In Situ Bioremediation of Contaminated Groundwater

More than any other factor, it is likely that oxygen limitation occurs in groundwaters, and the most rapid biodegradation processes normally involve oxygenase enzymes. The supply of oxygen for in situ bioremediation adds considerably to the expense and the technical difficulty. This supply involves the use of either blowers or vacuum pumps to draw air through the system. A measure of the pivotal role of oxygen is in the use of hydrogen peroxide to enhance aeration. That its use has been considered at all is remarkable: concentrations of H2O2 above 100 to 200 mg/L are toxic to microorganisms; it can be consumed very quickly, limiting treatment to the regions near the injection well; a groundwater circulation system must be created; and good soil permeability is essential. Recent advances in the knowledge of anaerobic biodegradation microbiology have opened up the prospect of greater acceptance of anaerobic bioremediation where maintaining aerobic conditions is not feasible. The existence of microorganisms capable of coupling the anaerobic reduction of Fe3+ to the oxidation of organic compounds shows promise. Virtually our entire knowledge of anaerobic metabolism of hydrocarbons has been gained since around 1990 (for reviews, see References 7 and 8). Several alkylbenzenes, alkanes or alkenes, are anaerobically biodegraded by denitrifying, ferric iron-reducing or sulfate-reducing bacteria. Another group of anaerobic hydrocarbon-degrading bacteria are “proton reducers” that rely on syntrophic associations with methanogens. As some of the most significant groundwater pollutants are chlorinated solvents, then a role for microbial reductive dehalogenation is feasible. Although slow, even the reductive dechlorination of dioxins is possible. A recent discovery has shown that even benzene can be oxidized completely under anaerobic conditions by pure cultures, using nitrate as the electron acceptor. Benzene is a particular problem in groundwater because it is relatively soluble and mobile. Keywords: aerobic; anaerobic; bioventing; biosparging; permeable reactive barrier