Methyl Tertiary Butyl Ether (MTBE)

Methyl tertiary butyl ether (methyl tert-butyl ether, methyl t-butyl ether, or MTBE) is a chemical compound once commonly used in the United States as a fuel additive in gasoline. It is an oxygenate, meaning it increases the oxygen content of the gasoline, and is also an octane rating enhancer in gasoline. MTBE and other oxygenates were commercially introduced into fuels in 1979 as anti-knock compounds following the phase out of tetraethyl lead in gasoline. The Clean Air Act Amendments of 1990 resulted in the widespread use of MTBE in those parts of the country where oxygenated gasoline was required, either by federal or state law. Refiners primarily added MTBE to gasoline to meet the Clean Air Act requirement that areas with severe problems in attaining the National Ambient Air Quality Standard for ozone use Reformulated Gasoline (RFG) containing 2% oxygen by weight. In 2005, Congress passed the Energy Policy Act, which removed the oxygenate requirement for RFG. At the same time, Congress also instituted a renewable fuel standard. In response, refiners made a wholesale switch, removing MTBE and blending fuel with ethanol. According to EPA's RFG Survey Data, MTBE has not been used in significant quantities in RFG areas since 2005. A similar decrease in MTBE use has also been observed in conventional gasoline areas (EPA 2013).

While the use of MTBE as a fuel additive in gasoline helped to reduce harmful air emissions, accidental releases and spills have contaminated surface and groundwaters. MTBE is detected in water more often and at higher concentrations in areas of the country where RFG was sold.

The presence of MTBE in drinking water sources presents two major problems. The first concern is that MTBE contamination may render water supplies unusable as drinking water. MTBE has an offensive taste and odor that can be detected in water even at low levels. Because of the taste and odor problem, MTBE contamination has resulted in the loss of certain drinking water sources. For example, high levels of MTBE found in groundwater wells that supply Santa Monica's drinking water led that city to close its wells, forcing it to purchase drinking water from another public water supplier. In addition, MTBE detections found in groundwater wells that supply South Lake Tahoe forced the South Tahoe Public Utility District to close 8 of its 34 wells despite detections below EPA's advisory levels. An additional four wells were closed as a precautionary measure due to their proximity to the existing MTBE plumes.

The second major concern involves uncertainty regarding the level of risk to public health from the chronic exposure of large numbers of people to low levels of MTBE in drinking water. While inhalation of MTBE in high concentrations has been shown to cause cancer in laboratory animals, the Agency concluded in 1997 that there is little likelihood that MTBE in drinking water would cause adverse health effects at levels that cause taste and odor problems. There is still much uncertainty about the extent of the health risks associated with chronic, low-level exposures to MTBE in drinking water. The Agency is continuing to review and update its analysis of the potential health risks posed by MTBE.

Once MTBE contaminates a drinking water source, its chemical nature makes it difficult, expensive, and time-consuming to remediate. For example, it is much harder and more expensive to remove MTBE from drinking water than it is to remove other organic components of gasoline. Furthermore, while MTBE biodegrades under both aerobic and anaerobic conditions, these conditions require specific microbial species and physical/chemical environments that are not found at all sites.

NOTE: EPA archived the Agency's original MTBE website, and no longer updates the content there. The Office of Underground Storage Tanks maintains an MTBE website, and this CLU-IN MTBE website is also maintained and periodically updated by EPA contractors.

Additional Information

Fuel Oxygenates
D. Barcelo (ed.).
Springer, New York. ISBN: 978-3-540-72640-1, Handbook of Environmental Chemistry, Vol. 5: Water Pollution, Part 5R, 411 pp, 2007

Contains papers on the characterization, chemistry, and behavior in the subsurface of fuel oxygenates, primarily MTBE, as well as information on the occurrence of MTBE and other fuel oxygenates in source water and drinking water of the United States. New molecular tools are reported for assessing the biodegradability of oxygenates by microflora from MTBE-contaminated sites, as is compound-specific isotope analysis to characterize degradation pathways and to quantify in situ degradation of fuel oxygenates and other fuel-derived contaminants. Adsorption and abiotic degradation of MTBE is also discussed, along with a toxicological review of methyl- and ethyl tertiary butyl ethers.

Groundwater Information Sheet: Methyl Tertiary-Butyl Ether (MTBE)
California State Water Resources Control Board, Division of Water Quality, GAMA Program, 10 pp, 2017

The information in this fact sheet was compiled by the staff of the Groundwater Ambient Monitoring and Assessment (GAMA) Program to provide general information regarding MTBE. The information was pulled from a variety of sources and relates mainly to drinking water. References are provided as sources of additional information.

Trends in Methyl Tert-Butyl Ether Concentrations in Private Wells in Southeast New Hampshire: 2005 to 2015
Flanagan, S., J. Levitt, and J. Ayotte.
Environmental Science & Technology 51(3):1168-1175(2017) [Abstract]

In southeast New Hampshire, where reformulated gasoline was used from the 1990s to 2007, MTBE concentrations ≥ 0.2 µg/L were found in water from 26.7% of 195 domestic wells sampled in 2005. In 2015 (8 yr after MTBE was banned), 10.3% continued to have MTBE. Most wells (140 of 195) had no MTBE detections (concentrations < 0.2 µg/L) in 2005 and 2015. On average, MTBE concentrations decreased 65% among 47 wells, whereas MTBE concentrations increased 17% among 4 wells between 2005 and 2015. Percent change in detection frequency from 2005 to 2015 was lowest in areas of high population density and highest where population densities were low. Bedrock composition was also a relevant factor. Additional information: Slide presentation