Methyl Tertiary-Butyl Ether (MTBE)

Methyl tertiary butyl ether (MTBE) is used as a gasoline additive for several purposes. It has been added in relatively low concentrations to increase octane ratings in premium grade fuels since the late 1970's. Beginning in the early 1990's MTBE has been added in much higher concentrations (up to 15 percent) to enhance gasoline combustion and reduce tailpipe emissions. Gasoline producers are required to add a fuel oxygenate (chemicals that aid combustion by increasing available oxygen) to California Phase II Reformulated Gasoline to reduce air pollution, especially carbon monoxide. MTBE is the most common fuel oxygenate, used in more than 80 percent of oxygenated fuels. MTBE is credited with contributing to significant reductions in carbon monoxide and ozone levels in many of these areas.

Potential and documented contamination of water resources by MTBE has become a cause for major public concern and increasing controversy. MTBE readily dissolves in water, can move rapidly through soils and aquifers, is resistant to microbial decomposition and is difficult to remove in water treatment. The US Environmental Protection Agency (USEPA) has classified MTBE as a potential human carcinogen. Finally, MTBE can give water an unpleasant taste and odor. These factors have caused widespread concern that drinking water supplies and human health may be at risk. On March 25,1999 California Governor Gray Davis issued an Executive Order "...for the removal of MTBE from gasoline at the earliest possible date, but not later than December 31, 2002."

The USEPA has not established drinking water standards for MTBE. In December 1997 the USEPA issued a "Drinking Water Advisory" for MTBE of 20 to 40 micrograms per liter (µg/L), primarily for taste and odor considerations. Based on present knowledge, USEPA believes that this provides a wide margin of safety.

The California Department of Health Services (DHS) is developing and implementing MTBE drinking water standards. The DHS has an action level for MTBE of 35 µg/L. In January 1999 a secondary Maximum Contaminant Level (MCL) of 5 µg/L was implemented. Secondary MCLs are based on aesthetics, in this case taste and odor. Primary MCLs are health related, and DHS is expected to adopt a MTBE primary MCL later in 1999.

Surface-water MTBE concentrations in California have been reported up to 12 µg/L in water bodies where motorized recreation activities are allowed. MTBE in California urban storm runoff samples generally has been less than 2 µg/L. Higher concentrations have been reported in heavily urbanized parts of the New York Metropolitan area.

Ground water used as a drinking water source in the Santa Monica area has been reported with MTBE concentrations of more than 600 µg/L. This has been traced to leaking underground gasoline storage facilities. MTBE concentrations greater than 30 µg/L usually can be attributed to a leaking tank or pipeline facility. Low MTBE concentrations, less than 3 µg/L, are more likely to result from atmospheric sources. Concentrations of MTBE in ground water between 3 and 30 µg/L may indicate that a leak or spill contaminant plume is affecting the well, in which case increasing concentrations may be anticipated. Ground water sampling in rural areas has a low frequency of detection, and concentrations of MTBE are generally low when detected. In California and in other States there is a high correlation between urban land use, motor vehicle traffic and population density, and the frequency of detection and water concentrations of MTBE. This could be anticipated given that gasoline is the only source of MTBE.

The relatively short period during which MTBE has been widely used as a gasoline oxygenate has resulted in major improvements in air quality. But in these same areas of high MTBE use, low concentrations of MTBE in most urban storm runoff water and its detection in urban ground water samples cause concern that increasing amounts of the water resources in California may have detectable concentrations of MTBE over time. While increasing amounts of information on MTBE is available, the trends and implications are not yet fully understood. Scientists at USGS and other organizations are working to better document, evaluate and understand the environmental behavior and fate of this compound.