What is MTBE?
Methyl tertiary Butyl Ether or Methyl tert Butyl Ether is an oxygenate added to gasoline in high concentrations (approximately 15% on a volumetric basis) to increase octane levels and enhance combustion, thereby reducing emissions of carbon monoxide (CO) and other organic compounds.  The California Air Resources Board predicted that the addition of oxygenates to fuels would reduce ozone precursors by 15%, reduce benzene emissions by 50% and reduce CO emissions by 11%; these reductions are equivalent to removing emissions from 3.5 million vehicles. 

Several events raised concern over the safety of MTBE.  In 1996, the city of Santa Monica closed some of its major drinking water wells after discovering MTBE contamination.  In addition, the U.S. Geological Survey reported MTBE to be the second most common contaminant in shallow urban aquifers.  Three percent of the USGS shallow urban wells** tested exceeded the minimum US EPA draft health advisory level of 20µg/l.

After review of the pros and cons of continued use of MTBE as the oxygenate in California gasoline, on March 15, 2002, Governor Gray Davis issued Executive Order D-52-02. The Order allowed California refineries up to 12 additional months for the transition from MTBE to ethanol in the production of gasoline. As of December 31, 2003, California refiners have successfully phased out the use of MTBE. Ethanol is now used as the primary oxygenate in those areas requiring an oxygenate additive under Federal law.

**Note these wells were not drinking water supplies.

Properties of MTBE
Relatively high vapor pressure (i.e. the vapor pressure of MTBE at 25°C is about 1/3 of an atmosphere) and water solubility allows MTBE to transfer easily from gasoline to air and then to water, or from gasoline directly to water.  MTBE will reversibly transfer back and forth between water and air depending upon which stream has the relatively higher (non-equilibrium) concentration.  MTBE only weakly partitions to soils and generally moves along with groundwater in the subsurface.

Health Concerns
Short term (acute) data on MTBE exposures indicates that it is not highly toxic.  However, there is insufficient knowledge about the long-term (chronic) toxicity, including carcinogenicity and reproductive toxicity, of MTBE on humans, animals and ecosystems. 

Sources of MTBE
MTBE is not harmful if completely combusted, rather contamination of water and air can occur if gasoline is allowed to evaporate or leak into the ground.  Spills and leaks may occur any time gasoline is transported, used, or stored.  Leaking underground storage tanks, lines, fueling stations and gasoline-powered recreational watercraft are known sources of introduction of MTBE into the environment.