SITE INFORMATION AND CORRESPONDENCE_1985-2004

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Staff Report City of Manteca and City of Lathrop Wastewater Quality Control Facility ammonia concentrations in their discharge. The study's objectives included: 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 river flows and pH, and 6) the extent of the zone of measurable ammonia downstream to a point where ammonia concentration do not exceed upstream. The CDO allowed time for the Discharger to conduct necessary bioassay and stream studies and to construct the treatment plant expansion to meet ammonia toxicity requirements. The City conducted a receiving water study(dated 9 July 1999)to analyze acute ammonia toxicity in the receiving water(San Joaquin River) near the point of discharge. The analysis for acute ammonia toxicity consisted of conducting 96-hour static bioassay tests using receiving water collected immediately downstream from the point of discharge. The results of these bioassay tests showed no toxicity within the River in tests conducted from December 1998 through May 1999. The City's effluent ammonia concentration ranges between 10 mg/1 and 37 mg/l, depending upon the season, weather conditions, flow and operational mode. In order to maintain the bioassay sample ammonia concentration at 10 mg/l or less,the plant effluent must be diluted with river water at a ratio of 2.8 parts river water to 1 part plant effluent. By application dated 9 July 1999, the discharger specifically requested a permit revision allowing dilution of their effluent samples used for acute toxicity tests because a diluted sample more accurately represents the actual conditions in the receiving water. The request is for a mixing zone in the receiving water to allow dilution of the toxic effects of the ammonia contained in their treated effluent, thereby eliminating the violation of their permit and achieving compliance with their CDO. The Board's Water Quality Control Plan for the San Joaquin River Basin allows for application of water quality objectives at the edge of mixing zones if such mixing zones are specified in the waste discharge requirements. The Discharger submitted additional information documenting their request on 22 July 1999, 23 November 1999, 3 December 1999, 11 January 2000 and 14 January 2000. The Discharger has concluded that construction of an effluent discharge diffuser and effluent storage basins will minimize the acute and chronic ammonia mixing zones during worst case flow periods. The Discharger proposes to mitigate the toxic effects of the effluent by constructing an effluent discharge diffuser in the River to enhance mixing and to minimize toxicity to benthic organisms; and by utilizing existing of holding ponds during critical San Joaquin River flow conditions. The diffuser will be designed to prevent plume contact with the River bottom and channel side banks. The existing wastewater holding ponds will hold up to two days of treated effluent while maintaining two feet of freeboard.