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detected in groundwater samples, even though the depth to groundwater has not been less than 38 <br /> feet since this investigation began This suggests that groundwater was shallower than 35 feet at <br /> some time in the past and was impacted by gasoline within the channel deposit at that time <br /> 5.3 Volume of Contaminated Soil <br /> The volume of contaminated soil can be calculated from Figure 6 For ease of calculation, the <br /> BTEX plume is considered to be cylindrical in shape (see Figure 7), with a radius of 25 feet and a <br /> height of(35-15) =20 feet Hence, <br /> Vs.., = n r2 h =7r (25) (20) = 39,270 ft' = 1,454 yds 3 <br /> 5.4 Estimated Volume of Gasoline Remaining in Place <br /> The volume of gasoline (in gallons) that remains to the soil can be computed by multiplying the <br /> volume of contaminated soil by the average soil porosity and the average hydrocarbon <br /> concentration The mean porosity of the samples tested by Sierra Tesing is 43% (Appendix C), <br /> and the mean hydrocarbon concentration, estimated from Figure 5, is 1,000 ppm <br /> Vgas = Vso,l x mean porosity (0) x mean concentration (ppm) x 7 48 gal/ft3 <br /> = 39,270 ft3 x 43 x 1,000 x 10-6 x 7 48 gal/ft3 <br /> = 126 gallons <br /> 6.0 GROUNDWATER RESULTS <br /> Two samples of groundwater were collected from boring HP-4 and analyzed for petroleum <br /> hydrocarbons Sample HP-4 was collected with a hydropunch at a depth of 39-42 feet, whereas <br /> sample HP-4-75w was collected at a depth of 75 feet with a Simulprobe As shown in Table 4, <br /> both samples contained gasoline compounds, but neither contained oxygenated additives As <br /> would be expected, concentrations were much higher in HP-4 In fact, the concentrations exceed <br /> those detected to the nearest soil sample (HP-4-32'), and are about an order of magnitude higher <br /> than those that had been detected previously in VM-2 <br /> 9 <br />