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r ! November'l2, 1996 Page 10 i Lower we16 —MOlecular components of gasoline volatilize more ` - '� readily than the higher molecular weight components As a result the g g p , volatilization rate of TPPH-g decreases with time even though hydro- carbon-impacted soils remain within the radius of influence of the SVE wells i Clays, silts, and organic carbon, components of soils found beneath the site, tend to hold hydrocarbon impact in place This phenomenon limits the fraction of soil bound hydrocarbons that may be removed by physical means Remaining Petroleum Hydrocarbon Impact Available data (groundwater monitoring and site analytical data) indicate that the primary sources of petroleum hydrocarbons were the former gasoline USTs (northeastern portion), and the southern end of the product island located northeast of the former gasoline USTs (Figure 2) These site improvements were removed in 1988, consequently, all primary sources have been removed from the site The secondary sources of petroleum hydrocarbon residuals include (1) soils impacted with petroleum hydrocarbons, and (2) groundwater containing dissolved petroleum _ hydrocarbons The current concentration and physical extent of these residual concen- trations have been defined and significantly reduced by the application of approximately - 2 years of SVE which has been performed at the site The operation of the former SVE system was terminated when the petroleum hydrocarbon concentration in the influent stream reached an asymptotic condition 1 4 � The concentration of the residual hydrocarbons in soils are characterized by the presence of the relatively high molecular weight compounds TPPH-g concentrations range in concentration from less than 10 to 10,000 ppm Benzene with few exceptions has not been detected Residual hydrocarbons as dissolved hydrocarbons are primarily affected by (1) hydrocarbon-affected vadose-zone soils typical r-of source areas, (2) residual hydro- carbons left in soil by groundwater fluctuation, (3) operation of the SVE system, and (4) intrinsic remediation (mechanisms including biodegradation, chemical oxidation, and adsorption/absorption) Dissolved hydrocarbon concentrations in groundwater samples collected from Wells MW-4 and MW-9, located in the source area, have generally decreased with the overall increase in groundwater elevation This trend indicates that SVE was effective in remediating hydrocarbon-affected soils in the source area Dissolved hydrocarbons were not detected in groundwater samples collected from ` 4 3201337BiCLOSLTRE2 I I ' 4 f