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detector which appears as an electrical signal. The signal is <br /> amplified, integrated, and recorded as a chromatographic peak. <br /> r <br /> Vapor samples are injected into the gas chromatograph, separated <br /> on an analytical column, sensed by the detector, integrated, and <br /> reported as individual compounds on chromatograms. The gas chro- <br /> matograph is operated in backflush mode to prevent contamination <br /> r of the analytical column with high concentrations of interfering <br /> compounds. The chromatograph was standardized with 10 ppm each <br /> of benzene and toluene vapor. Compound retention time and <br /> response data are stored in the instrument integrator and sub- <br /> sequently used to make identifications and to quantify unknowns <br /> in samples. Blanks were run to ensure that the system was free <br /> of contamination. As necessary, the instrument was re-calibrated <br /> by injecting standards and by running ambient air blanks approxi- <br /> mately every two hours through the day. This re-calibration <br /> ensured that the system was operating consistently and that para- <br /> metric changes caused by temperature changes through the day were <br /> accounted for. These data, along with multiple standard runs, <br /> ensure system reproducibility. The SVCA data are listed in Table <br /> 3 , and isoconcentration contours are plotted for benzene, <br /> toluene, and constituents eluting before benzene in Figures 4, 5, <br /> and 6. <br /> r <br /> 2 . 3 DRILLING AND MONITORING WELL INSTALLATION <br /> r <br /> Four boreholes were drilled on the site, located on the basis of <br /> _ the results of the SVCA survey and on previous investigations. <br /> The boreholes were drilled with a hollow-stem auger of 6" inside <br /> and 11 " outside diameter to a depth of 20 feet (MW4 , MWS , and <br /> ` MW6 ) and 32 . 5 feet (MW7 ) . <br /> From each borehole soil samples were collected at 5-foot <br /> intervals to the water table for soil classification and chemical <br /> r analysis. Two samples were also collected from below the water <br /> table where a strong odor of hydrocarbons was detected. Samples <br /> were collected with a split spoon. Samples of 3" I.D. , 18" long, <br /> L <br /> 13 <br />