SITE INFORMATION AND CORRESPONDENCE

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INFORMATION SHEET OR&NO. R5-2004-0028 17 CITY OF MANTECA, CITY OF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY The monthly average receiving water pH and temperature from the Mossdale monitoring station, and ammonia concentrations collected from the R-1 sample location during 2002 were evaluated to determine if concentrations have been observed in the receiving water above the chronic criteria. The TSD method demonstrated a reasonable potential for the receiving water to exceed the chronic ammonia toxicity. The maximum ammonia concentration of 1.4 mg/1 also demonstrates that there are times when there is no assimilative capacity in the receiving water for additional ammonia. 10.4 Effluent Ammonia Limits Based on the above discussion of reasonable potential, daily and monthly effluent ammonia limitations are required to protect aquatic organisms from ammonia toxicity. The USEPA TSD recommends that statistical permit limit derivations be used to develop chemical specific limitations.for NPDES permits. Effluent limitations are calculated as shown in Table 8. Because of the seasonal variation in pH and temperature of the receiving water and the sensitivity of the ammonia criteria to these conditions, seasonal limitations are established. For the warm weather months from 1 June to 30 September, the maximum permitted monthly average effluent pH is 8.0, the maximum historical monthly average receiving water pH is 9.1, the maximum historical monthly average effluent temperature is 27.2 F, and the maximum historical monthly average receiving water temperature is 25.7 F. The pH and temperature at the edge of a 4:1 mixing zone were estimated utilizing the USEPA DESCON program. These estimations are utilized in Table 8 to calculate effluent limitations that maintain compliance with chronic aquatic criterion in the receiving water outside of the mixing zone. Effluent limitations compliant with acute criteria for conditions at the end-of-pipe are also determined,but the more restrictive chronic criteria determine the final effluent limitations. Table 8 provides a daily maximum effluent limitation of 4.4 mg/1 ammonia as N and a 30-day average effluent limitation of 2.1 mg/l. As defined by the 1999 criteria, the 4-day average CCC ammonia concentration shall not exceed 2.5 times the value of the 30-day CCC. However, considering the maximum daily limitation is less than 2.5 times the CCC in all cases, the 4-day average cannot exceed the maximum daily limitation. For the cool weather months from 1 October to 31 May, the maximum permitted monthly average effluent pH is 8.0, the maximum historical monthly average receiving water pH is 8.5, the maximum historical monthly average effluent temperature is 25.2 F, and the maximum historical monthly average receiving water temperature is 19.6 F. The pH and temperature at the edge of a 4:1 mixing zone were estimated utilizing the USEPA DESCON program. These estimations are utilized in Table 8 to calculate effluent limitations that maintain compliance with chronic aquatic criterion in the receiving water outside of the mixing zone. Effluent limitations compliant with acute criteria for conditions at the end-of-pipe are also determined. In this case,the more restrictive acute criteria determine the final effluent limitations. Table 8 show that the acute criteria using the maximum permitted effluent pH of 8.0 provides a daily maximum effluent limitation of 5.6 mg/1 ammonia as N and a 30-day average effluent limitation of 2.8 mg/l.