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16 October 2006 AGE-NC Project No. 98-0520 ' Page 9 of 10 • Historical high water table seasons are correlated with the highest concentrations of ' hydrocarbon-impacted ground water. • Mass and volume calculations of the hydrocarbon remaining in the soil at the site presented in the AGE-prepared Quarterly Monitoring Report - First Quarter 2006 estimated only 0.524 gallons of gasoline remaining at the site. • The soil vapor extraction pilot test performed by AGE in September 2005 indicated an effective radius of influence from'vapor extraction well VW-I of approximately 80 feet,yet soil vapor samples collected during the pilot test reported no detections of any constituents of concern. • Based upon historical soil samples collected at the site, the highest concentrations of ' hydrocarbon-impacted soil appear to be located at approximately 17 to 20 feet bsg, which is also the transition zone at the site for a sandy silt/silty sand permeable zone that extends from the surface to 20 feet bsg. A less permeable silty clay zone begins at approximately 20 feet bsg and extends 10 to 15 feet further to approximately 30 to 35 feet bsg. ' 0 Vapor extraction well VW-1 is appropriately located within the source region; properly screened from 11 feet to 21 feet bsg within the vadose zone; and fully functionally as evidenced by the 80-foot effective radius of influence. • The small amount of volume of aged gasoline remaining at the site appears to be located and trapped from approximately 17 to 20 feet bsg within the sandy silt zone located directly above the silty clay zone. The aged gasoline appears to have volatilized the most volatile gasoline constituents of benzene and toluene. Lesser volatile gasoline constituents of ethyl-benzene and total xylenes remain in the soil. • SVE does not readily extract the lesser volatile aged TPH-g and the constituents ethyl- benzene and total xylenes as was observed during the 8-hour soil vapor extraction pilot test ' performed by AGE in September 2005. Remediation options to remove a small amount of non-migratory aged TPH-g and its constituents located at 17 to 20 bsg are extremely limited.Potentially,dynamic underground stripping-thermally heating the subsurface through the injection of steam in conjunction with SVE may enhance the ' SVE process by increasing the vapor pressure and volatilization rates of absorbed hydrocarbons to soil.This technique poses several risks and is costly.Risks include volatilizing hydrocarbon vapors deeper into the soil strata or upward to the surface and soil fracturing. If the site cannot be closed, ' then AGE recommends a more aggressive remedial approach using SVE with dynamic underground stripping. 1 . Advanced GeoEnvironmental,Inc. 1