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Technology Assessment Branch
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Methyl Tertiary Butyl Ether (MTBE)
- Policy and Guidance
- Chemistry and Behavior
- Environmental Occurrence
- Detection and Site Characterization
- Treatment Technologies
- Conferences and Seminars
- Additional Resources
Methyl tertiary butyl ether (methyl tert-butyl ether, methyl t-butyl ether, or MTBE) is a chemical compound used as a fuel additive in gasoline. It is an oxygenate, meaning it increases the oxygen content of the gasoline, and is also a source of octane in gasoline. It is widely used in those parts of the country where oxygenated gasoline is required, either by federal or state law. Refiners primarily have added MTBE to gasoline to meet the Clean Air Act (CAA) requirement that areas with severe problems in attaining the National Ambient Air Quality Standard (NAAQS) for ozone use Reformulated Gasoline (RFG) containing 2% oxygen by weight. Many states have voluntarily chosen to use RFG as a means of addressing marginal, moderate, or serious ozone nonattainment, and some refiners use MTBE to boost the octane of gasoline. MTBE also is used sometimes to meet the requirements of the CAA Wintertime Oxygenated Fuel (Wintertime Oxyfuel) program, which specifies that gasoline must contain 2.7% oxygen by weight during the wintertime in areas not in attainment for the NAAQS for carbon monoxide. Purely as a fuel additive, MTBE performs successfully, but environmental problems with the compound led the EPA in March 2000 to provide an advance notice of its intent to initiate a rulemaking pursuant to section 6 of the Toxic Substances Control Act (TSCA) to eliminate or limit the use of MTBE as a fuel additive.
While the use of MTBE as a fuel additive in gasoline has helped to reduce harmful air emissions, it also has caused widespread and serious contamination of the nation's drinking water supplies. Current data on MTBE levels in ground and surface waters indicate widespread and numerous detections at low levels of MTBE, with a more limited number of detections at higher levels. Given MTBE's widespread use as a gasoline additive and the large volumes of gasoline that are stored, transported, and used in all areas of the country, releases of MTBE to the nation's ground and surface waters occur in a number of ways. Leakage from the gasoline storage and distribution system is a major source of contamination, but the contamination also comes from spills, emissions from marine engines into lakes and reservoirs, and to some extent from air deposition. It can end up in drinking water supplies even when there are no indications of other gasoline components. MTBE is detected in water much more often and at higher concentrations in areas of the country where RFG is sold.
The presence of MTBE in drinking water sources presents two major problems. The first concern is that MTBE contamination may render water supplies unuseable as drinking water. MTBE has an offensive taste and odor which 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 ground-water 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 ground-water 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, MTBE does not biodegrade as readily as other components of gasoline.
Given the numerous and diverse sources of potential release into the environment and the problems associated with cleaning it up once it is released, EPA believes a comprehensive approach to such risk must include consideration of either reducing or eliminating the use of MTBE as a gasoline additive. EPA has developed a Methyl Tertiary Butyl Ether (MTBE) website that provides extensive general information about the contaminant.
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, 10 pp, 2010.
This brief groundwater information sheet provides general information (fate and transport, health effects, testing and remediation methods) and identifies where high levels of the compound are found in California. The information is pulled from a variety of sources, and a bibliography is provided.
Methyl Tertiary Butyl Ether (MTBE): Advance Notice of Intent to Initiate Rulemaking Under the Toxic Substances Control Act to Eliminate or Limit the Use of MTBE as a Fuel Additive in Gasoline; Advance Notice of Proposed Rulemaking
Federal Register, Vol 65 No 58, p 16093-16109, 20 Mar 2000.