SITE INFORMATION AND CORRESPONDENCE_2005-CURRENT

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t potential to cause or contribute to exceedance of Basin Plan objectives for the dissolved concentration of those constituents, then the permit could be revised to remove the present effluent limitations. In the alternative, the Regional Board explains that the City can submit a site-specific translator study so that revised effluent limitations for total iron and manganese can be developed.27 The Regional Board's approach to establishing effluent limitations for iron and manganese is consistent with guidance provided by the USEPA for NPDES permitting actions," and the State Board concludes that the effluent limitations are appropriate based on the information presently available. As suggested in the Regional Board's response to the petition, however, revision of the effluent limitations may be appropriate if the City submits data that establish that there is no reasonable potential for exceedance of the water quality objectives for dissolved concentrations of iron or manganese, or if the City submits data to support establishment of site-specific translators. C. Mixing Zones and Dilution Credits Contention: The City disputes the Regional Board's denial of mixing zones and dilution credits for use in establishing the permit effluent limitations for MBAS and arsenic. The City also disputes the Regional Board denial of dilution credits and a mixing zone in establishing effluent limitations for compliance with the acute toxicity water quality objectives for copper and cyanide. The City contends that (1) the Regional Board should have established a mixing zone for MBAS and arsenic and allowed a dilution credit of 222:1 based on the harmonic mean flow2' at the Vernalis gage, and (2) the Regional Board should have established effluent limitations based on allowance of dilution credits and a mixing zone of 600 feet from the point of discharge for compliance with the acute toxicity objectives for copper and cyanide. 2' NPDES permits are subject to revision pursuant to the applicable provisions of federal regulations based on the availability of new information that was not available when the permit was issued or based on changes in applicable standards. (40 C.F.R. § 122.62 and Cal. Code Regs.,tit. 23, § 2235.2) 2S "Technical Support Document for Water Quality-Based Toxics Control,"USEPA,March 1991,p. 111. Z' The term"harmonic mean flow,"as used for purposes of determining dilution credits is defined in Appendix 1 of the State Board"Policy for Implementation of Toxics Standards for Inland Surface Waters,Enclosed Bays and Estuaries of California,2000." 16.