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PROBLEM ASSESSMENT REPORI74NTERIM REMEDIAL ACTION PLAN -16- Former Beacon Station No. 494, 2185 East Fremont Street Stockton, CA—AMV Project No. 19005.07 Because migration control 1s essential for implementation of this alternative, in situ bioremediation is not viable at this time given the uncertainties with regard to direction of ground water gradient. ' 7.2.3 Var Extraction Though typically considered a soil remediation technology, utilization of vapor extraction for ' removing dissolved hydrocarbons from ground water can be viable under certain conditions. Applying vacuum at a well screened in the water table, and thus extracting vapors from pores in the surrounding soil matrix, can result in volatilization of dissolved hydrocarbons into the ' vapors as they pass over the water table surface. In addition, vapor extraction can promote natural biodegradation of dissolved hydrocarbons by providing a continual source of fresh oxygen to stimulate indigenous microorganisms, which convert the hydrocarbons to carbon ' dioxide and water. At the same time, vapor extraction would be removing the hydrocarbons in the soil above the water table that presumably impacted ground water in the past, and could potentially impact ground water again. A possible limitation of vapor extraction is inability to control migration of dissolved hydrocarbons in ground water. Because of the prospect for removing the source of contamination, and for removing dissolved ' hydrocarbons without having to recover and dispose of produced ground water, vapor extraction can also complement efforts to remediate ground water. Use of vapor extraction in combination with other ground water remediation strategies (ground water pumping) will result in enhanced ' oxygenation of the vadose zone and the capillary fringe, stimulating in situ organisms and resulting in enhanced hydrocarbon destruction. ' 7.2.4 VapQr Extraction With Air S ar in The use of air sparging can enhance the effectiveness of vapor extraction for removing dissolved ' hydrocarbons from ground water. Sparging air into the water table within the zone of influence of the vapor extraction well(s) can speed remediation by means of air stripping dissolved hydrocarbons from the ground water as the air passes through the ground water enroute to the ' vapor extraction well(s). Furthermore, introduction of the air via sparging would provide additional oxygen for enhancing the biologic breakdown of hydrocarbon compounds in the subsurface. With strategically located sparge points, air sparging has the additional possible ' benefit of controlling the migration of dissolved hydrocarbons in ground water. ' rptoo5.-i