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placed at the northern edge of the site. These sampling locations were <br /> chosen based on site topography, meteorological data, and the pattern of <br /> land use at CHSL. In addition, a wind direction-controlled ambient air <br /> sampler was placed immediately adjacent to each 24-hour sampler. <br /> In ambient air sampling proceedings, benzene, methylene chloride, <br /> tetrachloroethylene, carbon tetrachloride, 1,1,1-trichloroethane, <br /> trichloroethylene, and chloroform were detected in one or more samples. <br /> Benzene, carbon tetrachloride, and chloroform concentrations were below <br /> their regulatory detection limits of 2 ppb, 0.2 ppb, and 0.8 ppb, <br /> respectively. Field blanks collected during the ambient air sampling <br /> events contained benzene, methylene chloride, 1,1,1-trichloroethane, <br /> trichloroethylene, and chloroform (2). The presence of these compounds <br /> in field blanks suggests that laboratory contamination of samples may <br /> have occurred. For detailed ambient air sampling results, see <br /> Appendix D. <br /> Ambient air samples collected on March 2 and 3, 1988 generally have <br /> higher methylene chloride concentrations than the February 17, 1988 <br /> samples. Examination of field blanks for the March sampling revealed <br /> methylene chloride, suggesting that the sample was contaminated during <br /> collection or analysis (2). <br /> Comparison of upwind and downwind VOC concentrations indicates that <br /> upwind concentrations were similar to downwind concentrations with a few <br /> exceptions. Downwind concentrations of methylene chloride (58 parts per <br /> billion) and 1,1,1-trichloroethane (38 parts per billion) were higher <br /> than upwind concentrations for one sample (Sample AA-13 in Appendix D) <br /> collected on March 3, 1988 but EMCON has stated that this may have been <br /> due to a low sample volume caused by an equipment malfunction. EMCON <br /> did not explain why concentrations of other compounds were not similarly <br /> elevated in this particular sample. <br /> Another downwind sample collected on March 3, 1988, contained 4.0 ppb of <br /> methylene chloride and 3.7 ppb of TCA--over three times the upwind <br /> concentrations for TCA. In addition to the 24-hour ambient air study, <br /> 3-13 <br /> sa/tm/chl/si <br />