Potential for the Success of Bioattenuation at MTBE-Impacted Sites
Rula A. Deeb, Ph.D. and Michael Kavanaugh, Ph.D., P.E., Malcolm Pirnie, Inc

Frederick J. McGarry, P.E., DEE, State of New Hampshire Department of Environmental Services

Although methyl tertiary butyl ether (MtBE) was introduced in 1995 in reformulated gasoline to reduce air pollution from internal combustion engines, this gasoline additive, when it reaches groundwater, has frequently resulted in widespread contamination of both public and private water supplies. Contamination of public water supplies by MtBE at some level has been reported in all six New England states, with the greater number and level of contamination occurring in those states that are part of the reformulated gasoline (RFG) program. This paper will provide the most recent information on the number of public water supplies affected by this contaminant and will compare the frequency of detection in the non-RFG, Maine, Vermont, and portions of New Hampshire, with the remaining New England states. The paper will also review funding available in each state to pay the cost of remediation and providing treatment systems for water supplies affected by MtBE.

Richard H. Veeh, Eric A. Kern, Heiko W. Langner, Richard E. Macur, Alfred B. Cunningham, Montana State University

Methyl tert-butyl ether (MTBE) is an important fuel oxygenate that has been identified as a widespread point and non-point source contaminant in groundwater. MTBE has been shown to be very mobile and recalcitrant in groundwater systems, thus enhancing concern about associated health risks. Our study was undertaken to assess the potential for bacterial degradation of MTBE as part of the overall natural attenuation that may be occurring at a gasoline release site near Ronan, Montana. Initially, we employed standard batch enrichment techniques to develop an MTBE-degrading culture, using soil collected from the contaminated site as inoculum and added ¹⁴C-MTBE and 2-propanol as co-substrates. Several MTBE-degrading bacterial strains (in the presence of 2-propanol) were isolated from this mixed culture. Subsequently, we identified these bacterial isolates and documented changes in the mixed culture through several successive enrichments. Using molecular techniques including PCR, DGGE, and DNA sequencing, we have now identified most of the members of the mixed consortium. With MTBE as the sole carbon source, we determined that the isolates obtained initially were apparently declining over time relative to a more robust MTBE-degrading bacterium.

Michael Hyman, North Caroline State University and Kirk O’Reilly, Chevron Research and Technology Company

Methyl tertairy butyl ether (MTBE) is used in gasoline as an octane enhancer and as an oxygenate. The majority of MTBE enters the environment as part of gasoline and the fate of MTBE and other gasoline components are therefore intimately intertwined. We have demonstrated that a wide variety of microorganisms that can metabolize gasoline hydrocarbons can fortuitously degrade MTBE without using this compound as a growth substrate. This type activity is referred to as cometabolism and may represent an important mechanism for MTBE degradation in the environment. For example, we have demonstrated that bacteria grown of n-alkanes (C₂-C₈) rapidly degrade MTBE. Identified metabolic intermediates include tertiary butyl formate and tertiary butyl alcohol. In pure culture studies we have also shown a strong correlation between the ability of bacteria to cometabolize MTBE and their ability to grow on simple branched alkanes. In microcosm studies the predominant branched alkane in gasoline, isopentane, also consistently promotes MTBE degradation in a variety of soil types. Recently, we have demonstrated that certain benzene- and toluene-utilizing bacteria can also cometabolically degrade MTBE while these same compounds are potent inhibitors of MTBE degradation by alkane-utilizing bacteria. Our current research is directed at understanding how MTBE-degrading activity is regulated in alkane-utilizing bacteria. Our evidence suggests that products derived from the partial aerobic degradation of gasoline hydrocarbons can stimulate MTBE cometabolism in alkane-utilizing bacteria. This effect may explain the addition of oxygen to anaerobic MTBE-contaminated groundwater can lead to MTBE degradation in the apparent absence of organisms that can grow on MTBE. This presentation will summarize our studies into microbial degradation of MTBE and will attempt to identify environments where gasoline hydrocarbons can potentially promote or inhibit cometabolic MTBE degradation.

Rula A. Deeb, Ph.D. and Michael Kavanaugh, Ph.D., P.E., Malcolm Pirnie, Inc

Uncertainty regarding continued reliance on natural attenuation processes for remediation at gasoline-contaminated sites has increased over the past several years due to the addition of methyl tert-butyl ether (MTBE) to gasoline and its subsequent detection in groundwater on a national scale. Contrary to early reports of MTBE recalcitrance, a review of recent literature and of on-going studies suggests that this compound is biodegradable by a wide range of microorganisms. Both mixed and pure bacterial and fungal cultures have been shown to partially degrade or completely mineralize MTBE. MTBE and its major metabolic intermediate, and tert-butyl alcohol (TBA), can either be utilized as sole sources of carbon and energy or degraded cometabolically by cultures grown on alkanes or aromatic compounds. In laboratory studies, MTBE degradation has been shown to occur under aerobic conditions with half-lives ranging from 0.04 to 29 days. In addition, field scale ex-situ fixed-film and suspended growth bioreactors have been shown to successfully remove MTBE with efficiencies ranging from 83 to 96% with hydraulic residence times ranging from 0.3 hours to 3 days. While in some instances MTBE and TBA were degraded in aquifer microcosms from gasoline-contaminated sites, the evidence to date suggests that the bioattenuation of MTBE in subsurface environments is mostly a function of site-specific conditions. Specifically, the rates of MTBE and TBA biotransformation in the field appear to be heavily correlated to dissolved oxygen concentrations and to groundwater velocities.

This paper will focus on the most frequently asked questions regarding the potential for success of intrinsic biological processes as an attenuation mechanism for MTBE and TBA in the field. The current understanding of the factors limiting MTBE bioattenuation in the environment will be evaluated. Important parameters for optimizing MTBE and TBA biodegradation rates will be discussed as well as limitations that may impede the application of successful bioremediation strategies. Several key issues will be reviewed in detail including the potential accumulation of rate limiting intermediates during MTBE degradation, the effect of co-contaminants on MTBE biotransformation rates and the observed dependency of MTBE and TBA biodegradation on dissolved oxygen concentrations. Finally, bioattenuation will be compared to in situ conventional and emerging technologies for MTBE and TBA removal in the context of effectiveness for all contaminants of concern, reliability, risk reduction potential and cost.