons per day(mgd) of treated combined domestic and industrial wastewater to land.and partially to surface water. The City of Manteca and the City of Lathrop were both named as dischargers on Order No. 97-115. Land disposal of effluent is maximized by seasonally discharging effluent at agronomic rates to existing City-owned irrigation fields and other leased property. Excess flows are discharge to the San Joaquin River. 3. With adoption of the new Order in June 1997,the City of Manteca was given stricter toxicity effluent limitations than were contained in their previous permit. Specifically,the bioassay parameters (for acute toxicity)was revised to require compliance with the latest testing procedures contained in EPA/600/4-90/027. Furthermore, the bioassay test parameters were restricted to two of the allowable test species, fathead minnows and golden shiners. When testing with fathead minnows or golden shiners,the new EPA procedure requires the use of these species in the larval stage (0 to 14 days old) instead of the previous method of using juveniles (15 to 30 days old). Larvae are much more sensitive to ammonia levels than the juvenile species. City of Manteca did not operate their treatment plant to nitrify and therefore high ammonia levels were passed through their plant and discharged in their effluent. 4. Prior to the renewal of the permit, the City consistently met the bioassay effluent limitations of the permit. Since the renewal of the permit, and due to the difficulties meeting the Bioassay requirements, the City of Manteca has retained the services of a consultant. The City has found that their problems were associated with high ammonia concentrations in their effluent(20 to 25 mg/1) and with elevating pH levels in the static non-renewal testing tank. The EPA methods do not allow for pH corrections during the test, however, ammonia becomes more acutely toxic in higher pH environments. The combination of high ammonia levels,rising pH conditions, and using a more sensitive species required by the test resulted in the City's consistent failure of the bioassay test since changing over to the new test methods in July 1997 until April 1998.