SITE INFORMATION AND CORRESPONDENCE_1985-2004

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STAFF REPORT CITY OF MANTECA AND CITY OF LATHROP WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY INTRODUCTION Manteca has requested issuance of a revised NPDES permit establishing a mixing zone for ammonia toxicity to resolve current biotoxicity violations at the treatment plant. The City of Manteca's wastewater treatment plant accepts wastewater flows from Manteca and certain areas of the City of Lathrop. The City of Lathrop is also named in this Order due to their inclusion in the NPDES permit. The City of Manteca is solely responsible for the wastewater treatment facility. The Board, on 20 June 1997, adopted Order No. 97-115 (NPDES No. CA 0081558)which prescribe requirements for a discharge of an average flow of 5.3 million gallons per day(mgd) of treated combined domestic and industrial wastewater to land and partially to surface waters. Land disposal of effluent is maximized by discharging effluent at agronomic rates seasonally to existing City-owned irrigation fields, and additional City leased property. Excess flows are discharged to the San Joaquin River. The Board is being asked to consider a revision to this NPDES Order. The City of Manteca (hereafter Discharger)has requested modification of their permit to allow a change of their effluent toxicity requirements through utilization of a mixing zone in the receiving water. Proposed revisions to the Waste Discharge Requirements Order No. 97-115 are limited to acute effluent toxicity requirements, effluent ammonia limits, and effluent monitoring in the monitoring and reporting program. The proposed revision to the NPDES Order requires the construction of a diffuser to disperse the effluent throughout the cross sectional area of the river. Discharge and river flow conditions are prescribed to limit the size of the mixing zone in which aquatic life will not be fully protected. The acute and chronic mixing zones are estimated to be 2-3 % of the cross sectional area of the river at the maximum width of the mixing zone. The revision to this Order requires that the Discharger has the diffuser fully operational and therefore in compliance with the NPDES permit (Order No. 97-115)by 1 December 2001. BACKGROUND Order No. 97-115 (NPDES NO. CA 0081558)prescribes requirements for an average discharge of 5.3 million gallons per day(mgd) of treated combined domestic and industrial wastewater to land and partially to surface water. This Order, adopted in June 1997, gave the City of Manteca stricter toxicity effluent limitations than were contained in their previous permit. Specifically,the bioassay parameters(for acute toxicity)were revised to require compliance with the latest testing procedures contained in EPA/600/4-90/027F. Furthermore, the bioassay test parameters were