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4 <br /> based on historic information,and the zone of source removal effected by the AS/SVE <br /> system,with a 25 to 30 foot radius of influence. <br /> Discussion <br /> Biodegradation of petroleum hydrocarbons results in predictable geochemical <br /> consequences. Interpretation of these geochemical consequences is based on an <br /> understanding of the role of electron acceptors in hydrocarbon biodegradation. <br /> Petroleum hydrocarbon biodegradation involves the oxidation of dissolved organics <br /> coupled with the reduction of electron acceptors through the biological process of <br /> respiration. Because oxygen is the most thermodynamically efficient electron acceptor, <br /> petroleum hydrocarbons are generally most readily biodegraded under aerobic conditions. <br /> However,oxygen flux into the subsurface is limited.When sufficient oxygen is unavailable, <br /> soil micro-organisms are able to use other electron acceptors in hydrocarbon <br /> biodegradation.The order of electron acceptor utilization that occurs in petroleum <br /> hydrocarbon biodegradation is: <br /> Aerobic respiration(oxygen is the electron acceptor) <br /> • Nitrate reduction <br /> Iron reduction <br /> • Sulfate reduction <br /> • Methane fermentation(carbon dioxide is the electron acceptor) <br /> By determining the changes in the concentration of electron acceptors and/or associated <br /> metabolic by-products in groundwater at a specific site,the predominant processes <br /> involved in hydrocarbon biodegradation at the site can be identified,and an estimate of the <br /> site's expressed and potential hydrocarbon biodegradation capacity can be expressed. <br /> Dissolved oxygen is consumed in the aerobic degradation of BTEX at the outer fringe of the <br /> plume. Figure 4 shows a dissolved oxygen depression in the area around the zone of <br /> significant contamination <br /> In the degradation of BTEX,nitrate is converted to aqueous nitrogen by a process known as <br /> denitrification where nitrate is-the electron acceptor.The pattern of declining nitrate <br /> concentration around the zone of significant contamination in Figure 4 provides evidence of <br /> nitrate as an electron acceptor at the Disco site. Background nitrate concentration appears to <br /> be on the order of 6 to 8 mg/L. The nitrate result for MW DIS-8S dropped from 4.4 mg/L <br /> last quarter to 0.04 mg/L. <br /> Fe+3 accepts an electron and becomes Fe"in the degradation of BTEX. Figure 4 shows <br /> elevated levels of Fe 'in the zone of significant contamination.The elevated level of W' <br /> found in MW-DIS-4 last quarter was not found again this quarter.We suspect that last <br /> quarters iron result was incorrect. <br /> Sulfate is converted to aqueous hydrogen sulfide in the degradation of BTEX. Figure 4 <br /> shows depressed levels (2.0 mg/L)of sulfate in the zone of significant contamination in <br /> contrast to 10 to 30 mg/L outside the zone. The sulfate concentration in MW-DIS-2 <br /> decreased from 7.88 mg/L last quarter to 1.98 this quarter. All other values were consistent <br /> { with last quarter's results for sulfate. <br /> 1 SFMCAPROJECTSOELMONTE10961001_DOC <br />