SITE INFORMATION AND CORRESPONDENCE_1985-2004

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INFORMATION SHEET OR&NO. 115-2004-0028 • 6 CITY OF MANTECA, CITY OF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUINCOUNTY 2.7 Evaluation of Available Dilution for Priority Pollutant Human Carcinogen Criteria The human health-based criteria for carcinogens, other than arsenic, are based on safe levels for lifetime exposure and utilize the harmonic mean flow to represent the receiving water flow. The harmonic mean flow at Vernalis is 1976 cfs. The current annual average discharge rate is 5.72 mgd (8.9 cfs). A steady state analysis utilizing the harmonic mean flow provides a dilution of 222:1. The Regional Board is not required to grant a mixing zone or allocate the full assimilative capacity of the receiving water. For limitations based on human health criteria, dilution is limited to that required to maintain compliance. Where the ambient background concentrations are lower than the applicable human health criterion, the dilution credits determined in Table 12 of the Information Sheet apply for the determination of effluent limitations for carcinogens. 3 Biosolids Management The City of Manteca currently discharges biosolids that has been dewatered in drying beds to City-owned farmland adjacent to the treatment plant at agronomic rates, as described in the Order. New limitations on metal concentrations in sludge/soil mixtures and new conditions for sludge use as a soil amendment have been established. This new permit requires the City to reevaluate the sludge and effluent application rates to land and submit a land application plan. 4 Pretreatment Program The Discharger submitted a draft pretreatment program to the Regional Board for approval. The Regional Board, in an October 2001 Pretreatment Audit, identified areas of the program that were deficient or not implemented. The Regional Board staff, on 22 January 2003, provided comments to the Discharger identifying provisions of the City's Waste Ordinance and the Interjurisdictional Agreement between the City of Manteca and the Lathrop County Water District that are deficient. This Order provides a compliance schedule for the Discharger to submit a pretreatment program that corrects the deficiencies noted in the October 2001 Pretreatment Compliance Audit and in the 22 January 2003 letter. The Regional Board will reopen this Order to approve the pretreatment program upon submittal of a program that corrects the deficiencies. This Order requires full compliance with all pretreatment program requirements by 1 October 2004. 5 Ground Water Domestic wastewater contains constituents such as total dissolved solids (TDS), specific conductivity, pathogens,nitrates, organics, and metals. The Discharger's use of unlined ponds and the application of wastewater to land may result in an increase in the concentration of these constituents in groundwater. The increase in the concentration of these constituents in groundwater must be consistent with Resolution 68-16. Any increase in pollutant concentrations in groundwater must be shown to be necessary to allow wastewater service necessary to accommodate housing and economic expansion in the area and must be consistent with maximum benefit to the people of the State of California. Some degradation of groundwater by the Discharger is consistent with Resolution 68-16 provided that: a. The degradation is confined to a specified area;