Impact of Ethanol on the Natural Attenuation of Benzene, Toluene, and o-Xylene in a Normally Sulfate-Reducing Aquifer

Side-by-side experiments were conducted in a sulfate-reducing aquifer at a former fuel station to evaluate the effect of ethanol on biodegradation of other gasoline constituents. On one side, for ∼9 months we injected groundwater amended with 1−3 mg/L benzene, toluene, and o-xylene (BToX). On the other side, we injected the same, adding ∼500 mg/L ethanol. Initially the BToX plumes on both sides (“lanes”) extended approximately the same distance. Thereafter, the plumes in the “No Ethanol Lane” retracted significantly, which we hypothesize to be due to an initial acclimation period followed by improvement in efficiency of biodegradation under sulfate-reducing conditions. In the “With Ethanol Lane”, the BToX plumes also retracted, but more slowly and not as far. The preferential biodegradation of ethanol depleted dissolved sulfate, leading to methanogenic/acetogenic conditions. We hypothesize that BToX in the ethanol-impacted lane were biodegraded in part within the methanogenic/acetogenic zone and, in part, within sulfate-reducing zones developing along the plume fringes due to mixing with sulfate-containing groundwater surrounding the plumes due to dispersion and/or shifts in flow direction. Overall, this research confirms that ethanol may reduce rates of biodegradation of aromatic fuel components in the subsurface, in both transient and near steady-state conditions.