Environ. Sci. Technol. 2007, 41, 2015-2021
Impact of Ethanol on the Natural Attenuation of MTBE in a Normally Sulfate-Reducing Aquifer D O U G M A C K A Y , * ,† N I C K D E S I E Y E S , † MURRAY EINARSON,‡ KEVIN FERIS,† ALEX PAPPAS,† ISAAC WOOD,† LISA JACOBSON,† LARRY JUSTICE,† MARK NOSKE,† JOHN WILSON,§ CHERRI ADAIR,§ AND KATE SCOW† Department of Land, Air & Water Resources, University of California, Davis, California, Geomatrix Consultants, Oakland, California, and U.S. Environmental Protection Agency, Ada, Oaklahoma
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 ∼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. 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) under the 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 pseudofirst-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.
Introduction Ethanol is an increasingly common component of automobile fuels, and thus it is important to evaluate groundwater impacts of spills of ethanol-containing fuels. Ethanol is expected to degrade rapidly and without any acclimation period under most redox conditions unless present at very high concentrations, such as might occur adjacent to fresh spills (1, 2). Ethanol nonetheless is expected to impact the in situ biodegradation of other fuel components due to its rapid preferential degradation causing (1) depletion of ready available electron acceptors and/or (2) alterations in the fraction of the native microbial community able to degrade the various contaminants (1-5). In the case of the BTEX compounds (benzene, toluene, ethylbenzene and xylene * Corresponding author e-mail:
[email protected]. † University of California, Davis. ‡ Geomatrix Consultants. § U.S. Environmental Protection Agency. 10.1021/es062156q CCC: $37.00 Published on Web 02/06/2007
2007 American Chemical Society
isomers), it has been shown that ethanol can slow or stop their biodegradation in situ and in microcosms (2-5). Oxygenates such as methyl-tert-butyl ether (MTBE) have proven less susceptible to in situ biodegradation than other fuel constituents such as the BTEX species. MTBE contamination remains in the subsurface at many fuel release sites and thus could be impacted by newer releases of ethanolblended fuels which could create and sustain methanogenic conditions in groundwater. Studies have provided evidence of transformation of MTBE to tert-butyl alcohol (TBA) and other intermediates, both in microcosms under highly reduced (including methanogenic) conditions and in the methanogenic zones near fuel spills (6-9). Laboratory studies measuring fractionation of stable isotopes during anaerobic metabolism of MTBE implicate acetogenic bacteria in the process (8, 9). Laboratory studies have shown the concurrent transformation of MTBE and BTEX compounds in aquifer sediment (5, 7). It has been hypothesized that molecular hydrogen produced by the fermentation of BTEX compounds may support anaerobic metabolism of MTBE by acetogenic microorganisms (7); more recently it has been suggested that the transformation of MTBE to TBA may be an abiotic hydrogenation reaction driven by biologically produced hydrogen (10). In either case, as BTEX and other more readily degradable compounds in the spill are depleted over time by in situ biodegradation, production of hydrogen and thus anaerobic transformation of MTBE may slow or stop altogether. We hypothesize that new spills of ethanol-blended fuels at such sites could change the balance between soluble electron donors and acceptors and shift the microbial community toward hydrogen producers. Under such conditions, fermentation of ethanol and/or other species could generate products which may once again support acetogenic microbial communities and thus re-establish conditions supporting the transformation of pre-existing MTBE to TBA and other intermediates. Our goals were to conduct controlled field experiments at a typical fuel station to confirm that ethanol release would stimulate in situ transformation of pre-existing MTBE contamination, and to provide estimates of the reaction rate and lag time before onset of the reaction. Since most fuelcontaminated sites in the US overlie aquifers in which the dominant electron acceptor is sulfate (11), we selected a sulfate-dominated site for this study. Thus, the conditions in the aquifer at the study site are normally sulfate-reducing unless a significant amount of labile carbon, such as ethanol, is added. A prior publication (2) addressed impacts of ethanol on in situ biodegradation of BTEX species at the same site.
Site and Pre-Experimental Conditions Site 60, Vandenberg Air Force Base (VAFB), CA, has been described in detail previously (2). A gasoline leak was noted in 1994 at the fuel station; tanks and piping were excavated in 1995, and the excavation was backfilled with relatively permeable media. Figure 1 depicts the surface boundary of the excavated area and location of monitoring wells installed before and during the experiments. Figure 2 is a vertical schematic of the subsurface in and around the backfill based on characterization efforts described previously (2). Downgradient of the backfill, several thin, essentially horizontal, sandy layers exist within 8 m of ground surface. These include the S2, within the zone of water table fluctuation, and the S3 and S4, always fully saturated. Layers of less permeable silt or clayey silt separate these sandy layers. Since the S2 VOL. 41, NO. 6, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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S3 aquifer. Our monitoring above the S3 confirmed this conceptual model, depicted in Figure 2. Prior to the experiments MTBE concentrations were quite high in the S2, with a maximum detected value of 72900 µg/L. The maximum detected MTBE concentration in the S3 was much lower (1260 µg/L). TBA was detected in both the S2 and the S3, yet the range (0-97 µg/L, and 0-116 µg/L, respectively) and mean values (17.2 µg/L and 16.6 µg/L, respectively) were similar in samples collected from the two sands.
FIGURE 1. Map of the experimental area immediately around and downgradient of the former gas station. EUG to EK are transects of monitoring wells. The arc of background monitoring wells contains two (circled) used to supply water for injection. Injection occurred in two sets of 3 wells in the ER transect to create two “lanes” of injected water (“No Ethanol Lane” and “With Ethanol Lane,” depicted as blue (left) and red (right) outlined arrows, respectively). The dashed line S-S′′ shows the location of the vertical schematic in Figure 2. Reprinted with permission from Mackay et.al. (2). Copyright 2006, American Chemical Society.
FIGURE 2. Conceptualization of the subsurface. Screened sections of wells are shown as blue or red rectangles. Red shading depicts our conceptualization of the distribution of contaminants, primarily MTBE, in strata shallower than the S3 aquifer, and illustrates the hypothesis that contaminants migrating in the S3 aquifer are introduced to the S3 groundwater from above by diffusion or slow advection. Reprinted with permission from Mackay et al. (2). Copyright 2006, American Chemical Society. does not extend north of Monroe Street, the S3 aquifer is the primary pathway for downgradient advection of water and contaminants originating in or near the backfill. Outside of the backfill, vertical movement of water is impeded by low permeability surficial and deeper layers. For years prior to our experiments, there has been very little BTEX or other petroleum hydrocarbon contamination remaining in the source area or migrating downgradient in the S3 aquifer, apparently due to excavation of the spill and past/ ongoing biodegradation under predominantly sulfatereducing conditions. Pre-experimental BTEX concentrations in the S3 were very low (most
