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REGIONAL BOARD RESPONSE (SffRCB10CC FILE A-1483) -8 PETITION FOR REVIEW OF WASTE DISCHARGE REQUIREMENTS ORDER NOS.R5-2002-0083 AND R5-2002-0084 CITY OF STOCKTON REGIONAL WASTEWATER CONTROL FACILITY contains an implementation policy("Policy for Application of Water Quality Objectives")that specifies that the Regional Board`will, on a case-by-case basis, adopt numerical limitations in orders which will implement the narrative objectives." Basin Plan p. IV-17.00. It also states that the Regional Board will consider"relevant numerical criteria and guidelines developed and/or published by other agencies and organizations." With respect to constituents listed in the Califomia Toxics Rule or National Toxics Rule, the Regional Board applies the State Board "Policy for Implementation of Toxics Standards for Inland Surface Waters, Enclosed Bays, and Estuaries of California (2000)" (SIP) in making the reasonable potential analysis. With respect to other constituents, the Regional Board applies USEPA guidance"Technical Support Document for Water Quality-Based Effluent Limitations" (TSD) in making the reasonable potential analysis. With respect to ammonia, the Regional Board applied the USEPA guidance in making the reasonable potential analysis because it is not listed in the CTR or NTR. 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.2 The Regional Board determined that it was appropriate to use the USEPA ambient water quality criteria for ammonia as authorized by 40 CFR 122.44(d)(1)(vi)(B). The USEPA 1999 Update of Ambient Water Quality Criteria for Ammonia provides the latest applicable water quality criteria for ammonia. Section 4.3.3 of the TSD allows the consideration of exposure duration in evaluating toxicity to organisms passing through a mixing zone. When evaluating either an acute or chronic mixing zone for ammonia, the pH of the mixture of effluent and receiving water should be used to determine appropriate criteria to be applied within that mixing zone. The pH in the mixing zone will be a function of the effluent pH and the ambient dilution water pH being mixed together. The pH is an important factor because toxicity of ammonia increases logarithmically as pH increases. It is important to note that the lack of receiving water data(pH,temperature and ammonia concentrations) near the vicinity of the outfall and within a projected mixing zone, limits the accuracy of the evaluation of the acute and chronic ammonia criteria. However, reasonable worst-case assumptions can be made to evaluate both acute and chronic ammonia criteria. The most stringent acute ammonia criteria occur when salmonids are present within the water columna The SJR at Stockton is a migratory path for salmon moving to and from the SJR watershed. Salmon are likely to be present in the river at any time of the year3. The chronic ammonia criteria are most stringent when early life stages (ELS) of aquatic species are present. In response to a request for information regarding the time of year ELS of aquatic species are present in the SJR near the DWSC, a Department of Fish and Game memorandum3 states ELS of multiple fish and invertebrates species are present in the SJR in the vicinity of the RWCF outfall year-round. Therefore, the approach for evaluating both acute and chronic ammonia toxicity is based on the assumption that both salmonids and ELS of aquatic species are present in the SJR near the RWCF outfall year-round. r 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." 3 Department of Fish and Game memorandum, dated 27 February 2001 (Administrative Record,Binder 3,Item 12 1)