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07 July 2003 AGE-NC Project No 95-0142 IPage 4 of 7 collected from the deep screened well MW-6 (screened 70 to 80 feet bsg), located at the former UST TK1-90 area, the benzene was not detected from sample collected during the 03 March 2003 sampling, as compared to the initial value of 7,000 µg/1 detected on 11 December 1998 ' The lateral extent of TPH/BTEX-impacted acted ground water has been assessed down-gradient p and cross-gradient of the former UST area While the lateral and vertical extent of MTBE- impacted ground water has not assessed below a concentration of 20 µg/1 down-gradient of the former UST area (Figure 4) SITE REPORTING In 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 prepared,ground water 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 lie impacted ground water (i e MTBE) when other technologies (e g , air-sparging) may not be controllable or effective Since the approval of the CAP in December 1995, the local and regional ' ground water elevation has risen significantly (20 feet) and has maintained a depth-to-water of approximately 25 feet bsg The vertical extent of hydrocarbon-impacted soil had not been fully assessed when the CAP was developed The mass of adsorbed hydrocarbons now known to be within ' the saturated zone is significant, recent evaluation of soil and ground water below 50 feet bsg has documented significant concentrations of dissolved and adsorbed hydrocarbons that must be satisfactory mitigated to achieve site closure AGE concluded that due to the sustained Increase of ' ground water elevation, yielding a reduction In thickness of the Impacted vadose zone and an increase in thickness, and volume, of impacted soil (and ground water) in the saturated,zone, had significantly decreased the effectiveness of the proposed SVE/IAS method as a stand alone treatment ' for impacted soil and ground water Additionally, the impacted soil is primarily fine grained with discontinuous sand units, which makes the upward and lateral path of sparged air through the saturated zone unpredictable and stripped hydrocarbons unrecoverable,with the added potential for ' dispersion of the dissolved MTBE The dissolved hydrocarbon concentrations are known to extend to 80 feet bsg, where low concentrations suggest that 80 feet bsg is the dissolved plume's vertical extent The anticipated limited overall effectiveness and reliability of IAS technology to remediate ' the Impacted soil and ground water require a re-evaluation of this proposed method In Ground Water Extraction Pilot Test Report dated 31 January 2002 AGE recommended ' Implementation of ground water extraction as an alternative ground water remediation technique aat the site Ground water extraction should provide adequate capture of the dissolved hydrocarbons, Advanced GeoEnvironmental,Inc