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RECEIVED <br /> [� <br /> r�/ GAVIN NEWSOM <br /> JUL. p 2 2Qc6 COVFHNOH <br /> cAuroArl• i/ 1. V YANA GARCIA <br /> Water Boards ENVIRONMENTAL HEALTH ENVIRO ME FOR <br /> ENVIRONMENTAL PROTECTION <br /> DEPARTMENT <br /> Central Valley Regional Water Quality Control Board <br /> 24 June 2026 <br /> Mr. Atir Chak, P.E. Mr. Manpreet Singh and <br /> Contract Program Manager Ms. Sukhwinder Kaur <br /> Phillips 66 Company 5918 Allen Avenue <br /> 1660 West Anaheim Street San Jose, CA 95123 <br /> Wilmington, CA 90744 <br /> FIRST QUARTER 2026 GROUNDWATER MONITORING REPORT, <br /> TOSCO/UNOCAL #05643 AKA: #4409, 1502 NORTH EL DORADO STREET, <br /> STOCKTON, SAN JOAQUIN COUNTY <br /> Dear Mr. Chak, Mr. Singh, and Ms. Kaur: <br /> Central Valley Regional Water Quality Control Board (Central Valley Water Board) staff <br /> reviewed the Semi-Annual Groundwater Monitoring Report First Quarter 2026 <br /> (1 Q2026 Report) dated 29 April 2026, and the Semi-Annual Groundwater Monitoring <br /> and Status Report Third Quarter 2025 (3Q2025 Report) dated 30 October 2025, both <br /> submitted by Atlas Technical Consultants LLC (Atlas) on behalf of Phillips 66 Company <br /> (Phillips 66). The Tosco/Unocal #05643 AKA: #4409 property is located at 1502 North <br /> EI Dorado Street in Stockton (Site). <br /> During the first.quarter 2026, Atlas detected petroleum hydrocarbons in groundwater <br /> samples at concentrations of up to 2,500 micrograms per liter (ug/L) total petroleum <br /> hydrocarbons as gasoline (TPH-G), 97 ug/L benzene, 34 ug/L methyl tertiary butyl ether <br /> (MTBE), and 6.9 ug/L 1,2-dichloroethane (1,2-DCA). During the third quarter 2025, <br /> Atlas detected petroleum hydrocarbons in groundwater samples up to 5,500 ug/L TPH- <br /> G, 1.9 ug/L benzene, 85 ug/L MTBE, and 9.5 ug/L 1,2-DCA. On 9 September 2025, <br /> Atlas collected a water sample from domestic well 10, which contained 32 ug/L TPH-G <br /> and 4.2 ug/L 1,2-DCA. <br /> Atlas observed well construction information written on the side of the well housing that <br /> stated the domestic well's total depth is 146 feet below ground surface (ft bgs), that the <br /> casing ends at 92 ft bgs, and that the well has a 4 inch diameter casing. Atlas does not <br /> know if the well is screened or an open hole below 92 ft bgs. Atlas could not confirm the <br /> pump depth. Atlas interviewed the wells owner, who stated the well is actively used to <br /> supply water to the property and for a cooling system for the residence, and that the <br /> NICHOLAS AVDIS, CHAIR I PATRICK PULUPA, EXECUTIVE OFFICER <br /> 11020 Sun Center Drive,#200, Rancho Cordova,95670-6114 1 www.waterboards.ca.gov/centralvaIley <br />