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

Abstract

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 approximately 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 approximately 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.

Effect of Ethanol on Microbial Community Structure and Function During Natural Attenuation of Benzene, Toluene, and o-Xylene in a Sulfate-reducing Aquifer

“…General hydrogeology and well construction is described elsewhere. 3 Wells were placed in close proximity to enable highresolution characterization of the subsurface (1−3 ft between wells). A persistent relatively continuous MTBE plume appears to originate from higher concentrations of contaminants remaining in a shallower, less permeable, and horizontally restricted layer.…”

“…Experimental amendments started on May 13, 2004 (day 0) and continued for a total duration of ∼9 months, with relatively continuous injection only for the last 7 months due to system refinements and regular maintenance. For a more in-depth description of the injection system see Mackay et al 3 …”

“…B, T, o-X, MTBE, and ethanol), dissolved methane, sulfate, pH, ferrous iron, and sulfide levels were measured as described elsewhere. 3 Detection limits for these compounds were <0.010 mg/L for B, T, o-X, and MTBE, accessed by following the link in the citation at the bottom of the page. …”

“…1,2 Functional changes may include preferential degradation of more labile carbon substrates over other biodegradable and more toxic compounds present (e.g., benzene (B), toluene (T), xylene (X), and ethylbenzene (E)). 3 A growing body of work has examined the influence of ethanol on the natural attenuation and biodegradation of BTEX under both aerobic and anaerobic conditions [4][5][6][7][8] These studies suggest that simultaneous introduction of ethanol and BTEX will result in decreased BTEX attenuation rates and longer contaminant plumes. The majority of previous results, however, are based on microcosm or column experiments 4,5,8 or on small sets of observational field data comparing contaminated aquifers exposed or not exposed to ethanol.…”

Enthalpies of Formation and Bond Dissociation Energies of Lower Alkyl Hydroperoxides and Related Hydroperoxy and Alkoxy Radicals

“…1 Recent work 2 has also shown that seepage of bioethanol into the water table exarcebates the existing difficulty posed by leaks of methyl tert-butyl ether (MTBE) from faulty underground storage tanks 3 and reduces the rates of biodegradation of spilt aromatics. 4 Note that MTBE rose from almost total obscurity to become one of the most produced organic compounds in the world because it was believed that it could simultaneously replace the lead compounds in gasoline and improve urban air quality. It has since been banned as a gasoline additive in some areas of the United States and is in the process of being replaced by ethanol elsewhere.…”