CORRESPONDENCE_AGENCY CORRESPONDENCE

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Tosco/Unocal #05643 AKA #4409 - 2 - 24 June 2026 property is not connected to city water. Atlas does not know the drinking water source for the property occupants. Site wells are screened in four zones. A zone wells are screened between 21 and 50 ft bgs. B' zone wells are screened between 50 and 68 ft bgs. B zone wells are screened between 65 and 83 ft bgs. C zone wells are screened between 103 and 121 ft bgs. Based on the domestic well construction information observed by Atlas, the domestic well is screened below the B zone and extends past the C zone. Atlas concluded that based on Site data, that the 1,2-DCA reported in well 10 did not likely migrate from the Site, and that more likely there are regional TPH-G and 1,2-DCA plumes from off-Site sources. Atlas recommended continued semi-annual groundwater monitoring and evaluation of Site conditions against the State Water Resources Control Board's Low Threat Closure Policy. Central Valley Water Board staff has the following comments: 1. Please add domestic well 10 to the analytical data tables in future reports. Inclusion of this information will allow for better transparency of Site data to the public when reviewing reports. 2. Please sample domestic well 10 during the third quarter 2026 monitoring event to confirm 1,2-DCA and TPH-G detections. Please submit the third quarter 2026 groundwater monitoring report by 30 October 2026. 3. Based on a review of data from closed leaking underground storage tank cases in the area that are upgradient of domestic well 10, we concur that there appear to be other contributing sources to the 1,2-DCA concentrations detected in the well. The UNOCAL #2859 case, located at 1665 Pacific Avenue in Stockton, contained 1,2-DCA in groundwater at up to 280 ug/L in 2002, and their most down-gradient well contained 10 ug/L at the time of closure in 2010. Arco #4932, located at 16 East Harding Way in Stockton, contained 1,2-DCA in groundwater at up to 440 ug/L. During the last groundwater monitoring event in 2013, Arco #4932 wells contained 1,2-DCA at up to 8.8 ug/L, and at up to 2.5 ug/L in the three most down-gradient wells. There may be other cases that didn't analyze for 1,2-DCA that could also be contributors. Your Site has a historic maximum 1,2-DCA concentration of 47 ug/L, detected in 2007. Based on these cases having higher 1,2-DCA concentrations than your Site and the concentrations in down-gradient wells at these Sites at the time of closure, we suspect that the UNOCAL #2859 is the most likely contributor to the impacts observed in domestic well 10. However, we also believe that your Site, and ARCO #4932 may also be likely contributors. Staff requests that you notify us at least five business days before resampling Domestic Well 10 so that staff can collect a split-sample.