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i <br /> (:VYJ%TC <br /> 1117 Lone Palm Ave,Suite 201 <br /> Modesto,CA 95351 <br /> i Phone:209-579-2221 <br /> w Fax: 209-579-2225 <br /> fuel blended constituent concentrations that may serve as a secondary source for groundwater <br /> contamination and to prevent human exposure. <br /> Groundwater <br /> The specific remedial objectives for groundwater beneath the site are to reduce concentrations of <br /> rdissolved petroleum hydrocarbons and fuel blended constituent concentrations that are greater <br /> L; than the proposed groundwater quality objectives and to protect the potential use of the water- <br /> bearing zone underlying the site. A secondary objective is to control migration of the dissolved <br /> contaminant plume. <br /> 3.1 Soil Remediation Alternatives <br /> 3.1.1 Natural Attenuation and Groundwater Monitoring(NA) <br /> Natural attenuation, also known as passive bioremediation or intrinsic bioremediation, is a <br /> passive remedial approach that depends upon natural processes to degrade and dissipate <br /> petroleum constituents in soil and in groundwater. Through biodegradation the actual reduction <br /> of contaminant mass is realized. <br /> Several detailed field studies have been performed examining indicators of intrinsic <br /> bioremediation and identify factors, which significantly effect the rate and extent of <br /> bioremediation (Buscheck and others, 1993; McAllister and Chiang, 1994; Borden and others, <br /> 1995; Buscheck and Alcantar, 1995). Through these studies and on-going research on the <br /> factors which control biodegradation, it is now understood that dissolved petroleum <br /> hydrocarbons in groundwater will biodegrade, without artificial enhancement, due to the <br /> presence of naturally occurring (indigenous)microorganisms. <br /> i <br /> Intrinsic bioremediation utilizes indigenous microorganisms to degrade contaminants, which <br /> have been released into the subsurface. The biodegradation of the contaminants is essentially an <br /> 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 j <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 j <br /> the electron acceptors. Anaerobic microorganisms use other compounds such as nitrate, iron <br /> oxides (ferric iron), and sulfate as electron acceptors. <br /> Monitoring at the site would continue until concentrations of petroleum hydrocarbons reached <br /> acceptable levels. The advantages of intrinsic bioremediation include: <br /> • Readily available equipment and easy installation. <br /> L • Implementation with minimal disturbance to aboveground activities. <br /> • Cost effective under optimal conditions. <br /> • Can be used under buildings and other inaccessible locations. <br /> L s:\environmental\22493\reports\feasibility.doc 7 <br /> Lf <br />