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WASTE DISCHARGE REQU&MENTS ORDER NO. R5-2004-0020 -23- CITY OF MANTECA, CITY OFLATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY b. The Bay/Delta Plan prescribes a minimum DO concentration of 6.0 mg/1 in the San Joaquin River inside the reach from Turner Cut to Stockton during the period 1 September through 30 November. This higher DO concentration was imposed to enhance aquatic conditions during critical migration periods for salmon. 56. The DO objectives are frequently not met in the San Joaquin River, leading to the Clean Water Act section 303(d) listing. In 1998,the Regional Board classified the DO impairment within the San Joaquin River as a Toxic Hot Spot, making it a high priority problem for correction. Since the spring of 1999, the Discharger and other stakeholders have participated in the steering committee for the development of the DO TMDL for the San Joaquin River in the Deep Water Ship Channel(DWSC). A TMDL implementation plan was submitted to the Regional Board in February 2003. Staff has developed and submitted to the USEPA in June 2003 a TMDL report for controlling the problem. The existing low DO conditions in the Stockton DWSC are partially the result of channel morphology, and point and non-point sources that are beyond the control of the Discharger. The Discharger will make a significant reduction in the magnitude of its contribution to the DO problem by implementation of more stringent ammonia and BOD effluent limitations through the construction of nitrification, denitrification, and tertiary coagulation and filtration facilities. These facilities will prevent ammonia toxicity and reduce the nitrogenous and carbonaceous biochemical oxygen demand that is presently exerted on the San Joaquin River. Based on the above information, further action by the Discharger to reduce its impact on the San Joaquin River DO concentration,beyond the requirements of this permit, will not be required by the Regional Board until such time as the TMDL for DO has been developed and approved by USEPA. This Order contains a provision to allow for the permit to be reopened to consider modification of effluent limitations after the DO TMDL is finalized. DISINFECTIONXILTRATION 57. The beneficial uses of the San Joaquin River include municipal supply,water contact recreational uses and agricultural irrigation supply, and there is, at times, less than 20:1 dilution. Recreational uses identified in the immediate vicinity of the WQCF outfall include boating, skiing, swimming, and fishing. A number of agricultural diversions have been identified through a search of the State Board, Water Rights Division database. Within an approximate one-mile radius of the outfall, there are approximately five agricultural diversions identified in the database. One of the agricultural diversions is just downstream and in the immediate vicinity of the outfall. To protect these beneficial uses, the Regional Board finds that the wastewater must be disinfected and adequately treated to prevent disease. The principal infectious agents (pathogens) that may be present in raw sewage may be classified into three broad groups: bacteria,parasites, and viruses. Tertiary treatment, consisting of chemical coagulation, sedimentation, and filtration, has been found to remove approximately 99.5% of viruses.