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developed and sampled monitoring wells MW-1, MW-2 and MW-3. Analytical results of the soil and groundwater samples collected indicated elevated concentrations of petroleum hydrocarbons were present in the soil and groundwater downgradient of the former gasoline UST locations. On September 24 and October 2, 1996, Smith Technology conducted an additional subsurface investigation at the site. A total of 9 soil borings were advanced to depths up to 15 feet below ground surface. Soil, soil vapor, and groundwater samples were collected from the borings. Analytical results of the soil and groundwater samples collected indicated the general extent of total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and xylenes (BTEX), and methyl tertiary butyl ether (MTBE) in soil and groundwater under the site. Analytical results for geochemical parameters in soil and groundwater indicated bioremediation is occurring in the plume of petroleum hydrocarbons in groundwater beneath the site. Analytical results of soil vapor samples indicated the availability of oxygen, carbon dioxide, nitrogen and methane in the vadose zone. Also, results of laboratory geophysical testing on soil samples from the site indicated that the potential receptor on-site, the water supply well, would not be affected by the plume of petroleum hydrocarbons in groundwater within the next 20 years, in the worst case. INTRINSIC BIOREMEDIATION OF GROUNDWATER 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, = _l 1995). Through these studies and ongoing research of 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 is reduced (accepts electrons). There are several compounds that can serve as electron acceptors including oxygen, nitrate, iron oxides (Fe III), sulfate, water, and carbon dioxide(Borden and others, 1995), and also manganese (Mn IV) and sulfide. Aerobic microorganisms use oxygen as the electron acceptors. Anaerobic microorganisms use other compounds such as nitrate, iron oxides (ferric iron), manganese oxide (Mn IV), sulfate and carbon dioxide as electron acceptors. Oxygen is the most preferred electron acceptor in groundwater because microorganisms gain more energy from these reactions, however, this process usually results in the depletion of oxygen with an increase in carbon dioxide in the subsurface. Therefore low concentrations of dissolved oxygen and corresponding high concentrations of carbon dioxide within hydrocarbon plume indicate biodegradation is taking place (Borden and others, 1995). : i w:1951221Reporisl4QMR1996.doc 2 • smTH