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REGIONAL BOARD RESP&SE(SWRCB/OCC FILE A-1483) -2- STATE BOARD DRAFT WATER QUALITY ORDER PETITION FOR REVIEW OF WASTE DISCHARGE REQUIREMENTS ORDER NOS. R5-2001-0083 AND R5-2002-0084 CITY OF STOCETONREGIONAL WASTEWATER CONTROL FACILITY discharge. The SJR system is complex. Channels have been rebuilt. Natural flows are stored and diverted. Pumping in the South Delta by the Central Valley Project and State Water Project actually can make the net river flow go backwards. Tides flow in and out, often reversing the direction of the river four times daily so wastes are dumped into the same parcel of water as it moves back and forth, sometimes for days and weeks at a time. Some limited dilution may exist for many pollutants much of the time. However, pollutants combine together in complex ways to increase environmental impact over the impact found for single pollutants. The assimilative capacity of the lower SJR is already diminished due to the stress of low oxygen, high temperature, and pollutants from other sources. Based on a thorough review of actual receiving water flow data, there are critical times when net flows are minimal resulting in little or no dilution. The available flow data are from wet years, so the flows and resulting dilution would be less during critically dry years, for which the NPDES permit must provide protection. For these reasons, the Regional Board determined that no dilution credit be granted for almost all constituents. The City's petition contains substantial information regarding their evaluation of dilution, including water quality models. However, models are only tools for estimating flow or dilution. Real-time river flow data has been measured at Stockton and shows very little flow available for dilution during above average wet years. The water quality models proposed by the City are poorly documented, have not been calibrated or validated, and technical peer reviews were not performed. They should not be used in lieu of real-time data. The evaluation of actual flow measurement at the Stockton USGS flow monitoring station during above average wet years,with reasonable assumptions for critical dry year conditions, is more dependable and protective, and was determined to be appropriate AMMONIA The Regional Board determined that discharges of ammonia had the reasonable potential to cause or contribute to an excursion above the"narrative toxicity water quality objective"in the Basin Plan.' The Regional Board determined that it was appropriate to use USEPA ambient water quality criteria for ammonia as authorized by 40 CFR 122.44(d)(1)(vi)(B). Since ammonia is not listed in the California Toxics Rule or the National Toxics Rule, the Regional Board applied USEPA guidance' to evaluate reasonable potential. To ensure the proper application of USEPA's guidance, Regional Board staff worked closely with USEPA in developing a reasonable potential evaluation. Reasonable potential was clearly demonstrated by four separate methods: (1) identification of toxicity in the RWCF effluent, (2) In part the"narrative toxicity objective"states: "All waters shall be maintained free of toxic substances in concentrations that produce detrimental physiological responses in human,plant,animal, or aquatic life." The Clean Water Act mandates that NPDES permits include effluent limitations as stringent as necessary to meet water quality standards established pursuant to state or federal law. Pursuant to 40 CFR section 122.44(d)(1)(i),NPDES permits must contain limits that control all pollutants that"are or may be discharged at a level that will cause,have the reasonable potential to cause,or contribute to an excursion above any state water quality standard,including state narrative criteria for water quality." With respect to narrative objectives,the Regional Board must establish effluent limitations using one or more of three specified sources, including EPA's published water quality criteria(40 CFR 122.44(d)(1)(vi)(A),(B), or(C). = Technical Support Document for Water Quality-based Toxics Control(EPA/505/2-90-001)