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Former Discovery Chevrolet June 6, 1997 <br /> Tracy,California Report of Additional Subsurface Investigation <br /> petroleum hydrocarbon degrading bacteria (McKee et al., 1972). McAllister and Chiang <br /> (1994)provide evidence that aerobic biodegradation occurs at ambient DO levels as low as 1 <br /> to 2 ppm. DO readings from the five wells at this site ranged from 2.9 to 4.9 ppm (Table 2). <br /> Aerobic degradation takes place more rapidly than anaerobic degradation and is favored by <br /> microorganisms. Complete conversion of toluene (and many other hydrocarbons) to carbon <br /> dioxide and water requires approximately 3 micrograms per liter (ppb) of oxygen for each <br /> ppb of hydrocarbon. Since groundwater saturated with oxygen contains from 6,000 to <br /> 12,000 ppb oxygen, complete conversion of petroleum hydrocarbon concentrations between <br /> 2,000 and 4,000 ppb can be accomplished by strictly aerobic process (Borden, et at., 1995). <br /> Based on the DO concentrations (Table 2), it appears that there is sufficient oxygen for <br /> aerobic bioremediation to be occurring in the shallow groundwater beneath this site and that <br /> aerobic bioremediation has possibly occurred. It also appears that the DO is being <br /> . k <br /> replenished in MW-1 and MW-5. <br /> �U <br /> Anaerobic Bioremediation <br /> If sufficient oxygen is not present to act as an electron acceptor in the petroleum <br /> hydrocarbon biodegradation reaction (anaerobic conditions), other compounds may act as <br /> electron acceptors. These compounds include nitrate (NO3), iron oxides [e.g., Fe(OH3)], <br /> sulfate (SO4), water (1120), and carbon dioxide (COD. Because of the energy released to the <br /> organism, anaerobic bacteria prefer to utilize species in the oxidation-reduction reactions in <br /> the order listed above; the most preferred compound is nitrate followed by ferric oxide and <br /> sulfate. The least preferred compounds for use in the reduction of petroleum hydrocarbons <br /> are water and carbon dioxide (Borden, et. al., 1995). <br /> Nitrate as an Electron Acceptor <br /> Nitrate is used as an electron acceptor to convert the gasoline hydrocarbon compounds <br /> toluene, ethylbenzene, and total xylenes to carbon dioxide, water, and free nitrogen. There <br /> is debate over whether benzene is biodegradable in the presence of nitrate (Borden, et. al., <br /> 1955; Kuhn, et. al., 1988; Hutchins, et al., 1991; Mihelcic and Lundy, 1991). <br /> 300I-1 I.rpt $ HORIZON ENVIRONMENTAL INC. <br />