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l 4 Intrinsic Bioremediation Demonstration Samples were collected from monitoring wells MW-DIS-2,3,4,8S,8D,and 9 by the minimum aeration protocol presented in the Appendix of the PAR/FRP(CH2M HILL, April 1995). In brief,the intent of this demonstration is to evaluate the geochemical indicators of natural biodegradation and assess the potential for self containment and abatement of the Disco property gasoline plume. Analytical Results The geochemical indicators of intrinsic bioremediation are summarized in Table 4 for the current and previous monitoring events. Average concentrations and values are tabulated in the right column for each sampling location. { Figure 3 shows the ITB parameters associated with each of the wells monitored in December 1996.The dashed lines in the central lower third indicate the "zone of significant contamination"based on historic information,and the zone of source removal effected by the AS/SVE wells,with a 25 to 30 foot radius of influence. 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 f respiration. Because oxygen is the most thermodynamically efficient electron acceptor, petroleum hydrocarbons are generally most readily biodegraded under aerobic conditions. However,oxygen flux into the subsurface is limited. When sufficient oxygen is unavailable, soil micro-organisms are able to use other electron acceptors in hydrocarbon . biodegradation. The preferential order of electron acceptor utilization that occurs in petroleum hydrocarbon biodegradation is: • Aerobic respiration (oxygen is the electron acceptor) t Nitrate reduction Iron reduction • Sulfate reduction • Methane fermentation(carbon dioxide is the electron acceptor) t 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 quantified. Dissolved oxygen (D.O.)is consumed in the aerobic degradation of BTEX at the outer fringe of the plume. Figure 3 shows a dissolved oxygen depression in the area around the zone of significant contamination. The shallow groundwater D.O. concentration,in background r well MW-DIS-4 is 4.6 mg/L. In and around the BTEX plume,D.O. is less than 0.5 mg/L. 4 } SFOIG:IPROJECTSIDELMONTEIPLMT 3314RPT2_ AQTR-REP.DOC 12