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� v A S S o C 1 A T E S I N C Several detailed field studies have been performed examining indicators of intrinsic bioremediation and identify factors which significantly effect the rate and extent of ' bioremediation (Buscheck and others, 1993, McAllister and Chiang, 1994, Borden and others, 1995, Buscheck and Alcantar, 1995) Through these studies and on-going research on the factors ' which control biodegradation, it is now understood that dissolved petroleum hydrocarbons in groundwater will biodegrade, without artificial enhancement, due to the presence of naturally occurring (indigenous) microorganisms Technical protocols for implementing and monitoring ' intrinsic bioremediation studies in groundwater have been developed by the US Air Force and Chevron Corporation ' Intrinsic bioremediation in brief, is the use of indigenous microorganisms to degrade contaminants which have been released into the subsurface The biodegradation of the contaminants is essentially an oxidation-reduction (redox) reaction where the hydrocarbon is ' oxidized (donates an electron) and an electron acceptor (i e oxygen) is reduced (accepts electrons) There are several compounds that can serve as electron acceptors including oxygen, nitrate, iron oxides, sulfate, water, and carbon dioxide (Borden and others, 1995) Aerobic ' microorganisms use oxygen as the electron acceptors Anaerobic microorganisms use other compounds such as nitrate, iron oxides (ferric iron), and sulfate as electron acceptors Groundwater samples collected from each of the site's monitoring wells have not been analyzed for the indicator parameters noted above It is likely that intrinsic bioremediation is occurring ' within the dissolved petroleum hydrocarbon plume at the subject site to some degree but it is not known at what the rate of reduction is ' 552 Groundwater Extraction with Soil Vapor Extraction Groundwater extraction with soil vapor extraction, or dual-phase extraction, uses pumps to remove a combination of impacted groundwater and hydrocarbon vapor from the subsurface Dual-phase extraction systems are typically designed to maximize extractions rates but this method also stimulates biodegradation by increasing the supply of oxygen in the subsurface Pilot testing to evaluate the effectiveness of vapor extraction has already been performed at the site Results indicated that vapor extraction is a feasible alternative for remediation of the site In order to determine the effectiveness of groundwater extraction, pilot testing to evaluate the effectiveness of groundwater extraction would have to be performed at the site Data provided by the pilot studies could be used to properly design a full-scale groundwater extraction and soil vapor extraction system 5`3 In-Situ Air Sparging with Soil Vapor Extraction ., In-situ air sparging with soil vapor extraction has proven effective in reducing VOCs and certain semi-volatile compounds In brief, air is injected into the saturated zone The air forms bubbles that rise to the unsaturated zone, carrying trapped and dissolved contaminants Vapor extraction W 1625961reportslpdotcap doe 8