Side-by-side experiments were conducted in an aquifer contaminated with methyl-tert-butyl ether (MTBE) at a former fuel station to evaluate the effect of ethanol release on the fate of pre-existing MTBE contamination. 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. The fates of BToX in both sides ("lanes") were addressed in a prior publication. No MTBE transformation was observed in the "No Ethanol Lane." In the "With Ethanol Lane", MTBE was transformed to tert-butyl alcohol (TBA) underthe methanogenic and/or acetogenic conditions induced by the in situ biodegradation of the ethanol downgradient of the injection wells. The lag time before onset of this transformation was less than 2 months and the pseudo-first-order reaction rate estimated after 7-8 months was 0.046 d(-1). Our results imply that rapid subsurface transformation of MTBE to TBA may be expected in situations where strongly anaerobic conditions are sustained and fluxes of requisite nutrients and electron donors allow development of an active acetogenic/methanogenic zone beyond the reach of inhibitory effects such as those caused by high concentrations of ethanol.

SynopsisIn a continuous release field experiment, ethanol altered microbial community structure/function, lowered the redox state, and slowed biodegradation of coreleased BTo-X in an anaerobic aquifer. AbstractEthanol (EtOH) is a commonly used fuel oxygenate in reformulated gasoline and is an alternative fuel and fuel supplement. Effects of EtOH release on aquifer microbial ecology and geochemistry have not been well characterized in situ. We performed a controlled field release of petroleum constituents (benzene (B), toluene (T), o-xylene (o-X) at ∼1-3 mg/L each) with and without EtOH (∼500 mg/L). Mixed linear modeling (MLM) assessed effects on the microbial ecology of a naturally sulfidic aquifer and how the microbial community affected B, T, and o-X plume lengths and aquifer geochemistry. Changes in microbial community structure were determined by quantitative polymerase chain reaction (qPCR) targeting Bacteria, Archaea, and sulfate reducing bacteria (SRB); SRB were enumerated using a novel qPCR method targeting the adenosine-5′-phosphosulfate reductase gene. Bacterial and SRB densities increased with and without EtOH-amendment (1−8 orders of magnitude). Significant increases in Archaeal species richness; Archaeal cell densities (3-6 orders of magnitude); B, T, and o-X plume lengths; depletion of sulfate; and induction of methanogenic conditions were only observed with EtOH-amendment. MLM supported the conclusion that EtOH-amendment altered microbial community structure and function, which in turn lowered the NOT THE PUBLISHED VERSION; this is the author's final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Shallow, small-rate releases of ethanol-blended fuels from underground storage tanks (USTs) may be quite common and result in subsurface CH 4 generation. However, vadose zone transport of CH 4 generated from these fuel releases is poorly understood, despite the potential to promote vapor intrusion or create explosion hazards. In this study, we simulated shallow CH 4 generation with a controlled subsurface CH 4 release from July 2014 to February 2015 to characterize subsurface CH 4 migration and surface emissions and to determine environmental controls on CH 4 fate and transport. July 2014 through November 2014 was an extended period of drought followed by precipitation during December 2014. Throughout the experiment, under varied CH 4 injection rates, CH 4 formed a radially symmetrical plume around the injection point. Surface efflux during the drought period of the experiment was relatively high and stable, with approximately 10 to 11 and 34 to 52% of injected CH 4 reaching the ground surface during the low-and high-rate injections, respectively. Following the period of precipitation and increased soil moisture, efflux dropped and stabilized at approximately 1% of injected CH 4 , even as soil moisture began to decrease again. Tracer and inhibitor experiments and estimates of soil diffusivity suggest that microbial CH 4 oxidation was responsible for the observed drop in efflux. The decrease in efflux only after soil moisture increased suggests a strong environmental control over the transport and oxidation of vadose zone CH 4 .