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.
Widespread contamination of groundwater by methyl tertiary butyl ether (MTBE) has triggered the exploration of different technologies for in situ removal of the pollutant, including biostimulation of naturally occurring microbial communities or bioaugmentation with specific microbial strains known to biodegrade the oxygenate. After laboratory studies revealed that bacterial strain PM1 rapidly and completely biodegraded MTBE in groundwater sediments, the organism was tested in an in situ field study at Port Hueneme Naval Construction Battalion Center in Oxnard, California. Two pilot test plots (A and B) in groundwater located down-gradient from an MTBE source were intermittently sparged with pure oxygen. Plot B was also inoculated with strain PM1. MTBE concentrations up-gradient from plots A and B initially varied temporally from 1.5 to 6 mg MTBE/L. Six months after treatment began, MTBE concentrations in monitoring wells down-gradient from the injection bed decreased substantially in the shallow zone of the ground-water in plots A and B, thus even in the absence of the inoculated strain PM1. In the deeper zone, downstream MTBE concentrations also decreased in plot A and to a lesser extent in plot B. Difficulties in delivery of oxygen to the deeper zone of plot B, evidenced by low dissolved oxygen concentrations, were likely responsible for low rates of MTBE removal at that location. We measured the survival and movement of strain PM1 in groundwater samples using two methods for detection of DNA sequences specific to strain PM1: TaqMan quantitative polymerase chain reaction, and internal transcribed spacer region analysis. A naturally occurring bacterial strain with > 99% 16S rDNA sequence similarity to strain PM1 was detected in groundwater collected at various locations at Port Hueneme, including outside the plots where the organism was inoculated. Addition of oxygen to naturally occurring microbial populations was sufficient to stimulate MTBE removal at this site. In some cases, however, inoculation with an MTBE-degrading culture may be warranted.
Microbial communities in subsurface environments are poorly characterized and the impacts of anthropogenic contamination on their structure and function have not been adequately addressed. The release of contaminant(s) to a previously unexposed environment is often hypothesized to decrease the diversity of the affected community. We characterized the structure of microbial communities along a gradient of benzene, toluene, ethylbenzene, and xylene (BTEX) and methyl-tert-butyl-ether (MTBE) contamination, resulting from a petroleum spill, within a shallow sandy aquifer at Vandenberg Air Force Base (VAFB) in Lompoc, CA. Differences in microbial community composition along the contaminant plume were assessed via a combinatorial approach utilizing denaturing gradient gel electrophoresis (DGGE), cloning and sequencing, intergenic transcribed spacer analysis (ITS), and comparative phylogenetic analysis of partial 16S rDNA sequences. Substantial bacterial sequence diversity, similar levels of species richness, and similar phylo-groups (including the Cytophaga-Flavobacterium-Bacteroidetes group and numerous members of the alpha-, beta-, gamma-, delta-, and epsilon-groups of the proteobacteria) were observed in both uncontaminated and contaminated regions of the aquifer. High-resolution measures (ITS fingerprinting and phylogenetic inference) readily separated communities impacted by the original petroleum spill (in source zone) from those in other parts of the aquifer and indicated that communities exposed to MTBE only were similar to communities in uncontaminated regions. Collectively, these data suggest that petroleum contamination alters microbial community structure at the species and subspecies level. Further study is required to determine whether these changes have an impact on the functioning of this subsurface ecosystem.
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