SITE INFORMATION AND CORRESPONDENCE_2005-CURRENT

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relied was not calibrated to site-specific conditions.35 The Regional Board found that the City's modeling did not adequately account for local flow conditions,tidal influences, and the discharge of Brown Sand, Inc. immediately downstream. In concluding that dilution credits should not be allowed in establishing compliance with acute toxicity criteria,the Regional Board cited the limited mixing of the City's side-bank discharge near the City's outfall,the commingling with the adjacent NPDES discharge of Brown Sand, Inc., the periods of slack tide that can occur during low flow conditions, and the one-hour exposure criteria that the acute toxicity criteria are intended to protect. The evidentiary record and the findings in Order R5-2004-0028 support the Regional Board's decision to establish effluent limitations for copper and cyanide based on compliance with acute toxicity criteria at the point of discharge. D. Restrictions on Periods of Discharge Contention: The permit contains a condition limiting discharges from the City's WQCF to periods of outgoing tides. The City contends that the condition should allow discharges during periods of net downstream flow of 0.5 foot per second rather than restricting discharges to periods of outgoing tides. Findings: Tidal influences can cause periods of reverse flow or no net flow in the river at the point of the City's discharge. Therefore, it was reasonable to limit the City's discharges to periods when flow conditions do not result in a build-up or concentration of pollutants in the immediate vicinity of the City's discharge. The requirement limiting discharges to periods of outgoing tides was based on a condition described in the City's environmental impact report (EIR). However, the minor change requested in the City's petition would more directly address the objective of preventing accumulation of discharged effluent in the vicinity of the outfall. The conditions of the permit should be revised to allow for discharge during periods of net downstream flow of 0.5 feet per second or more as proposed by the City. E. Cost of Compliance With Effluent Limitations Contention: The City contends that the Regional Board gave inadequate consideration to the high cost of permit compliance in view of the limited water quality benefits that are expected to occur. " The City's August 29,2002,response to comments from the Regional Board on the City's permit application states: "the RMA-10 model was not calibrated since there are no comprehensive data sets at low flows" 19.