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4. 6 Extent of Vadose Zone Contamination <br />' Contours of TPH-g for all soil samples, including those collected <br /> by previous consultants, define a vadose-zone contaminant plume <br />' that is narrow and nearly vertical in the northeast-southwest <br /> direction (Figure 8) . The plume is centered near B1, in the <br /> vicinity of the former gasoline tank cavity. The plume is larger <br /> and somewhat multi-lobed in the northwest-southeast direction <br /> (Figure 9) . The plume has a narrow "neck" where it passes through <br /> the thick silty clay bed that lies at a depth of 25-35 feet, and <br /> forms two lobes in the underlying sand bed. The lobe beneath Bl is <br /> I evident, as is a deeper lobe beneath MW-4 and MW-14 . This lobe is <br /> located beneath the former dispenser island. <br /> Figures 10--14 are contour maps of the vadose zone plume. Above 30 <br /> feet, the plume forms a simple ellipse centered near B1 (Figure <br /> 10) . Where the plume divides into two lobes between 30 and 50 feet, <br /> the northern lobe is larger. Below 50 feet, the southern lobe <br /> (centered between B3 and B4 ) becomes dominant. <br /> Due to low concentrations , contour maps for benzene are less <br /> reliable than those for TPH. Benzene concentrations appear to be <br /> generally proportional to those of TPH, and therefore it is assumed <br /> that benzene contours roughly follow those for TPH. The benzene <br /> concentration map for the 40-45 foot depth interval (Figure 15) , <br /> showing a two-lobed plume, takes this assumption into account. <br /> 4 . 7 Volume of Contaminated Soil <br /> The volume of contaminated soil can be estimated by measuring the <br /> area encompassed by the zero line of contamination for each of the <br /> 10-foot depth intervals depicted in Figures 10-14 . The calculations <br /> are shown below: <br /> Area within zero line: <br /> 20-30 feet: 3 , 600 sq. ft. x 10 ft. <br /> 30-40 feet: 6, 300 sq. ft. " <br /> 40-50 feet: 13 , 275 sq. ft. " <br /> 50-60 feet: 91900 sq. ft. " <br /> Total: 330, 750 cu. ft. 27 cu. ft. /cu. yd. <br /> 12 , 250 cu. yds. <br /> I4 .8 Extent of Groundwater Contamination <br /> Figure 15 shows contours of TPH in groundwater, and Figure 16 shows <br /> contours of benzene in groundwater. The contaminant plumes appear <br /> to be elongated northeast-southwest, parallel to the interpreted <br /> trend of channel deposits underlying the site. The orientation of <br /> 5 <br />