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, - 9 O <br /> Irr <br /> � 3 <br /> r 2 <br /> �I <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 /> y <br /> IL <br /> �a <br /> t The potential subsurface release of hydrocarbons at this facility became <br /> apparent when a soil-gas survey, conducted by Target Environmental Services in <br /> March 1:989 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 rear the <br /> pump islands. <br /> In April 1990 four groundwater monitoring wells were constructed by IT <br /> r� Corporation. A groundwater survey found that groundwater occurred at about 23 <br /> feet below grade, flowed northeast, and contained Total Petroleum Hydrocarbons <br /> as gasoline (TPN-g) at concentrations up to 200 ug/L (ppb). Analysis of soil <br /> samples collected during drflling 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 /> j� 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 /> witnessed by San Joaquin County Public Health Services Department personnel. <br /> ( � Well de.truction is discussed in Section 3.1. <br /> USTs were removed from the site on February 8, 1991, following removel 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 /> encountered in any of the excavations. Gasoline appeared to be widespread in <br /> the fuel UST excavation and absent in the rotor and used oil UST excavation <br /> (Figure 2). A third, new excavation_ (this excavation was into native alluvial <br /> material, rather than backfill) for new fuel USTs also appeared to be free of. <br /> hydrocarbons. Fuel and oil piping was removed on February 21 and fuel <br /> h!A:ENG:9199/O41591 <br /> ti <br /> f : <br />