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dissolved petroleum hydrocarbons in the sandy portion of the aquifer will travel 4.69 feet/year. Calculations are presented in Appendix V. At this time, it is uncertain the extent of dissolved petroleum hydrocarbons in the plume. Further investigation is required. However, the rate of flow in the sandy portion of the aquifer is not significant. These flow rates may explain the &W limited size of the plume since the source of the contaminants (the former gasoline USTs) were removed in 1986. L 6.0 INTRINSIC BIOREMEDIATION OF GROUNDWATER �_J Discussion LM 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 L 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. 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 L w:195122TIumerpt 12 • Sion L