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FACT SHEET ORDER NO. R5-2002- -35- CITY OF STOCKTON REGIONAL WASTEWATER CONTROL FACILITY SAN JOAQUIN COUNTY LATE REVISIONS err-e an the balanee, and the PP-1#1 The mean background concentrations of dibromochloromethane, 1,1-DCE, PCE, bromodichloromethane, and bis (2-ethyhexyl) phthalate exceeded the CTR criteria; therefore, the receiving water has no assimilative capacity for diluting these constituents. However on closer examination, it is apparent that the method reporting level for each of these constituents was greater than the criteria itself. Provision G.9 of this Order requires the Discharger to conduct additional monitoring to evaluate CTR priority pollutants, including organic human carcinogens, using appropriate reporting limits. Based on the findings of the study and the mixing zone study, discussed below, Regional Board staff will evaluate "reasonable potential' for the constituents to exceed criteria pursuant to the SIP, Section 1.3, Step 8, and reopen the permit to establish final effluent limitations. The final limitations may need to be adjusted appropriately such that the combined relative carcinogenic risk for all of the identified constituents is below 1 at a dilution of 10:1, riveneffluent, respectively. The SIP at 1.4.2.2.B requires, among other things, that when a mixing zone/dilution credit is granted, the permit must specify the point in the receiving water where the applicable criteria/objectives must be met. As of this time the Discharger has yet to perform such an analysis over a variety of flow conditions. However, considering the long-term averaging period for human carcinogens, the infrequency of critical conditions and worst-case effluent concentrations, and the fact that there are no drinking water intakes for numerous miles down- or up-stream of the discharge, the Regional Board finds the lack of a detailed mixing zone study is not significant enough to postpone the imposition of final effluent limitations for chloroform, dichloromethane, and TCE. The Discharger shall include in the Human Carcinogen Impact Study as described in Provision G.5, a mixing zone analysis of the effluent discharge into the SJR performed over various flow conditions to delineate the extent of the corresponding"human carcinogen" mixing zone (s). Additional interim reporting requirements, as discussed in Provisions G.6 and G.7, are as follows: • Preparation of a pollutant prevention and minimization program in compliance with CWC 13263.3(d)(3); and • Performance of an engineering treatment feasibility study examining the feasibility, costs and benefits of treatment to remove pollutants from the discharge. 11.4.3 Receiving Water Monitoring for Priority Pollutants Human Carcinogens Receiving water monitoring of human carcinogens will be required to provide assurance that water quality criteria are being met downstream of the mixing zone and that the beneficial use of municipal supply is being protected. Although a mixing zone analysis has not been performed to delineate the specific boundaries of the mixing zone for human carcinogens, the Regional Board finds that receiving water samples collected at the existing R-1 and R-8 receiving water monitoring locations should provide adequate information during the interim