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_ _ <br /> MEMO <br /> 1.0 INTRODUCTION <br /> Seven underground storage tanks (USTs) were excavated on February 8, 1990, and <br /> ►- replaced in February and March, 1990, at BP Oil Station No. 11192, located at <br /> 1403 Country Club Boulevard in Stockton, California (Figure 1). This work was <br /> performed as part of a general renovation of the facility. <br /> i <br /> The potential subsurface release of hydrocarbons at this facility became <br /> apparent when a soil-gas survey, conducted by Target Environmental Services in <br /> i a <br /> March 1989 confirmed the presence of benzene, toluene, ethylbenzene, and <br /> xylene (BTEX), and other volatile organic compounds in soil-gas. These <br /> detections were interpreted to represent gasoline and were greatest near the <br /> pump islands. <br /> In April 1990 four groundwater monitoring wells were constructed by IT <br /> j Corporation. A groundwater survey found that groundwater occurred at about 23 <br /> feet below grade, flowed northeast, and contained Total Petroleum Hydrocarbons <br /> w.N as gasoline (TPN-g) at concentrations up to 200 ug/L (ppb). Analysis of soil <br /> samples collected during drilling revealed the presence of benzene and <br /> ethylbenzene at levels below 1.0 mg/kg (ppm), at a depth of 20 feet in the <br /> downgradient vicinity of the fuel USTs. <br /> Monitoring well MW-2 was destroyed on February 4, 1991, shortly after it was <br /> determined that it was within an area which was to be excavated (Figure 2) for <br /> new fuel USTs (the "new" excavation). Well destruction was permitted and <br /> �s <br /> witnessed by San Joaquin County Public Health Services Department personnel. <br /> Well destruction is discussed in Section 3.1. <br /> USTs were removed from the site on February 8, 1991, following removal of <br /> pavement, pump islands, and a sales building. Fuel USTs appeared in excellent <br /> condition and oil USTs appeared corroded but intact. Groundwater was not <br /> A <br /> encountered in any of the excavations. Gasoline appeared to be widespread in <br /> the fuel UST excavation and absent in the motor and used oil UST excavation <br /> 3 (Figure 2). A third, new excavation (this excavation was into native alluvial <br /> -- — — -* ---- materials rather than-backfill -for-new-fuel-USTs also ape — — -- -- --- <br /> <`. } pp.ared._..to.,.be...free-o f -- <br /> hydrocarbons. Fuel and oil piping was removed on February 21 and fuel <br /> �,TA:ENG:9199/041591 <br />