SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

RESPONSE TO WRITTEN COMMENTS -34- City Of Stockton-Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting remaining assimilative capacity for toxicity. Therefore, the Permit fails to comply with Clean Water Act mandates to implement a water quality standard protective of aquatic life for toxicity. RWQCB Response: The permit includes effluent limitations for ammonia that do not allow toxicity. The permit recognizes that the Discharger cannot meet the limitations, and therefore a Cease and Desist Order is proposed. The allowance to modify the acute bioassay to remove ammonia from consideration is not to allow ammonia toxicity in the receiving water, but to allow more reliable determination of any other constituents in the discharge that might cause acute toxicity. In addition, the permit requires the Discharger to conduct a toxicity identification/toxicity reduction evaluation to reduce constituents causing chronic toxicity. DeltaKeener Comment No. 8: Available dilution for priority pollutant human carcinogen criteria was improperly calculated. The period of UVM data represents the wettest cycle in California over the last 200 years. It ignores frequent drought cycles. It ignores average water years. It is entirely improper to use such a truncated data set (November 95 through September 2000) to compute a short-term harmonic mean for purposes of establishing human health carcinogen criteria. Please explain why it is proper to allow a 10:1 dilution ratio based totally on wet year flows? Why were dry year flows and the drier early decades of the century ignored? A reasonable, prudent and conservative dilution credit for human carcinogens, based on long-term flows, would be in the area of 4 or 5:1. RWQCB Response: Two data sets were available to calculate harmonic dilution: 1) 70+years of flow estimates at Stockton modeled from Vemalis flow data which included wet and drought cycles; and 2) 5 years of actual flow data measured at Stockton which included no dry year data. River flows predicted at Stockton from Vemalis flow data frequently do not agree with flows actually measured at Stockton. Harmonic mean flows based on actual measured data was judged more reliable, and so was used for development of effluent limits for human carcinogens. This may over estimate available dilution, however it is very unlikely that a human health risk would exist because: 1) statistically estimated effluent concentrations are well above measured effluent concentrations; 2) there are no drinking water intakes within many miles of the discharge point; and 3) the receiving water standards assume 70 years of water consumption. These effluent limitations will be recalculated in future permits as better flow data sets are developed through time. DeltaKeener Comment No. 9: The permit violates the state and federal anti-degradation policy by allowing mercury and lindane mass loadings to increase pending the uncertain establishment of a pollutant trading system, and allows diazinon and chlorpyriphos mass loadings to increase without restriction. RWQCB Response: The permit contains performance-based mass limitations for mercury, lindane and diazinon, intended to prevent further degradation pending compliance with final effluent limitations. The lindane final effluent limitation, based on the Basin Plan non-detect limitation, is effective within the permit term and is not tied to a pollutant trading system. Chlorpyrifos has not