SITE INFORMATION AND CORRESPONDENCE_2005-CURRENT

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cause nuisance or adversely affect beneficial uses. Non-contact recreation, including aesthetic enjoyment, is identified as a beneficial use in the Basin Plan. There is evidence in the record of foaming in the vicinity of the City's discharge. Considering the relationship between foaming and MBAS in concentrations exceeding the secondary MCL, and in view of the presence of foaming at the City's point of discharge, the Regional Board correctly applied the effluent limitation for MBAS at the point of discharge, and did not provide for dilution credit and establishment of a mixing zone. Arsenic: The City's permit includes monthly average effluent concentration limits of 10 ug/1 based on the Basin Plan numeric water quality objective for arsenic in the receiving water." The permit does not provide for dilution credit and a mixing zone. The Regional Board found that the plume from the City's discharge would overlap with the plume of the discharge from Brown Sand, Inc.,which also includes arsenic and is located 50 feet downstream. The SIP provides, among other criteria, that a mixing zone shall not overlap a mixing zone from different outfalls and that a regional water quality control board shall not approve mixing zones in violation of the conditions in the SIP." Establishing a mixing zone for arsenic that would overlap the downstream plume from Brown Sand, Inc. would not comply with the provisions of the SIP. In this instance, the problems associated with allowance of a mixing zone and dilution credits for arsenic would be compounded during low flow and reverse flow conditions in the river at the point of discharge. The record supports the Regional Board's denial of a mixing zone and dilution credits in establishing the effluent limitations for arsenic.34 Copper and Cyanide: In establishing the effluent limitations in the City's permit for compliance with acute toxicity criteria for copper and cyanide, the Regional Board did not allow for the dilution credits and mixing zone requested by the City. The Regional Board found that the modeling results submitted by the City were questionable due to a lack of data to calibrate and validate the model. In August 2002, the City conceded that the model on which it 'Z The 10 ug/1 concentration limit for arsenic in the City's effluent, as established in the permit. is equal to the numeric water quality objective of 0.01 mg/I for arsenic in the receiving water as specified in Table III-1 of the Basin Plan. " SIP,P. 15. 34 The record indicates that the City's proposed treatment of their source water will result in producing effluent with an average arsenic concentration of 8 ug/l,which is below the average monthly permit effluent limitation of 10 ug/l. 18.