EPA Grant Number: R83-0633-010
Title: Identifying the Signature of the Natural Attenuation of MTBE in Goundwater Using Molecular Methods and "Bug Traps"
Investigators: K.L. Sublette, L Ford, D. White, A. Peacock, R. Kolhatkar
Institution: University of Tulsa
EPA Project Officer: Bala Krishnan
Project Period: 3-1-03 to 2-28-04
Project Amount: $148,887
Research Category: Intrinsic bioremediation
The use of fuel oxygenates like methyl tert-butyl ether (MTBE) as octane enhancers has greatly muddied the waters, so to speak, in the use of risk-based management of gasoline-contaminated groundwater resulting from leaking underground storage tanks (USTs). In the absence of MTBE a risk-based approach to management of these sites depends on the natural attenuation of the more soluble components of the gasoline (BTEX) through intrinsic bioremediation. Microbial degradation combined with a tendency for these compounds to be somewhat retarded in their movement by interaction with soil organic matter have frequently combined to make risk- based management of these sites both cost-effective and protective of human health and ecological receptors. However, in oxygenated gasoline MTBE is present in higher concentrations that benzene, is more water soluble than BTEX, sorbs weakly to soil and aquifer material, and seems to be more recalcitrant to biodegradation. These properties combine to make MTBE more environmentally persistent than other gasoline components and, therefore, make a risk-based management option more difficult to implement. A defensible risk-based management approach to MTBE contaminated sites would be a major benefit to the domestic petroleum industry. There has been tantalizing field evidence that intrinsic bioremediation of MTBE and its major metabolite, tert-butyl alcohol (TBA), do indeed occur. Most observations link intrinsic bioremediation of these compounds to methanogenic conditions in the subsurface. However, both laboratory microcosm studies and field observations are inconsistent. Some sites appear to support MTBE biodegradation and some do not and the sites are not sufficiently distinct to ascertain why MTBE biodegradation occurs in some cases and not in others. The signature of MTBE biodegradation has yet to be found.
The Science Advisory Board of the U.S. Environmental Protection Agency has recently made three major research recommendations to the EPA Office of Research & Development on the subject of MTBE: 1) Determine the biodegradability of MTBE under various field conditions (for example, various electron acceptors and mixtures of hydrocarbons substrates); 2) Improve the predictability of dissolution rates of MTBE and their fluxes exiting source zones; and 3) Monitor multiple representative and highly characterized sites to provide an information database on indirect measures of MTBE natural attenuation (as has been done with BTEX). Clearly the objective is to provide a scientific basis for risk-based decision making in the management of MTBE plumes. The Board specifically cites the need to determine the "footprint" of MTBE biodegradation.
This proposal seeks to address the third recommended research objective with the exception that we seek a direct, not indirect, measure or footprint of MTBE biodegradation. Specifically we propose to search for the signature of MTBE intrinsic bioremediation in the microbial ecology of the contaminated groundwater. BP, in collaboration with EPA, recently conducted an extensive survey of 74 BP retail sites with gasoline spills. Some of these sites indicated MTBE attenuation and others did not. Working with BP we will investigate and compare the in situ microbial ecologies of a subset of these sites using "bug traps" which concentrate organisms for analysis and provide a time-integrated picture of the subsurface microbial community. Further we will supplement this field work with an analysis of the microbial ecology of microcosms currently operated in the laboratory of Dr. Joe Suflita (University of Oklahoma). Microcosms positive for MTBE biodegradation will be sampled using the same bug traps used in the field sampling.
It is hypothesized that the in situ microbiota contain a signature of past and present MTBE exposure and utilization. A comprehensive measure of the in situ microbial ecology of a site will yield the identity of this signature. We maintain that a definition of MTBE natural attenuation in terms of a site's microbial ecology is direct and definitive, whereas a definition in terms of contaminant chemistry and/or geochemistry or hydrology is indirect and inferential. It is well established that indigenous microbes generate various compounds within their cellular structure (biomarkers) that reflect in situ conditions. Therefore, the microbial community contains a record of the sum microbial response to the environment that is written from changes in the biochemistry of individual organisms. We propose that these responses, in association with knowledge of available electron acceptors and donors of a site, will define the signature of a successful MTBE natural attenuation process.