Biodegradation of monoaromatic hydrocarbons by indigenous aquifer microorganisms under aerobic and denitrifying conditions.

Bioremediation of aquifers contaminated with benzene, toluene, ethylbenzene, and xylenes (BTEX) might be enhanced by the injection of nitrate as adjunct electron acceptor to oxygen in order to increase the electron acceptor pool. Indigenous microorganisms were generally effective in degrading alkylbenzenes, but not benzene, under anoxic denitrifying conditions. If molecular oxygen was present in concentrations that did not inhibit denitrification, both oxygen and nitrate were used in oxidative benzene biotransformations. Denitrifiers aided in the ultimate BTEX degradation by oxidizing metabolites of incomplete aerobic BTEX degradation. In addition, microbial-mediated oxidation of alkylbenzenes and BTEX-metabolites with nitrate lowered the apparent biochemical oxygen demand (BOD) of the system. This, in turn, might have reduced competition between benzene and other substrates for molecular oxygen, which would permit a greater extent of aerobic benzene biotransformations. Toluene and its potential metabolites, benzoate, protocatechuate, 3-methylcatechol, 4-methylcatechol, succinate, and adipate were degraded in anoxic denitrifying incubations seeded with four different aquifer materials. Ortho-xylene was partially removed cometabolically, with toluene acting as the primary substrate. Meta- and para-xylenes were degraded in denitrifying enrichments from one of the four aquifer samples. Catechol and gentisate were not degraded after four months of incubation under denitrifying conditions. Although benzene degradation was not observed under anoxic (O$\sb2