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C' 411d <br /> Chevron U.S.A. - Page lb <br /> During our review of the analytical reports prepared for the subsoil samples <br /> obtained from Dames & Moore's soil borings, a presence of benzene and toluene was <br /> observed in a number of samples analyzed from below the 26 foot -depth horizon. <br /> The concentrations observed were less than 1 part per million for each species <br /> below the 26 foot horizon. <br /> In soil borings B-1 and B-3, there was a distinct break between samples with <br /> the small concentrations of toluene, where non-detected samples were observed <br /> to occur vertically between samples with detectable concentrations. In soil <br /> boring B-2, toluene and benzene were detected to approximately ?Q feet below the <br /> existing grade. However, a notable attenuation in concentration was observed to <br /> occur :at approximately 26 feet below grade in the B-2 soil boring samples <br /> analyses. <br /> Review of the logs of soil borings were interpreted to suggest a zone of <br /> less permeable soils in the vertical regions where non-detected results of <br /> i <br /> chemical analysis were reported. The migration of low concentrations of benzene <br /> and toluene (less than 1 part per million concentration below 26 feet) through an <br /> interpreted less permeable horizon may have volatilized causing vapor-phase <br /> distribution of benzene and toluene below the interpreted less permeable horizon. <br /> In the Woodward-Clyde report dated September 2, 1988 for an investigation <br /> conducted at the subject property, a summary of data regarding soil samples <br /> obtained from exploratory drilling and sampling, and chemical analysis for <br /> petroleum hydrocarbons was presented. The results of those analyses had shown <br /> detectable concentrations of benzene in samples obtained from their soil boring <br /> DAMES & MOORE <br />