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INFORMATION SHEET OR NO. R5-2004-0028 15 CITY OF MANTECA, CITY LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY adequately nitrifies and denitrifies the waste stream to protect the beneficial uses of municipal and domestic supply. 10 Ammonia-Nitrogen This section provides a detailed discussion and evaluation of ammonia in the effluent. A review of the Discharger's monitoring reports from January 1998 through December 2002 shows an average ammonia effluent concentration of 18 mg/1, a minimum concentration of less than 0.1 mg/l, and a maximum concentration of 43 mg/l. The data indicate very little seasonal fluctuation. Receiving water monitoring(R-1) was conducted from January 2002 through December 2002 (see Table 4). The receiving water data showed an average of 0.2 mg/l with a minimum of less than 0.01 mg/l and a maximum of 1.4 mg/l. 10.1 Toxicity Criteria The USEPA 1999 Update of Ambient Water Quality Criteria for Ammonia provides the applicable water quality criteria for this pollutant. Ammonia is not a priority pollutant; therefore, USEPA guidance,rather than the SIP, is applicable for reasonable potential and effluent limitation calculations. Section 4.3.3 of the TSD allows the consideration of exposure duration in evaluating toxicity to organisms passing through a mixing zone. When evaluating either an acute or chronic mixing zone for ammonia,the pH of the mixture of effluent and receiving water should be used to determine appropriate criteria to be applied within that mixing zone. The pH in the mixing zone will be a function of the effluent pH and the ambient dilution water pH being mixed together. The pH is an important factor because toxicity of ammonia increases logarithmically as pH increases. 10.2 Consideration of Aquatic Organisms The most stringent acute ammonia criteria are applied when salmonoids are present within the water column. The San Joaquin River at Manteca is a migratory path for salmon, and they are likely to be present in the river at any time of the year. The chronic ammonia criteria are most stringent when early life stages (ELS) of aquatic species are present. In response to a request for information regarding the time of year ELS of fish are present in the San Joaquin River near the Deep Water Ship Channel (DWSC), a Department of Fish and Game memorandum, dated 27 February 2001, states ELS of multiple fish and invertebrates species are present in the San Joaquin River year-round. Therefore,both acute and chronic ammonia toxicity are based on the assumption that both salmonids and ELS of fishes are present in the San Joaquin River near the Manteca WQCF outfall year-round. 10.3 Reasonable Potential Evaluation The reasonable potential evaluation shows that the WQCF effluent has reasonable potential to cause or contribute to an in-stream excursion above USEPA acute and chronic water quality criteria for ammonia. This has been demonstrated by determining reasonable potential based on critical