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INFORMATION SHEET OR*NO. R5-2004-0028 . 16 CITY OF MANTECA, CITY OF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY conditions that are a combination of worst-case observations using effluent data and using receiving water data(see Table 7). Consistent with 40 CFR section 122.44(d)(vi)(A) and the Basin Plan"Policy for Application of Water Quality Objectives", this Order implements the Basin Plan narrative toxicity objective by applying USEPA's Ambient Water Quality Criteria for the Protection of Freshwater Aquatic Life for ammonia. This Order includes effluent limitations for ammonia,based on the narrative toxicity objective and the USEPA's Ambient Water Quality Criteria for the Protection of Freshwater Aquatic Life. The acute criterion or criteria maximum concentration(CMC) for ammonia is a function of receiving water pH and is stated as a 1-hour average concentration. A worst-case scenario occurs when there is little to no dilution of the effluent by the receiving water. This was discussed in the previous dilution section. Therefore, for the acute criteria,water quality objectives need to be achieved in the effluent at the end-of-pipe. As allowed by the TSD,this Order calculates the CMC using critical conditions that are a combination of worst-case observations. The acute criterion for ammonia is determined by evaluating the maximum effluent pH at the end of the pipe. The maximum allowable effluent pH is 8.0. The calculated CMC for this condition is 5.6 mg/l ammonia as N. The maximum effluent concentration,measured on 15 August 2001,was 42.8 mg/1 ammonia as N. This exceeds the calculated ammonia CMC value. Even using the mean effluent ammonia concentration of 17.7 mg/1 exceeds the CMC value under worst-case pH conditions. This scenario shows that there is reasonable potential for acute water quality objectives to be exceeded by effluent ammonia concentrations. The receiving water pH and ammonia concentrations were also evaluated to determine if there is reasonable potential to cause acute ammonia toxicity,based upon concentrations found in the receiving water. The acute criterion is determined using the receiving water pH. In July 2002, the receiving water reached a maximum pH of 9.3, as recorded by the City at the R-1 monitoring site. The receiving water ammonia concentrations determined by the discharger's monitoring during 2002 indicated a maximum concentration of 1.4 mg/1 with an average of 0.2 mg/l. As determined by the TSD approach, the receiving water at times may exceed the CMC for ammonia. The chronic criterion, or criteria continuous concentration(CCC), for ammonia is a function of both pH and temperature. For ammonia, the CCC is stated as a 30-day average concentration,with the highest 4-day average within the 30-day average not to exceed 2.5 times the CCC. As allowed by the TSD, the CCC is calculated using critical conditions that are a combination of worst-case observations. The highest receiving water 30-day average pH was 9.1, observed during June/July 1992 at the DWR Mossdale monitoring station. The maximum 30-day average temperature of 25.7 C (78.3 F)was observed during July 2002 at the DWR Mossdale monitoring station. The calculated CCC for this condition is 0.21 mg/l ammonia-N. The effluent 30-day average ammonia concentration during that same period was 14.1 mg/I ammonia as N and 17.7 mg/1 averaged over the past 5 years. The calculated CCC is exceeded which demonstrates that the effluent has the reasonable potential to cause or contribute to chronic ammonia toxicity in the receiving water. 'EPA Technical Support Document,March 1991,Chapter 3