SITE INFORMATION AND CORRESPONDENCE_1985-2004

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REVISED WASTE DISCHARGE REQUIREMENTS ORDER NO. 97-115 4 CITY OF MANTECA AND CITY OF LATHROP WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY effluent ammonia concentration of 31 mg N/1, effluent flow of 8.11 mgd, river flow velocity of 0.56 cfs, and river pH of 8.5 were used. 19. The USEPA water quality criteria for ammonia(updated August 1998) for determining in-stream ammonia acute toxicity indicates a 1- hour average ammonia toxicity limit of 2.14 mg N/1 for a pH of 8.5. (The San Joaquin River pH varies from a high of 8.47 to a low of 6.21 based on data from August 1994 to July 1999 taken from the closest City of Stockton's upstream surface monitoring station that is 12 miles downstream from the Manteca discharge point and confirmed by data obtained at Vernalis monitoring station located 11 miles upstream of the Manteca discharge point on the San Joaquin River). The USEPA water quality criteria for ammonia for determining in-stream ammonia chronic toxicity indicate a 30-day average ammonia toxicity limit of 0.569 mg N/1 for a pH of 8.5 20. The longest ammonia mixing zone is predicted to occur during the summer months at low river flow periods (641 cfs). For these conditions, the acute toxicity plumes are predicted to be 40 feet long. The cumulative cross sectional area of the plumes at the widest point would cover 1.4%of the river channel. The chronic toxicity plumes are predicted to be 51 feet long. The cumulative cross sectional area of the plumes at the widest point would cover 3.16% of the river channel. 21. These findings and permit limitations are based on worst case flow conditions and assuming a river pH maintained at 8.5. The residence time of an aquatic specimen within this zone is minimal, resulting in low potential for chronic or acute effects. Under most circumstances, toxic ammonia effects will be much less due to higher river flows, lower effluent flows until build out, and lower river pH conditions in the winter when ammonia levels are generally higher. 22. To validate the modeling results, the Discharger will be required to monitor the river for ammonia downstream of the acute and chronic mixing zones. If acute toxicity is found 40 feet beyond the discharge point or chronic toxicity is found 51 feet beyond the discharge point, then this Order will be reopened and effluent limitations added or amended for ammonia toxicity. 23. Due to the relative small size of the ammonia acute and chronic toxicity mixing zones with mitigation, the Board finds the discharge of ammonia at a concentration of 31 mg N/1 in summer and 35 mg NII in non-summer periods will not cause significant impacts on aquatic life, which is the beneficial use most likely affected by ammonia. The discharge of ammonia will also not cause a violation of water quality objectives outside the mixing zone described in the preceding findings. 24. The City-owned irrigation fields, Assessors parcel Nos. 241-310-16, 18, 32, 44, 48, 52, 53, 54, and 241-300-47, 48, 49, 50, total 283 acres. The city-leased irrigation fields, Assessor's Parcel Nos. 241-320-01 and 241-320-02, total 160 acres. 25. The City's biosolids are presently being reused at city's owned land (180 acre site) and applied at agronomic rates, and limited by crop utilization rates as well as 40 CFR 503 and Department of Health Services "California Manual of Good Practice for Landspreading of Sewage Sludge".