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WASTE DISCHARGE REQU*MENTS ORDER NO. R5-2004-002$ -16- CITY OF MANTECA, CITY OF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY WQCF effluent contains detectable levels of mercury below CTR priority pollutant criteria. Since the CTR criteria are not based on bioaccumulation,the discharge was evaluated based on the Basin Plan's narrative toxicity objective. Any loading of mercury from the discharge may have the reasonable potential to cause or contribute to an excursion above the narrative toxicity objective by causing bioaccumulation in fish tissue. Health advisories by the Department of Health Services remain in effect for human consumption of fish in the Delta, including the San Joaquin River at Manteca, due to excessive concentrations of mercury in fish flesh. These current warnings and available fish tissue data confirm that there is currently no assimilative capacity for mercury. Therefore,water quality based effluent limitations for mercury that properly address bioaccumulation are required. A TMDL for mercury is scheduled to be completed by December 2005. For situations like this, the SIP recommends that limiting mass loading of the bioaccumulative pollutant should be considered in the interim at representative, current levels pending development of the TMDL. A mass load limit for mercury is included in this Order. If the Regional Board determines that a mercury offset program is feasible for Dischargers subject to a NPDES permit,then this Order maybe reopened to reevaluate the interim mercury mass loading limitation(s) and the need for a mercury offset program for this Discharger. NONPRIORITY POLLUTANTS 40. Temperature: The State Board Water Quality Control Plan for Control of Temperatures in Coastal and Interstate Waters and Enclosed Bays and Estuaries of California(the Thermal Plan) is applicable to this discharge. For the purposes of compliance with the Thermal Plan, the discharger is considered to be an existing discharger of elevated temperature waste. Monitoring by the discharger indicates that the 20 T limitation of Objective 5.A.(1)a of the Thermal Plan is occasionally exceeded in winter months when the receiving water is at its lowest temperatures. Modeling conducted by Resource Management Associates (RMA), subject to the model limitations discussed in the information sheet, indicates that the current and the expanded flows with continuous discharge also exceed both the 1 degree and 4 degree requirements of Objectives 5.A.(1)b and 5.A.(1)c of the Thermal Plan. The modeling also demonstrates that a timed discharge, that is, discharging only on the outgoing tide, for the increased flow exceeds only the 4 degree requirement,not the 1 degree requirement. The Discharger has requested an exception to the 4 degree requirement of Objective 5.A.(1)c of the Thermal Plan which requires that the discharge shall not cause a surface water temperature rise greater than 4 T above the natural temperature of the receiving waters at any time or place and has also requested a one month averaging period to meet the 20 degree limit of Objective 5.A.(1)a. The Discharger has not yet implemented measures necessary to meet conditions for any exception, including installation of an in-stream real-time flow monitor in the vicinity of the outfall or construction of storage basins to provide for effluent discharges on out-going tides. Therefore, effluent and receiving water limitations are included to achieve compliance with the Thermal Plan. This Order requires that the discharger comply with the Thermal Plan and implement the necessary mitigations, such as discharging during the outgoing tide,to bring the existing discharge into compliance and maintain compliance when increasing its effluent discharge rate. This Order may be reopened to include alternative limitations for temperature if a resolution is adopted in the future that provides exceptions for particular objectives of the Thermal Plan.