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v <br /> %TC <br /> A S S O C I A T E S I N C <br /> Glover, the property owner, no gasoline constituents have been detected in this well above <br /> laboratory method detection limits. <br /> L- 4.0 BIOREMEDIATION-DISCUSSION AND FURTHUR EVALUATION <br /> 4.1 Discussion <br /> V <br /> Several detailed field studies have been performed examining indicators of intrinsic bioremediation <br /> and identify factors that significantly effect the rate and extent of bioremediation (Buscheck and <br /> others, 1993; McAllister and Chiang, 1994; Borden and others, 1995; Buscheck and Alcantar, <br /> 1995). Through these studies and on-going research on the factors, which control <br /> biodegradation, it is now understood that dissolved petroleum hydrocarbons in groundwater will <br /> biodegrade, without artificial enhancement, due to the presence of naturally occurring <br /> (indigenous) microorganisms. The US Air Force and Chevron Corporation have developed <br /> v <br /> technical protocols for implementing and monitoring intrinsic bioremediation studies in <br /> groundwater. <br /> Intrinsic bioremediation in brief, is the use of indigenous microorganisms to degrade contaminants <br /> that have been released into the subsurface. The biodegradation of the contaminants is essentially <br /> - an oxidation-reduction (redox) reaction where the hydrocarbon is oxidized (donates an electron) <br /> and an electron acceptor (i.e. oxygen) is reduced (accepts electrons). There are several <br /> compounds that can serve as electron acceptors including oxygen, nitrate, iron oxides, sulfate, <br /> water, and carbon dioxide (Borden and others, 1995). Aerobic microorganisms use oxygen as the <br /> electron acceptors. Anaerobic microorganisms use other compounds such as nitrate, iron oxides <br /> (ferric iron), and sulfate as electron acceptors. <br /> Oxygen is the most preferred electron acceptor in groundwater because microorganisms gain <br /> more energy from these reactions, however, this process usually results in the depletion of oxygen <br /> with an increase in carbon dioxide in the subsurface. Therefore low concentrations of dissolved <br /> oxygen and corresponding high concentrations of carbon dioxide within hydrocarbon plume <br /> indicate biodegradation is taking place (Borden and others, 1995). <br /> In anaerobic environments microorganisms may use other compounds such as nitrate, ferric iron <br /> and sulfate as electron acceptors. An increase in ferrous iron (i.e. reduced ferric iron), carbon <br /> dioxide and perhaps hydrogen sulfide and a corresponding decrease in nitrate and/or sulfate within <br /> a hydrocarbon plume indicate that anaerobic biodegradation is taking place. <br /> Additional indicators of anaerobic biodegradations include total alkalinity and redox potential <br /> (Buscheck and O'Reilly, 1995). The total alkalinity of a groundwater system is indicative of the <br /> water's capacity to neutralize acid. Alkalinity results from the dissolution of rock (particularly <br /> carbonate rocks), the transfer of carbon dioxide from the atmosphere, and the respiration of <br /> microorganisms. Therefore, an increase in alkalinity within a hydrocarbon plume is potentially an <br /> `—' indicator of bioremediation occurring (Buscheck and O'Reilly, 1995). The redox potential of <br /> w <br /> wA95122Vcports\ReVest for NFA-&c 9 <br /> V <br />