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903808-SS4 CON-43 <br /> 4.3.5.5.7 The source of the arsenic in the groundwater is not clear. It <br /> is ubiquitous in the soil , yet highly localized in the groundwater. This <br /> argues against the soil as the source. A possible explanation of the <br /> arsenic at the site is that it was used as a pesticide during the early <br /> operations of the Depot. <br /> 4.3.5.5.8 The significance of arsenic as a contaminant on the site is not <br /> totally clear, as Wells LM 1 through LM 46 have not been sampled for <br /> arsenic since the Radian investigation. The sampling round of all wells <br /> scheduled for February 1991 will include arsenic as an analyte. These data <br /> should receive careful examination for inclusion in the separate report on <br /> quarterly monitoring results. The salient point for the current <br /> investigation is that all concentrations detected were all below the MCL <br /> for arsenic (7 ug/l maximum detected versus 50 ug/1 MCL) . <br /> 4.3.5.6 Additional Organic Constituents <br /> 4.3.5.6.0 In addition to TCE and PCE, other organic constituents were <br /> occasionally detected in the groundwater samples collected as noted in <br /> Section 4.0.3. The results obtained from samples collected in 1987, 1988, <br /> 1989, and 1990 are discussed in the following sections. <br /> 4.3.5.6. 1 Purgeable Halocarbons (EPA 601) . The results for the <br /> purgeable halocarbon analyses are presented in Table 4.0-2. Of the <br /> compounds above detection limits, McCI and chloroform are common laboratory <br /> artifacts and are unlikely to be associated with site contamination. The <br /> compound 1,1-dichloroethene was detected in low concentrations (<9.0 ug/1 ) <br /> at five locations where TCE and/or PCE were also detected and may reflect <br /> degradation of either TCE or PCE. The compound 1,2-dichlorethene was <br /> observed only in two samples, with one of the two results measured as being <br /> below detection in a duplicate sample. Because of the low concentration of <br /> the remaining sample (LM 25, 0.8 ug/l 1,2-dichloroethene) , it is possible <br /> that this result may be either a laboratory or sampling artifact. Freon <br /> 113 (1,1,2-trichloro-2,2,1-trifluoroethene) was observed at low <br /> concentrations in four wells with repeated sampling in LM 24 and LM 42 <br /> being above detection. While this compound is commonly a laboratory <br /> 4-187 <br />