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r OWN Laboratory and field evidence suggests that micrc;b4al populations can utilize minute amounts of oxygen to initiate hydrocarbon oxidal-ion and that subsequent oxidation is sustained by alterna- tive electron acceptors, such as nitrate or sulfate. Swain et al. (1971) reported that Pseudomonas aeruginosa degraded octane by aerobic oxidation and denitrification if a trace amount of oxygen (<0.05 mg/L) was present. When oxygen was entirely excluded, however, degradation did not proceed. Kuznetsova and Gorlenko (1965) reported that aerobic Pseudomonas initiate attack on hydrocarbons at the upper edges or rims of oil fields, and that the partially oxidized products of this process are further oxidized by sulfate reducers with concomitant formation of H2S. In nature, electron acceptors are used preferentially in the order: 02 > NO3 > SO4 --2 > CO2 Oxygen is preferred over nitrate, nitrate over sulfate and sulfate over carbon dioxide (methane fermentation) . The reduced products of these electron acce.itors are water, nitrogen gas, hydrogen sulfide, and methane, respectively. Following depletion of oxygen in ground water, any nitrate pre- sent may be used as an electron acceptor, resulting in denitrifi- cation by the following reaction (Mitchell 1974) : C6H6 + 5NO3 --> 6CO2 + 2-1/2N2 + 3H20 On a weight ratio, approximately one part of benzene is degraded for every four parts of nitrate (the ratio would be about 1/1 for nitrate reported as nitrogen) . This biodegradation mechanism is the basis of remedial technology demonstrations in Germany (Battermann and Werner 1984) and Canada (Berry-Spark et al. 1986) . In both of these studies nitrates were added to i 17