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these contaminants in the capillary fringe are indicative of groundwater impact and advection. j�.nn S liL or 4. Petroleum hydrocarbons in groundwater were detected in samples collec d from borings MtV-16 SB4 and SBS and in wells MW 1 through MW5 and MW7 throu W9. he plumes of TPHg, benzene, and MTBE are west and south of the former locate of the USTs. The extent of MTBE contamination is defined to___he north. The extent of MTBE contamination to the south, west, and the eas are inferred. 5. The well located at the northeast corner of the State Hcspital is the closest well to the site, located approximately 800 feet to the west. Because this well is located upgradient from the site, it is unlikely that the well has been impacted or is threatened by petroleum hydrocarbons from the site. 6.0 REMEDIAL ALTERNATIVES Based on the results of previous investigations, three remedial alternatives for soil and groundwater are considered viable. Soil and groundwater remedial alternatives include l) natural attenuation with groundwater monitoring, 2) air sparging with vapor extraction, and 3) vapor extraction with groundwater extraction and treatment. A summary of each alternative is presented below: 6_1 Natural Attenuation with Groundwater Monitoring Natural attenuation, also known as passive bioremediation or intrinsic bioremediation, is a passive remedial approach that depends upon natural processes to degrade and dissipate petroleum constituents in soil and in groundwater. The process of biodegradation biologicically oxidizes the hydrocarbon substrate to carbon dioxide and water. Several detailed field studies have been performed examining indicators of intrinsic bioremediation and identify factors, which significantly affect 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 SA\22493 veports\CAP.doc 6