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40 DATA EVALUATION AND DISCUSSION 41 Petroleum Hydrocarbons in Soil Data obtained from soil borings VW-1, VM-1, and VM-2, utilized with previously obtained soil ' analytical data, provide reasonable delineation of TPHg within the vadose zone and capillary fringe soil zones beneath the site For the purposes of this discussion, vadose zone soil is defined as that portion of soil above the recent historical high groundwater level and seasonal hydrocarbon smear ' zone Capillary fringe soil is defined as the soil zone between the recent historical high and low groundwater levels within which the transport and distribution of hydrocarbon mass has been ' significantly influenced by vertical and lateral groundwater movement Based on laboratory analytical results, the vadose zone plume exists primarily beneath the former ' gasoline and diesel USTs, extending laterally and vertically away from the source location The majority of the TPHg mass in the vadose zone exists in the vicinities of borings VW-1, MW-4, and ' MW-14, with an associated plume volume of approximately 2,000 cubic yards Based on historical groundwater elevation data, the capillary fringe zone beneath the site exists ' between depths of approximately 40 and 65 feet in the vicinity of VW-1 Capillary fringe impacted soil is generally bounded by borings SB-18, AB-2 and MW-13 It is estimated that approximately 3,000 cubic yards of impacted soil exist within the described capillary fringe zone ' Based on an average TPHg concentration of 2,000 mg/kg in the vadose zone, it is estimated that there is approximately 10,000 pounds of TPHg mass in the described vadose zone beneath the site Based on an average TPHg concentration of 200 mg/kg in the capillary fringe zone, it is estimated 1 that there is approximately 1,000 pounds of TPHg mass in the described capillary fringe zone beneath the site The estimated lateral and vertical limits of residual TPHg in soil are depicted on ' Figures 3 and 6, respectively 42 Vapor Extraction Pilot Test Results of the VET indicated that an effective ROI of approximately 55 feet can be expected at an extraction flowrate of approximately 60 scfm and a vacuum of approximately 50 inches H2O This ' ROI estimate is based on a minimum required vacuum of approximately 0 25 to 0 50 inches H2O necessary for effective vadose zone hydrocarbon volatilization The vacuum responses observed in ' wells VM-1 and VM-2 (greater than 2 0 inches of H2O) are approximately an order of magnitude larger than the minimum vacuum response considered necessary for effective remediation Larger ROIs could be achieved by increasing the extraction well flowrate and induced vacuum by ' implementing a VES capable of generating and processing a flowrate greater than 60 scfm at a wellhead vacuum greater than 50 inches H2O Geocon estimates that the utilization a 100 scfm Project No 58100-06-18A -11- December 1996