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J yf, <br /> HYDRO <br /> ENVIRONMENTAL <br /> TUNNOLOGIES,INC. <br /> . 1.2 Background <br /> The site contained three underground storage tanks (UST's) which were installed in <br /> 1970 In July 1989, the tank containing regular leaded gasoline failed a tank test The <br /> station closed in August 1989 with no further testing performed The UST's and <br /> associated product lines were removed from the site in December 1989 Harken <br /> Marketing Company (HMC) submitted a work plan to the Department of Health <br /> Services of San Joaquin County to assess to possible unauthorized discharge of <br /> petroleum constituents <br /> In January 1991, Associated Soils Analysis (ASA) supervised the drilling of three soil <br /> borings and the installation of three monitoring wells (MW-1, MW-2 and MW-3) <br /> each to a depth of 25 feet below grade Detectable concentrations of petroleum <br /> hydrocarbons were present in soil and ground water samples collected from all <br /> monitoring wells Detailed results are presented in the report by ASA titled "Site <br /> Assessment Study for Petroleum Constituents in the Soil and Ground Water at <br /> HMC Location No 100878," dated April 1, 1991 <br /> In June 1992, Tank Protect Engineering (TPE) supervised the drilling of three soil <br /> borings and the installation of three monitoring wells (MW-4, MW-5 and MW-6) <br /> each to a depth of 31 5 feet below grade Detectable concentrations of petroleum <br /> hydrocarbons were present only in soil and ground water samples collected from <br /> monitoring well MSN-5. Depth to ground water beneath the site is approximately <br /> 20 feet below grade, and flows towards the north with a gradient of approximately <br /> 0 002 feet/feet. Detailed results are presented in the Preliminary Site Assessment <br /> Report by TPE, dated August 24, 1992 <br /> 2.0 FIELD ACTIVITIES <br /> All drilling, well construction, and sampling during this recent phase of <br /> investigation was performed in accordance with state and local agency guidelines A <br /> copy of HETI's standard field protocols is attached as Appendix A <br /> 2.1 Soil Borehole Drilling and Soil Sampling <br /> + A safety briefing was conducted prior to drilling on December 4, 1993, with B & F <br /> Drilling personnel of Rancho Cordova, California At the end of the briefing, all <br /> personnel reviewed and signed the Site Safety Plan prepared for this site (Appendix <br /> B) ` Prior to drilling, HETI obtained access for drilling from the current property <br /> owner Well ,installation permits were obtained from the San Joaquin County <br /> Public Health Services Environmental Health Division (Appendix C) <br />' B & F Drilling used a Mobile,B-61 ,drill rig to drill'two 8-inch diameter soil borings, <br /> designated MW-7 and MW-8,;each to' a 'total I depth of '30 feet 'below'grade A <br />+ i I 2 <br />