Analysis of Microbiological and Geochemical Processes Controlling Biodegradation of Aromatic Hydrocarbons in Anaerobic Aquifers

Background: Benzene, toluene, ethylbenzene and xylenes (BTEX) are among the groundwater contaminants of greatest concern because of their toxicity, their solubility in water, and their resistance to degradation in anaerobic environments. Prior to entry of hydrocarbon pollutants, shallow aquifers are often aerobic with relatively low levels of dissolved organic carbon. Indigenous aerobic bacteria readily metabolize hydrocarbon pollutants (e.g. fuels) entering these systems, and quickly deplete the available oxygen. Most enhanced bioremediation systems involve the addition of oxygen, however, the feasibility of aerobic biodegradation is often limited by its low water solubility; in addition, oxygen injection may induce iron oxide precipitation. Thus, bioremediation using alternate electron acceptors, such as NO₃^–, Fe⁺³, and SO₄^-2, has been the subject of much recent research. A better understanding of underlying mechanisms of these anaerobic processes is needed for them to be effectively employed for aquifer bioremediation. This requires identification of the organisms mediating the reactions, the metabolic pathways by which they degrade BTEX, and the effects of available electron acceptors (and other environmental factors) on their activities.

The study aquifer at Fort McCoy, Wisconsin, underlies a former Petroleum, Oils, and Lubricants Station, and is contaminated with BTEX from two 12,000-gallon leaking underground storage tanks (UST’s) that were installed in 1943 and removed in 1989. Residual non-aqueous phase liquid concentrated in a 0.3 to 0.6 m thick zone near the water table continuously supplies BTEX to groundwater. The current plume is approximately 100 m long and 30 m wide at its maximum, with BTEX concentrations generally less than 20 mg/1.