SITE INFORMATION AND CORRESPONDENCE_2005-CURRENT

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Findings: A mixing zone is an area within which water qualityobjectives do not apply. The.Basin Plan provides that the Regional Board may designate mixing zones provided that the discharger has demonstrated to the satisfaction of the Regional Board that a mixing zone will not adversely impact beneficial uses. (Basin Plan, p. N-16.00.) Guidance regarding use of mixing zones and dilution credits for toxic pollutants is provided by the State Board"Policy for Implementation of Toxics Standards for Inland Surface Waters,Enclosed Bays and Estuaries of California, 2000" (State Implementation Policy or SIP). The SIP provides that allowance of mixing zones is discretionary and shall be determined on a discharge-by-discharge basis.30 Other regulatory documents applicable to establishment of mixing zones and dilution credits are summarized in the Regional Board's Information Sheet for Order No. R5-2004-0028. As mentioned above, flows in the river near the City's discharge are influenced by tidal conditions. During critical low flow periods, upstream flows occur on the flood tide,no flow occurs during the slack tide, and downstream flows occur during the ebb tide. The result is that the receiving water may receive multiple doses of effluent as tidal conditions affect the direction of flow in the river. The Regional Board's evaluation of dilution availability in the vicinity of the City's discharge included consideration of the effluent constituents in question and the complex dynamics of the flows in the river, tidal flows, and the intermittent side bank discharges from the City and from the Brown Sand outfall located 50 feet downstream." The availability of dilution for the constituents specified in the City's petition is addressed below. MBAS: The City's permit includes a monthly average effluent concentration limit of 500 ug/l for MBAS based on the secondary MCL for foaming agents and the Basin Plan narrative objective for chemical constituents which prohibits floating material in amounts that 30 The SIP defines a dilution credit as"a numerical value associated with the mixing zones for the receiving water entrained into the discharge. The dilution credit is a value used in the calculation of effluent limitations: Dilution credits may be limited or denied on a pollutant-by-pollutant basis,which may result in a dilution credit for all,some or no priority pollutants in a discharge" (SIP,p. 13.) " Order No.R5-2004-0028,pp. 12 and 13,and Regional Board Inforrmtion Sheet for Order No.R5-2004-0028, pp. 1-6. The City submitted modeling results and an analysis developed by its consultants to support the City's conclusion that the discharge would have very little effect at downstream locations. The City's modeling results focused on thermal impacts of its discharge to the San Joaquin River. The Regional Board found that the model had not been calibrated for the location in question,that dilution and plume dimensions for pollutants were not determined under critical conditions that have occurred at the outfall,that the modeling was not run for sufficient time to properly account for tidal cycles and associated recirculation, and that there was no assurance that the plume dimensions of in-stream dilution information provided by the modeling were accurate for the City's discharge. 17.