SITE INFORMATION AND CORRESPONDENCE_1985-2004

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r REVISED WASTE DISCHARGE REQUIREMENTS ORDER NO. 97-115 2 CITY OF MANTECA AND CITY OF LATHROP WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY the use of larval stage (0 to 14 days old) fathead minnows or golden shiners instead of the previous method of using juveniles (15 to 30 days old). Larvae are much more sensitive to ammonia levels than the juvenile species. The City of Manteca cannot operate their treatment plant to nitrify, therefore high ammonia levels pass through their plant and discharge in their effluent. 5. Prior to the 1997 permit renewal, the City consistently met the permitted effluent bioassay limitations of the permit. A toxicity investigation evaluation (TIE) conducted by the City found the toxicity was caused by high ammonia concentrations in their effluent. The combination of high ammonia levels and the use of a more sensitive species, as required by the latest EPA methods, resulted in the City's consistent failure of the bioassay test since July 1997. 6. 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. 7. Data obtained from the City of Stockton's upstream sampling station on the San Joaquin River (Station R-1, 11 river miles downstream from Manteca) shows there have been only three samples with measurable ammonia concentration between August 1994 to July 1999. The monthly data during this time period typically show non-detectable concentrations of ammonia (<O.1 or<0.2 mg/1), except for concentrations of 0.2, 0.2, and 0.6 mg/l, collected in May 1995 (pH 7.21), March 1995 (pH 6.32), and January 1995 (pH 6.73). No ammonia toxicity is shown in this data, demonstrating the river has assimilative capacity for Manteca's discharge. 8. The City conducted numerous bioassay tests to determine the in-laboratory acute ammonia toxicity threshold for larval minnows in 96-hour static bioassay tests. It was found they are able to achieve full compliance with the acute toxicity requirements of their permit when ammonia concentrations were 10 mg/L or less. 9. The City's effluent ammonia concentration ranges between 10 mg/l 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. 10. 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 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. 11. This Order and the Basin Plan prohibit the discharge of toxic constituents in toxic amounts outside the small zone of mixing, or initial dilution, at the discharge outfall.