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RESPONSE TO WRITTEN COMMENTS -20- City Of Stockton - Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting calculated accounting for the available dilution." The Regional Board has not approved the findings of the Jones & Stokes report. Detailed comments on the report are found in Attachment B. i S!2 kton Comment,No. 1V.K(Cyanide): In establishing effluent limitations, no dilution was - - allowed. RWQCB Response: The Regional Board has not approved the findings of the Jones & Stokes dilution report. Detailed comments on the report are found in Attachment B. Stockton Comment No. IV.L (PCBs and Dioxin/Furans): The listing of the Deep Water Ship Channel as being impaired by PCBs and dioxin/furans from the McCormick and Baxter site in the Port of Stockton does not justify the requirement that the City collect additional data for these pollutants. RWQCB Response: The provision to provide monitoring data for PCBs and dioxin/furans is required by the SIP. Provision G.9 of the tentative order is intended to be consistent with the requirements of the technical report request issued by the Regional Board on 10 September 2001. As stated in the technical report request, the City is not required to monitor for constituents for which they have already monitored as long as the data meet the requirements of the request. Stockton Comment No. IV.M (Carcinogen Impact Study)' The proposed order seeks to require a Human Carcinogen Impact Study without complying with either Porter-Cologne Section 13267 or Sections 13000 and 13001. The legal and technical bases for this requirement are not known. The City objects to this requirement, as the Board has not adopted any provision stating that a 10-6 risk may not be exceeded cumulatively in an effluent. This is a major new requirement that must undergo rulemaking if it is to be enforced. As the City has already completed its mixing study and confirmed the extent of the chronic mixing zone, further study regarding the delineation of the human health mixing zone is not necessary. RTVQCB Response: Water Code section 13383 authorizes the Regional Board to require dischargers subject to NPDES permits to, among other requirements, sample effluent,retain records, and provide other information as reasonably required. The requirement to conduct a carcinogen impact study is consistent with Section 13383. It is also consistent with Water Code section 13267. The Basin Plan at IV-18.00 states, "Pollutants which are carcinogens or which manifest their toxic effects on the same organ systems or through similar mechanisms will be considered to have potentially additive toxicity". The human carcinogens to be studied may exhibit additive toxicity, and need to be addressed together. In addition, the extent of the mixing zone for human carcinogens is based on the harmonic mean dilution rather than the chronic mixing zone, and are not comparable. In order to grant a dilution credit for human carcinogens, a mixing zone must be granted. The SIP requires that the size of the mixing zone be delineated. The requirement is not based on any new rulemaking. No change was made based on the comment because the findings are adequate to explain the need for the study. Stockton Comment No. IV.N (Thermal Discharge Re(mirements): Finding 15 evaluated the City's compliance with Basin Plan thermal discharge requirements and determined that, although the