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REGIONAL BOARD RESPONSE (SWRCB/OCC F1LE A-1483) -24- PETITION FOR REVIEW OF WASTE DISCHARGE REQUIREMENTS ORDER NOS. R5-2002-0083 AND R5-2002-0084 CITY OF STOCKTON REGIONAL WASTEWATER CONTROL FACILITY reasonably required. The requirement to conduct a carcinogen impact study is consistent with section 13383. It is also consistent with Water Code section 13267. The Basin Plan at IV-18.00 states, "Pollutants which are carcinogens or which manifest their toxic effects on the same organ systems or through similar mechanisms will be considered to have potentially additive toxicity". The human carcinogens to be studied may exhibit additive toxicity, and need to be addressed together. In addition, the extent of the mixing zone for human carcinogens is based on the harmonic mean dilution rather than the chronic mixing zone, and is not assessed by any studies completed to date. In order to grant a dilution credit for human carcinogens, a mixing zone must be granted. The SIP requires that the size of the mixing zone be delineated. The requirement is not based on any new rulemaking. "115. The Permit includes a requirement to develop pollution prevention plans. (Provision G.6.) This requirement is unreasonable because it derives from lack of consideration of existing dilution, and is based on inaccurate conclusions regarding TDS." Regional Board's response concerning dilution is stated above in response to the First Basis for Review. Total Dissolved Solids: The southern one-third of the Delta is 303(d) listed as an impaired water body for total dissolved solids (TDS). The RWCF outfall is downstream of this impaired area, so the RWCF discharge will not impact this 303(d) listed area. There are no USEPA water quality criteria for protection of aquatic organisms for TDS. However, its presence in water can be growth limiting to certain agricultural crops and affects the taste of the water for human consumption. The secondary California maximum contaminant level (MCL) for TDS is 500 mg/1 as a recommended level, 1000 mg/I as an upper level, and 1500 mg/1 as a short-term maximum. The recommended agricultural water quality goal for TDS is 450 mg/1 as a long-term average. The Discharger's report of waste discharge indicates an annual average TDS effluent concentration of 726 mg/l, a lowest monthly average of 638 mg/l, and a highest monthly average of 792 mg/l. These concentrations exceeded the applicable objectives. Limited TDS data collected at receiving water sample location Rl from May 1994 through June 1998 indicate that the concentration of TDS in the receiving water ranged from 77 mg/i to 630 mg/1 and averaged 313 mg/1 in 13 sampling events. These receiving water monitoring data indicate there is somewhat limited receiving water assimilative capacity for TDS, and at times the receiving water exceeds these levels established as long-term averages. As water exported from the Delta by the State Water Project is, in part,mixed with Colorado River water to provide municipal water supply with an acceptable TDS, any increase in salt concentration effectively reduces the available water supply in Southern California. Some of the significant sources of TDS in the effluent include discharges from industrial users, and the Discharger's practice of acidifying the plant effluent to prevent ammonia toxicity in flow through bioassays. Pollution prevention is necessary to assure the receiving water achieves the water quality objectives for TDS. Therefore,pursuant to Water Code section 13263.3,Provision G.6 of Order R5-2002-0083 requires the Discharger to develop a pollution prevention plan to limit or reduce the amount of TDS in the effluent. The study will include assessment of the sources of TDS to the RWCF including sources to the collection system and chemical usage at the RWCF (e.g. acidification of effluent to meet lower acute ammonia toxicity). The study shall also include proposal and cost estimates of limiting significant TDS