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I Final Remedial Plan - Geweke ' 2 January 1992 Page 5 of 15 ito be implemented. :Technologically, non-detectable levels' of hydrocarbon contaminants - could be achieved. However, due to ' budgetary constraints, ,we propose to perform the mitigation -until an acceptable remaining contamination level is achieved. For vacuum extraction acceptable contaminant levels are reached ' upon the indication of a significant decrease in the concentration of volatile organics in the recovered vapor; typically at 500 parts per million (ppm) as' hexane. Subsequently, confirmatory soil sampling will . be conducted to 'document the concentration of the contamination left in place, if' any. Section 3 . 7 . 7 . covers the confirmatory sampling plan. The filtration goal of the water treatment program is to achieve the State Drinking Water standard (DWS) prior to discharging the treated water back into the aquifer. The--plan -ca-lls for treatment of between 40-,000 and 100,-000 gallons of water, at "which time -a point of diminishingreturns is anticipated, to. be encountered. Upon - reaching this-- point of - diminishing return, a passive monitoring program for the ground water needs to be implemented on a quarterly basis. 3 . SOIL REMEDIATION ' 3 . 1. PHILOSOPHY AND BACKGROUND OF THE CHOSEN TREATMENT METHOD Several soil mitigation methods for this site were considered; ranging from soil removal followed by aeration/treatment to in-situ ' treatment programs. Based ' upon the extended depth of the contaminated zone (approximately 60 feet) , an excavation method does not seem to be feasible. The operation and the integrity of the site could be severely impacted. Accordingly, we believe that an in-situ treatment is the most effective method of mitigation. Due to the strict air pollution regulations in Southern California, which were passed by the South Coast Air Quality Management District (SCAQMD) in 1987, GeoAudit has successfully designed and completed several soil treatment projects using vacuum extraction, ' both on the surface and in-situ. Our vacuum extraction unit uses an internal combustion engine to destruct hydrocarbon contaminants. The engine power is. used to drive the vacuum blower. Given sufficient vapor saturation and composition, the engine operation can be sustained by the recovered vapor. To maximize the destruction ratio, a provision to co-fire the engine with natural gas is provided. By destructing the contaminants rather than ' transferring them into another medium, there is no need to deal with spent filters, e.g. activated carbon, at the end of the mitigation period.