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ti► N%00 <br /> s <br /> bw METHODOLOGY <br /> SOIL VAPOR SURVEY <br /> 6" On June 24, 1987, a soil vapor survey was conducted by EA Engineering, <br /> Science, and Technology, Inc. at the Tracy service center, to aid in <br /> rno delineating the horizontal distribution of volatile petroleum hydrocarbons <br /> in the area surrounding the underground gasoline storage tank. <br /> iffm <br /> The survey involved driving a slotted sampling probe into the soil and using <br /> 60 a vacuum pump to obtain samples of air from soil pores above the water <br /> table. The air samples were collected from the drive tube with a <br /> microsyringe, and analyzed in the field for selected volatile components of <br /> ikW gasoline with a portable gas chromatograph. The gas chromatograph was field <br /> calibrated with 10 ppm standards of benzene and toluene. <br /> Figure 4 shows the sample locations of the soil vapor survey. Thirteen <br /> sample points were chosen in a grid pattern surrounding the tank area to <br /> provide the best estimate of potential migration pathways. A vertical <br /> profile of the vadose zone was initially sampled at location V1, next to the <br /> fuel tank. Vapor samples were extracted from the soil at 3, 5.5, 8, and 9.5 <br /> feet. No detectable compounds were observed at V1, so the survey proceeded <br /> ` to V2. Hydrocarbon vapors were detected at V2 at a depth of 7.5 feet, and <br /> all remaining samples were taken at that depth. This depth was slightly <br /> • above the water table elevation found at that time. <br /> imW Hydrocarbon vapors were detected at three sample locations. The highest <br /> vapor concentrations (instrument response was 11,600 volt-seconds in the <br /> benzene range) were detected at point V2, directly to the north-northeast of <br /> the underground tank. Much lower vapor concentrations were detected <br /> northeast of the tank at V7 (instrument response was 160 volt-seconds in the <br /> benzene range) and southwest of the tank at V5 (instrument response was 14 <br /> volt-seconds in the benzene range) . These results indicate that vapor phase <br /> 1► hydrocarbon migration occurred locally to the northeast and, possibly, the <br /> ` southwest of the tank location. <br /> 3286a/BAV100 11 <br />