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06/24/2002 15 42 2094671118 AGE 5TOCKTON PAGE 08 31 January 2002 AGE-NC Protect No 95-0128 Page7of8 screened interval located within a silt(Appendix K) The geotechnical analysis of the soil at the site indicated a best permeability of IX10'cm/s (Appendix J) • Laboratory analysis of extracted water during the pumping test detected elevated concenti ations of TPH-g, BTEX and MTBE (Appendix H} 5.0. RECOMMENDATIONS In Corrective Action Plan - November 1995, dated 15 November 1995, ,A GTE recoznmended implementation of soil vapor extraction in conjunction with in-situ air sparging for remediation of the petroleum hydrocarbon-impacted soil and gi ound wager at the site At the time the CAP was prepai ed,gi ound water exti action was generally not viewed as an effective and cost efficient method of giound watei remediation, but has gained more favor recently as a method to address impacted ground water, due to seduced treatment and disposal costs, where air-sp uging may not be conte ollable of effective Since the appi oval of the CAP in December 1995,the local and regional ground water elevation has nsexi significantly and has maintained an elevation at approximately 25 feet bsg AGE believes that the sustained inei ease of groundwater elevation has significantly decreased the potential effectiveness of the pi oposed SVE/IAS as a stand alone treatment fol impacted soil and ground watei due to a ;eduction of the thickness of the impacted vadose zone and an increased thickness, and volume, of impacted soil (and gi ound water) in the saturated zone The impacted soil is primarily fine grained, which may male the upward and lateral path of sparged air through the saturated zone unpi edictable and stripped hydi ocal bons uni ecovei able The vertical extent of hydrocatbon-unpacted 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 concenti ations of dissolved and adsoi bed hydrocarbons that must be satisfactory mitigated to achieve site elosw e At this time, dissolved hydi ocarbon concentrations are known to extend to 80 feet bsg, where low concentrations suggest that 80 feet bsg is close to the adsoibed plume margin The anticipated limited overall effectiveness and i eliability of LAS technology to remediate impacted soil and gi ound water requii e a re-evaluation of the pi oposed method to remediate ground water and seduce the hydrocaibon mass AGE recommends the use of ground watei extraction as an alternative ground water remediation technique at the site Gi ound water extraction should provide adequate capture of the dissolved hydrocaibons based on site-specific hydrogeologic conditions and hydrocarbon distribution The limited areal distribution of high concentiations of dissolved hydrocarbons near the source of the release £avois a high potential for capturing the major portion of the dissolved hydrocarbons on the site A, ground watei extraction system in the soui ee area and along the down-gradient portion of significantly impacted ground water should effectively address the plume of impacted ground water Advanced GwEnvironmcotal Inc