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0 <br />1.0 Introduction <br />A release of gasoline occurred at the property from an underground gasoline storage tank that <br />was operated from an unknown time in the twentieth century through the 1950s. Environmental <br />investigation of the release began when the UST was removed in 1996. Soil Vapor Extraction <br />(SVE) was conducted from February 2000 until October 2004 and has reduced the <br />concentrations and mass of gasoline in the unsaturated zone. <br />This work plan is based upon the following conceptual model of the site. <br />There may be a low -mobility mass of gasoline beneath the water table of the property because: <br />• Groundwater has become shallower since the fuel release occurred during and prior to <br />the 1950s. The released gasoline likely moved downward through the unsaturated <br />zone until it reached the water table. Some of the gasoline probably became trapped at <br />that depth when the water table became more shallow. <br />• Four and a half years of SVE have not reduced the groundwater or soil gas <br />concentrations of gasoline enough to discontinue remediation. This suggests that a <br />significant mass of gasoline source exists at the property beneath the water table. This <br />mass has been protected from direct removal by SVE because it has been submerged. <br />An immobile mass of separate -phase gasoline hydrocarbon (SPH) may be present. This mass is <br />probably immobile due to partial saturation of void space and other factors. The presence of <br />SPH would explain the ongoing concentrations of gasoline constituents in groundwater ("bleed <br />off' of constituents in dissolved phase) and the absence of SPH in site wells. <br />-Z:1sacDPIP0051P0188018800391A.5ad.dx 1 -1 <br />