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6 <br /> CLEARWATER <br /> G R O U P, I N C <br /> 10 <br />' 6.0 SUMMARY AND DISCUSSION OF SUBSURFACE CONDITIONS <br /> AND FEASIBILITY TEST RESULTS <br />' The extent of soil contamination has been delineated and can be described as a <br /> cylinder Centered near RW-1, which extends from approximately 10 to 17 feet below <br /> grade and has a radius of approximately 50 feet It appears that contaminants from <br /> the former underground storage tanks and associated piping migrated to the water <br /> table with little lateral migration Upon encountering the water table, the <br /> contaminants began to spread laterally The current distribution of soil <br /> contamination probably resulted from "smearing" as the water table fluctuates <br /> (historically between 10 and 16 feet below grade) The approximate weight of fuel <br /> hydrocarbons in the vadose zone and capillary fringe is 5,050 pounds (or -842 <br /> gallons of gasoline) <br /> Groundwater contamination has similar areal extent as soil contamination, and <br /> SPH has been detected in wells MW-1 and MW-2 intermittently Depth to <br /> groundwater has historically fluctuated between 10 to 17 feet below grade and flow <br /> has varied from the northwest to the northeast <br /> Analysis of aquifer test data suggests that transmissivity and hydraulic conductivity <br /> of the shallow water bearing zone are approximately 17,083 gal/day/ft and 1,139 <br /> gal/day/ft2, respectively Groundwater extraction from RW-1 at flowrates greater <br /> than 5 gpm can capture a significant portion of the dissolved plume However, to <br /> completely capture the known extent of the dissolved plume, a second recovery <br /> well, located proximal to MW-1, is needed Coeval pumping from RW-1 and a <br /> second recovery well should exert hydraulic control over SPH detected <br /> intermittently in wells MW-1 and MW-2 <br /> Results from the vapor extraction tests indicate that soil air permeability ranges <br /> from 3 to 5 darcies in the vadose zone and radii of influence for wells tested range <br /> from 17 to 35 feet Although these radii of influence together cover the majority of <br /> I soil contamination, an additional vapor recovery point, proximal to MW-1, would <br /> be needed to completely affect the northernmost extent of soil contamination At a <br /> vacuum of 60 in w c , flowrates from the test wells range from 10 to 60 cfm Results <br /> I from testing indicate that significantly greater air flowrates and radii of influence <br /> can be achieved by simultaneously depressing the water table and removing soil <br /> vapor (dual-phase groundwater and soil vapor extraction) than simply extracting <br /> vapors The contaminants below the site are susceptible to volatilization <br /> Relatively high hydrocarbon recovery rates and large radii of influence achieved <br /> during the tests suggest that soil vapor extraction is a viable remedial alternative It <br /> is likely that a well-designed groundwater and soil vapor extraction system will <br /> significantly enhance contaminant removal from the groundwater and soil beneath <br /> the site <br /> D-107, PAR/RAP 15 February eb y 21 196 9 <br /> I <br />