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1 6.2.2.2.2 Injection Wells None of the present wells at the site are suitable for air sparging. Use of this technology would require the installation of several injection wells, screened at a depth of between 80 and 85 feet in order to mobilize contaminants at 70 to 75 feet. We estimate that these wells ' could create a cone of influence 40 feet in diameter at the air-water interface (55 feet). Between 6 and 10 injection wells would likely be required in order to intercept the entire ' plume of contaminated groundwater. Some of the monitoring wells drilled to date are known to have encountered low- permeability beds that could cause channeling of air flow within the upper part of the saturated zone. Although there is a silty clay bed between 40 and 45 feet in the vicinity of RW-1 and MW-1, both wells have at least 10 feet of screen between this bed and the air- water interface. A thicker silty clay interval is present in MW-4 and MW-6 between the depths of 40 and 60 feet, and in MW-7 and MW-8 between 40 and 70 feet. These wells are screened through this bed. ' 6.2.2.2.3 Extraction Wells The injection wells would be located so as to encircle RW-1 and groundwater monitoring wells MW-1., MW-2, and MW-3. All of these are 4-inch diameter wells with long screen intervals above the water table and are located within the plume of contaminated groundwater, making them suitable as vapor extraction wells. The need for additional extraction wells could be evaluated after several months of monitoring. ' 6.2.2.2.4 Monitoring Activities One of the potential problems associated with air sparging is lateral spread of the contaminant plume. This is due either to mounding of groundwater above injection points or to lateral air flow away from sparge wells. To detect this problem, extraction and groundwater monitoring wells should be monitored weekly during the first three months of ' operation. This can be done as part of the monitoring of the related vapor extraction system. The depth to groundwater should be measured to detect mounding, and organic vapors and ' dissolved oxygen should be monitored to detect increases in these parameters. 6.2.2.2.5 Duration ' Air sparging in highly permeable aquifers generally results in rapid reduction of hydrocarbon concentrations in the saturated zone. Pilot tests have not been conducted at this site, but ' consideration of aquifer transmissivity, vapor extraction pressures, and the relatively low concentrations of contaminants (except in the vicinity of MW-1) all suggest that groundwater - remediation could be accomplished within 12 months. Geabyal AYdil Scrviti..l.a 3^1 ARA FAR/1-28.914A.47 !