SITE INFORMATION AND CORRESPONDENCE

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FROM 1997. .14 17:48 #888 P.13/24 February 14, 1997 pace 12 air flow. Screen intervals for each ofthe SVE wells extend from 14 to 44 feet bgs, but because of frictional resistance along the well screen, ' most of the vapor flow occurs over the Lipper portion of the well screen. Additionally, the effects of frictional resistance are compounded due to the fact that horizontal permeability to air flow is typically an order of magnitude greater the verticai permeability. The two factors mentioned would cause mass removal to be localized near the top of the screen interval which is at 14 feet bgs. Data suggest the greatest petroleum hydrocarbon impact currently occurs at the 25-to ,0-foot bgs interval. With respect to general variations in soil perme- ability to air flow, it is well known that the primary variable controlling in-situ remediation is soil permeability. For SVT, local variations in permeability create preferred flow channels which restrict direct contact between soil vapor flow and hydrocarbon impact. In the end, soil permeability will dictate the pace and completeness of active remediation. • Lower weight molecular components of gasoline volatilize more readily than the higher molecular weight components. As a result,the volatilization rate of TPPH-g decreases with time even though hydro- carbon-impacted soils remain within the radius of influence of the SVE wells. • 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, 988;consequently, all primary sources have been removed from the site. The secondary sources of petroleum hydrocarbon contamination include. (1) soils impacted with petroleum hydrocarbons, and(2)groundwater containing dissolved petroleum hydrocarbons. The current concentration and physical extent of these residual concentrations have been defined and significantly reduced by the application of approximately 2 years of SVE which has been performed at the site and documented biodegradation. The operation of the former SVE system was terminated when the J10IJJ1BICLAStrttA