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L. <br /> VvATc <br /> A S S O C I A T E S I N C <br /> toluene, and MTBE in groundwater, ATC did not perform mass balance calculations for <br /> groundwater. <br /> 4.0 REMEDIAL ALTERNATIVES <br /> Based on the results of previous investigations, ATC has considered three remedial alternatives <br /> for soil and groundwater. Soil and groundwater remedial alternatives include natural attenuation <br /> with groundwater monitoring, - dual-phase groundwater and soil vapor extraction, and <br /> groundwater extraction and treatment. A summary of each alternative is presented below: <br /> 1.1 Natural Attenuation with Groundwater Monitoring <br /> Natural attenuation, also known as passive bioremediation or intrinsic bioremediation, is a passive <br /> remedial approach that depends upon natural processes to degrade and dissipate petroleum <br /> constituents in soil and in groundwater. Through biodegradation the actual reduction of <br /> petroleum constituent mass is realized. <br /> Several detailed field studies have been performed examining indicators of intrinsic bioremediation <br /> and identify factors, which significantly effect the rate and extent of bioremediation (Buscheck and <br /> rte, 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 /> 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 <br /> contaminants, which have been released into the subsurface. The biodegradation of the <br /> contaminants is essentially an oxidation-reduction (redox) reaction where the hydrocarbon is <br /> oxidized (donates an electron) and an electron acceptor (i.e. oxygen) is reduced (accepts <br /> ` electrons). There are several compounds that can serve as electron acceptors including oxygen, <br /> nitrate, iron oxides, sulfate, water, and carbon dioxide (Borden and others, 1995). Aerobic <br /> microorganisms use oxygen as the electron acceptors. Anaerobic microorganisms use other <br /> compounds such as nitrate, iron oxides (ferric iron), and sulfate as electron acceptors. <br /> In order to determine the effectiveness of natural attenuation with groundwater monitoring, ATC <br /> conducted quarterly intrinsic bioremediation monitoring of selected monitoring wells from <br /> October 2002 to January 2004. For comparison purposes, MW3, MW4, and MW5 are assumed <br /> �. to be located beyond the hydrocarbon plume, and MWI and MW2 are assumed to be located <br /> inside the hydrocarbon plume. The data collected indicate intrinsic bioremediation is occurring <br /> within the groundwater plume. Specifically, the redox potential of groundwater is reduced, <br /> �- indicating that anaerobic biodegradation has taken place. Additionally, the levels of manganese are <br /> S:IEnvironmentaIN62577\SCMCAPAdddoc 4 <br /> 1r <br />