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r23 Apr112002 • AGE-NC Project No 95-0142 Page 3 of 16 • On 10, 11 and 12 January 2000 ground water monitoring wells MW-7 and MW-8 were ' installed on-site, after a failed sanitary seal (grout) had penetrated the transition seal (bentonite) and Infiltrated the sand filter pack ys ' To date a total of 18 ground water monitoring events have occurred at the site ' Assessment activities at the site to date Indicate that • The vadose zone at the site is composed of fine grained sediments,below the water table,the hydrologic structure is composed primarily of fine grained sediments with discontinuous sand units • The majority of the unsaturated, hydrocarbon-impacted soil resides below former UST ' TK1-90, to a depth of 30 feet below surface grade (bsg) Saturated hydrocarbon-impacted soil extends to a depth of 80 feet bsg, however, below 40 feet bsg, hydrocarbon concentrations generally decrease with depth The mass of hydrocarbons in the unsaturated soil is greater than the saturated soil (Figures 3a and 3b) • The highest concentration of hydrocarbon-impacted ground water has remained below '. former UST TKl-90 The maximum benzene concentration was 41,000 micrograms per liter (µg/1), detected in a water sample from vapor extraction well VW-1, located at the former UST area • TPH-g and BTEX concentrations have steadily declined in the ground water samples collected from the deep screened well MW-6 (screened 70 to 80 feet bsg), located at the t former UST TK1-90 area, the benzene concentration detected on 28 January 2002 was 2 1 µg/1 compared to the Initial value of 7,000 µg11 detected on 11 December 1998 ' The lateral extent of hydrocarbon-impacted ground water has been assessed down-gradient and cross-gradient of the former UST area(Figure 2) i24 FEASIBILITY TESTING OF ON-SITE GROUND WATER EXTRACTION iIn Corrective -Action Plan - November 1995, dated 15 November 1995, AGE recommended implementation of soil vapor extraction in conjunction with in-situ air sparging for remediation of the petroleum hydrocarbon-impacted soil and ground water at the site At the time the CAP was iprepared,groundwater extraction was generally not viewed as an effective and cost efficient method of ground water remediation,but this method has gained more favor recently as a method to address i impacted ground water when other technologies (e g , air-sparging) may not be controllable or effective, due in part to reduced extracted water treatment and disposal costs Advanced GeoEnvtironmental,Inc i