SITE INFORMATION AND CORRESPONDENCE_1985-2004

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t CEASE AND DESIST ORDER NO. 99-051 -2- CITY OF MANTECA AND CITY OF LATHROP SAN JOAQUIN COUNTY Board for modification of the Order based on new information." It has become evident that treatment plant expansion or modifications may be necessary to reduce ammonia concentrations in the effluent. Testing of the fathead minnows have shown that an ammonia concentration of 10 mg/1 or less is required in the effluent to meet acute toxicity requirements of Order No. 97-115. Manteca has proposed to consider full nitrification in its expansion of the treatment plant to reduce the toxicity of the proposed blended effluent. Manteca further proposes to evaluate the effects of the current discharge and determine the extent of the mixing zone currently effected by the acute ammonia concentrations. The study will include: 1) defining what range of effluent concentrations are acutely toxic in accordance with the 96 hour static bioassay; 2) what range of concentrations are acutely toxic in the first hour of toxicity testing; 3) defining the zone of initial dilution in the receiving water to the extent ammonia concentrations are acutely toxic to the 1 hour exposure level; 4) define the zone of initial dilution in the receiving water to the extent ammonia concentrations are no longer toxic,using bioassay protocol; 5) determine the zone in the receiving water which exceeds the EPA Water Quality criteria for various specific temperature and pH; and 6) the extent of the zone of measurable ammonia downstream to a point where ammonia concentrations do not exceed upstream concentrations. This Cease and Desist Order allows the City of Manteca time to conduct the necessary bioassay and stream studies and provides time to construct expansion of the treatment plant to meet ammonia toxicity requirements. This Order does not preclude consideration of modification of Order No. 97-115 based on the evaluation of the effects of the discharge on ambient receiving water or preclude resubmittal of the request for the use of different test species to be used in the acute toxicity bioassay test. 6. With adoption of Order No 97-115 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/600/4-90/027F. 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. 7. Prior to the renewal of the permit in 1997,the City consistently met the bioassay effluent limitations of the permit. A toxicity investigation evaluation(TIE) conducted by the City has found that their problems were associated with high ammonia concentrations in their eluent(20 to 25 mg/1). The combination of high ammonia levels and use of a more sensitive species required by the latest EPA methods resulted in the City's consistent failure of the bioassay test since changing over to the new test methods in July 1997. 8. Order No 97-115 Effluent Limitation C.5. states: "Survival of aquatic organisms in 96 hour bioassays of undiluted waste shall be no less than: Minimum for any one bioassay--- -----------70% Median for any three or more consecutive bioassays ------- 90%"