Methyl Tertiary Butyl Ether (MTBE)

Methyl tertiary butyl ether (MTBE, CAS No. 1634-04-4) is an ether compound made by combining methanol and isobutylene. The methanol typically is derived from natural gas; isobutylene can be derived as a byproduct of the petroleum refinery process.

MTBE is a liquid, colorless, volatile organic compound with a terpene-like odor, a vapor pressure of 249 mm Hg at 25° C, a water solubility of 51,000 mg/L, and a specific gravity of 0.74. When released to the air, it has a 5 to 6 day half life and degrades primarily by reacting with photochemically produced hydroxyl radicals to form tertiary butyl formate. The exposure to atmospheric UV light alone will not be sufficient to decompose this compound. The low log K_ow (1.24) suggests that MTBE will be very mobile in soils. MTBE has a Henry's Constant of 0.022 and hence is not likely to form a vapor plume in the vadose zone above a dissolved phase plume.

MTBE is capable of traveling through soil rapidly, is very soluble in water (much more so than BTEX), and moves at nearly the same speed as the groundwater (about twice as fast as benzene). MTBE can be degraded either aerobically or anaerobically but only by a relatively small number of species under specific site conditions (Key et. al 2013 and Davis and Erickson 2004). Wilson et. al (2005) postulates that bacteria are available at most sites that will degrade MTBE but they might not be abundant and are slow growing, hence evidence of degradation may have a significant lag time. Studies by Mace and Choi (1998) and Kamath et. al (2012) indicate in a majority of cases benzene and MTBE plumes stabilize and shrink over time and are comparable in length. They attribute this to biodegradation of both chemicals.

MTBE has a very unpleasant turpentine-like taste and odor that at low levels of contamination can render drinking water unacceptable for consumption. Studies conducted on the concentrations of MTBE in drinking water at which individuals can detect the taste and odor of the chemical have shown that human sensitivity to the taste and odor of MTBE varies widely.

Additional Information

Assessment of MTBE Biodegradation in Contaminated Groundwater Using 13C and 14C Analysis: Field and Laboratory Microcosm Studies (Abstract)
S. Thornton, S.H. Bottrell, K.H. Spence, R. Pickup, M.J. Spence, N. Shah, H.E.H. Mallinson, and H.H. Richnow.
Applied Geochemistry 26:(5)828-837(2011)

Radiolabelled assays and compound-specific stable isotope analysis (CSIA) were used to assess MTBE biodegradation in an unleaded fuel plume in a UK chalk aquifer, both in the field and in laboratory microcosm experiments. The 14C-MTBE radiorespirometry studies demonstrated widespread potential for aerobic and anaerobic MTBE biodegradation in the aquifer.

Biodegradation of MTBE by Bacteria Isolated from Oil Hydrocarbons-Contaminated Environments
Lalevic, B., V. Raicevic, D. Kikovic, L. Jovanovic, G. Surlan-Momirovic, J. Jovic, A.R. Talaie, and F. Morina.
International Journal of Environmental Research 6(1):81-86(2012)

Biodegradation of Methyl Tert-Butyl Ether by Isolated Bacteria from Contaminated Soils to Gasoline
A. Kariminik, J. Amini, and K. Saeidi.
International Research Journal of Applied and Basic Sciences 5(12):1566-1569(2013)

Three bacterial species isolated from gasoline-contaminated soils were capable of degrading MTBE as a sole carbon and energy source. The degradation rates of MTBE in 500 ppm concentration after 20 days with Micrococcus luteus, Bacillus subtilis, and B. megaterium was 93.2%, 60%, and 97.97%, respectively. The findings indicated that these bacteria were successfully adapted on MTBE and potentially can offer a suitable and efficient method to treat MTBE contaminated environments.

Biodegradation of the Gasoline Oxygenates Methyl tert-Butyl Ether, Ethyl tert-Butyl Ether, and tert-Amyl Methyl Ether by Propane-Oxidizing Bacteria
R. Steffan, K. Mcclay, S. Vainberg, C. Condee, and D. Zhang.
Applied and Environmental Microbiology, Vol 63 No 11, p 4216-4222, 1997.

Chapter 13: MTBE
U.S. EPA, Office of Water.
Regulatory Determinations Support Document for Selected Contaminants from the Second Drinking Water Contaminant Candidate List (CCL 2), EPA 815-R-08-012, 76 pp, 2008

This chapter discusses the status of EPA's evaluation of information and data on MTBE's physical and chemical properties, use and environmental release, environmental fate, potential health effects, and occurrence and exposure estimates.

Chemical and Physical Information
Toxicological Profile Methyl t-Butyl Ether (MTBE), Chapter 3.
Agency for Toxic Substances and Disease Registry, 2022.

