SITE INFORMATION AND CORRESPONDENCE_1985-2004

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INFORMATION SHEET ORDER NO. 97-115 (revised) -4- CITY OF MANTECA AND CITY OF LATHROP SAN JOAQUIN COUNTY in the summer and 1032 cfs for non-summer months were used. Based on a probability analysis, these river flows are expected to occur one month every 35 years. Historical river pH, river ammonia concentrations, treatment plant effluent pH and effluent ammonia concentrations were reviewed to determine what values of these parameters would most likely occur during the seasonal conditions modeled. Probability plots were generated to determine what values of these parameters would most likely occur during summer and non-summer conditions at frequencies of one month every 35 years for the worst case mixing zone. Receiving water pH data was obtained from the Vernalis monitoring station and from the City of Stockton's R-1 River monitoring station, which is approximately 12 miles downstream of the discharge point. Data from these two sources was quite consistent, strongly indicating they are representative of the River at the discharge point. High river pH conditions were assumed for modeling purposes because ammonia is more toxic to aquatic life at higher pH. For acute toxicity calculations, the highest recorded river pH in a three-year period was used. For chronic toxicity, which involves a longer period of exposure, the highest 7-day average pH calculated during the three-year period was used. A wastewater effluent flow of 8.1.1 mgd was used for all cases, which is the maximum permitted flow under Order 97-115. The values of treatment plant effluent pH and ammonia concentrations used for modeling purposes were compiled from daily effluent data for pH and ammonia for 1994 through 1997. Daily data was directly used for acute mixing zone modeling. The same data was averaged over a 30-day period for the chronic mixing zone calculations. Ammonia toxicity values as a function of pH were obtained from the August 1998 US EPA Receiving Water Criteria for ammonia in fresh water. EPA updated its ammonia toxicity data in December 1999, however there were no significant changes for acute ammonia toxicity and only slight temperature correction changes for chronic toxicity. According to the US EPA document, acute toxicity is defined as the ammonia concentration at which death to aquatic organisms occurs after one hour of exposure. Exposure to "acute"concentrations for periods of time less than one hour should not produce death, although they might produce"chronic" impacts. The document also provides ammonia concentrations at which"chronic" impacts (e.g., impacts other than death, such as reduced weight gain, lower reproduction, etc.)occur when the organisms are continuously exposed to those concentrations for 30 days. Exposure to "chronic" concentrations for periods less than 30 days should produce no physiologic impacts on the aquatic organisms.