Biodegradation of aromatic hydrocarbons in aquifer materials with mixtures of oxygen and nitrate as electron acceptors

Releases of crude oil, coal tar, gasoline from leaking underground storage tanks and associated petroleum products (comprised of aromatic hydrocarbons) have contaminated groundwater and subsurface sediments at numerous sites worldwide. In situ bioremediation is an effective and economically favorable technology for treatment of the aqueous phase plumes associated with aromatic hydrocarbon contamination. Aerobic bioremediation has been highly effective in the remediation of many fuel releases, however, oxygen is difficult to deliver to the subsurface and in many cases is prohibitively expensive. Providing some level of oxygen in combination with an alternative electron acceptor, such as nitrate, may prove more successful than traditional bioremediation schemes which rely on oxygen as the sole electron acceptor.; This study compares the biodegradation of benzene, toluene, ethylbenzene, m-xylene, naphthalene and phenanthrene (constituents of petroleum and coal tar) under aerobic, anaerobic and mixed oxygen/nitrate electron acceptor conditions. Denitrifying aquifer bacteria from a site contaminated with coal tar were used as inocula for batch microcosm and aquifer sediment column studies which evaluated the feasibility of mixed oxygen/nitrate bioremediation.; With the exception of toluene, oxygen controls the removal of aromatic hydrocarbons from the mixture of BTEX, naphthalene and phenanthrene. Benzene, ethylbenzene, m-xylene, naphthalene and phenanthrene require oxygen for their removal from the mixture. Increased levels of oxygen improve the extent of compound removal whereas nitrate concentration has no significant effect. Extremely high levels of oxygen (i.e. 30 mg O{dollar}sb2{dollar}/L) cause a significant lag in the onset of compound removal whereas moderate aerobic levels (7 mg O{dollar}sb2{dollar}/L) result in comparable removal at a faster rate.; Only toluene is biodegraded at oxygen levels below 1 mg/L. Regardless of the concentration of oxygen, nitrate is used as the electron acceptor for biodegradation of toluene. Naphthalene biodegradation occurs in addition to toluene when oxygen is above a threshold of 1.5 mg/L. Naphthalene removal requires nitrate in addition to oxygen for its removal under microaerophilic conditions (O{dollar}sb2 le{dollar} 2mg/L). We observed denitrification in the presence of oxygen ("aerobic denitrification") although the presence of anaerobic microsites may account for this result. In general, we found that nitrate enhances the aerobic biodegradation of this mixture of aromatic hydrocarbons by the consortia of denitrifying aquifer microorganisms.