Evaluating Potential Exposures to Ecological Receptors Due to Transport of Hydrophobic Organic Contaminants in Subsurface Systems
Ford, R.G., M.C. Brooks, C.G. Enfield, and M. Kravitz.
EPA 600-R-10-015, 69 pp, 2014

Detailed discussion of enhanced transport mechanisms is the focus of this technical paper. It recommends several types of screening assessments to evaluate site conditions for the potential to enhance transport of HOCs—PCBs, dioxins, fuels (including the influence of MTBE), and creosote and tar DNAPL—as well as site artifacts that result from inadequate well installation and sampling procedures within a groundwater monitoring network. These assessments are incorporated into a suggested three-tiered decision analysis process that provides a summarized view of the upland contaminant-source characteristics that need evaluation to establish whether facilitated transport of HOCs might occur at a given site.

Exceptionally Long MTBE Plumes of the Past Have Greatly Diminished
McDade , J.M., J.A. Connor, S.M. Paquette, and J.M. Small.
Groundwater, Vol 53 No 4, 515-524, 2015

Reviewers compiled recent groundwater monitoring records for nine exceptional plumes identified in studies from the late 1990s and early 2000s, sites with groundwater plume lengths ranging from 2,700 ft to 10,500 ft in length. Groundwater monitoring data compiled in this review demonstrate that these large MTBE plumes decreased in length over the past decade, with five of the nine plumes exhibiting decreases of 75% or more compared to their historical maximum lengths. MTBE concentrations within these plumes declined by 93-100%, with two of the nine sites showing such significant decreases (98% and 99%) that the regulatory authority found the sites require no further action.

Fate, Transport and Remediation of MTBE
Nancy E. Kinner.
University of New Hampshire, Bedrock Bioremediation Center, 16 pp, 2001.
Contact: Nancy Kinner, nancy.kinner@unh.edu

Methyl tert-butyl ether Pathway Map
Charlotte Rosendahl Pedersen and Carla Essenberg.
The University of Minnesota Biocatalysis/Biodegradation Database.
Contact: Lynda Ellis, lynda@tc.umn.edu, or Larry Wackett, wackett@cbs.umn.edu

Microbial Degradation of the Fuel Oxygenate Methyl Tert-Butyl Ether (MTBE)
Laura Youngster, Ph.D. dissertation, Rutgers, New Brunswick, NJ. 142 pp, 2009

Although microbially mediated biodegradation holds promise as a tool for remediation of MTBE-contaminated water supplies, MTBE biotransformation processes are slow, and it is difficult to isolate the appropriate degrading organisms. In this study, MTBE-degrading cultures were analyzed using a variety of isolation-independent techniques. A majority of the experiments used previously established anaerobic enrichment cultures that had been maintained on MTBE for several years. This work contributes to the current body of knowledge about MTBE degradation, and the data presented will be useful in many aspects of studying, enhancing, and monitoring MTBE degradation under a variety of conditions.

Monitored Natural Attenuation Technical Guidance
Site Remediation Program, New Jersey Department of Environmental Protection, 2012

This guidance contains an extensive discussion of MTBE biodegration mechanisms.

Physiological and Enzymatic Diversity of Aerobic MTBE Biodegradation Processes
M. Hyman. Second European MTBE Conference in Barcelona, Spain. 26 slides, 2006

A Review of Bioremediation and Natural Attenuation of MTBE
L. Davis and L. Erickson.
Environmental Progress 23(3):243-252(2004)

Using DNA-Stable Isotope Probing to Identify MTBE- and TBA-Degrading Microorganisms in Contaminated Groundwater (Abstract)
K.C. Key, K.L. Sublette, K. Duncan, D.M. Mackay, K.M. Scow, and D. Ogles. Groundwater Monitoring & Remediation, 33(4):57-68(2013)

Using Groundwater Age Distributions to Understand Changes in Methyl Tert-Butyl Ether (MTBE) Concentrations in Ambient Groundwater, Northeastern United States
Lindsey, B., J. Ayotte, B. Jurgens, and L. DeSimone.
Science of the Total Environment 579:579-587(2017) [Abstract]

MTBE use in the U.S. peaked in 1999 and was largely discontinued by 2007. Based on a national survey of wells selected to represent ambient conditions, temporal changes in MTBE concentrations in groundwater were evaluated in the northeastern United States, an area of the nation with widespread low-level detections of MTBE. Six well networks, each representing specific areas and well types (monitoring or supply wells), were each sampled at 10-yr intervals between 1996 and 2012. Concentrations were decreasing or unchanged in most wells as of 2012, with the exception of a small number of wells where concentrations continue to increase. Statistically significant increasing concentrations were found in one network sampled for the second time shortly after peak MTBE use, and decreasing concentrations were found in two networks sampled for the second time about 10 yr after peak MTBE use. Modeling and sample results showed that wells with young median ages and narrow age distributions responded more quickly to changes in the contaminant source than wells with older median ages and broad age distributions. Well depth and aquifer type affect these responses. Regardless of the timing of decontamination, all of these aquifers show high susceptibility for contamination by a highly soluble, persistent constituent.