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essentially an oxidation-reduction (redox) reaction where the hydrocarbon is oxidized (donates an electron) and an electron acceptor (i a 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 microorgamsms use oxygen as the electron acceptors Anaerobic nucroorganisms use other compounds such as nitrate, iron oxides (ferric iron), and sulfate as electron acceptors Natural Attenuation of the residual petroleum hydrocarbons at the project site should occur over ' time, however, shallow groundwater at the site and the documented soil contamination at the soil/water interface appear to reverse and threaten mass reduction of dissolved petroleum hydrocarbons in the vicinity of MW4 prolonging natural attenuation Due to benzene levels ' currently 34 times the MCL in well MW4 and with MW4's close proximity to the municipal supply well, it is questionable if this alternative would meet regulatory criteria for the protection ' of groundwater quality 10.2 Soil Excavation Excavation of shallow soil contamination and on-site aeration or off-site disposal to a certified waste disposal facility would remove the potential source for groundwater contamination Additionally, standing water in the open excavation can be remediated by pumping the water into an on-site holding tank for either on or off-site treatment and/or disposal As discussed in previous sections, underground utilities and above ground structures (municipal well compound, post office building) have impeded further excavation of the petroleum affected areas and extensive shoring and/or demolition of the structures would be required Additional soil excavation at the project site is not economically feasible 10.3 In-Situ Air SparRing with Soil Vapor Extraction In-situ air sparging with soil vapor extraction (SVE) has proven effective in reducing volatile organic compounds (VOCs) and certain semi-volatile compounds In brief, air is infected into the saturated zone The air forms bubbles that rise to the unsaturated zone, carrying trapped and dissolved contaminants Vapor extraction wells in the unsaturated zone capture sparged air If necessary the effluent air can be treated using a variety of vapor treatment options The advantages of in-situ airs arm with SVE include g p g g ' • Short treatment times, usually 6 months to 2 years under optimal conditions Readily available equipment and easy installation • Implementation with nummal disturbance to above ground activities • Requires no removal, treatment, storage , or discharge considerations for groundwater • Cost competitive • Can be used under buildings and other locations that cannot be excavated w 1943261repotu\sumrpt doc 8 1 r SWFH