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Mr. Soon Kim Page 15 December 19, 1995 Discussion Biodegradation of petroleum hydrocarbons results in predictable geochemical consequences. Interpretation of these geochemical consequences is based on an understanding of the role of electron acceptors in hydrocarbon biodegradation. Petroleum hydrocarbon biodegradation involves the oxidation of dissolved organics coupled with the reduction of electron acceptors through the biological process of respiration. Oxygen is the most thermodynamically efficient electron acceptor, thus petroleum hydrocarbons are generally most readily biodegraded under aerobic conditions. However,oxygen flux into the subsurface is limited. When sufficient oxygen is not available, soil microorganisms are able to use other electron acceptors inhydrocarbon biodegradation. The order of electron acceptor utilization that occurs in petroleum hydrocarbon biodegradation is: • Aerobic respiration (oxygen is the electron acceptor) • Nitrate reduction • Iron reduction • Sulfate reduction • Methane fermentation (carbon dioxide is the electron acceptor) By determining the changes in the concentration of electron acceptors and/or associated metabolic by-products in groundwater at a specific site,the predominant processes involved in hydrocarbon biodegradation at the site can be identified, and an estimate of the site's expressed and potential hydrocarbon biodegradation capacity can be developed. Dissolved oxygen is consumed in the aerobic degradation of BTEX at the outer fringe of the plume. Figure 9 shows a dissolved oxygen depression in the area around the zone of significant contamination. Elevated dissolved oxygen concentration is shown in the area of air sparging, were dissolved oxygen levels have been as high as 6.0 mg/L. In the degradation of BTEX, nitrate is converted to aqueous nitrogen by a process known as denitrification were nitrate is the electron acceptor. The pattern of declining nitrate concentration around the zone of significant contamination in Figure 5 provides evidence of nitrate as an electron acceptor at the Disco Site. Background nitrate concentration appears to be on the order of 6 to 8 mg/L. Fe" accepts an electron and becomes Fe'in the degradation of BTEX. Figure 6 shows { elevated levels of Fe'in the zone of significant contamination. The elevated level of Fe Z in MW-DIS-4 does not fit the expected trend. No explanation for this result is offered at this writing. Perhaps successive monitoring results will provide additional information to explain. this result. 4 i i