These websites have a lot of good information and useful links
MTBE (methyl-t-butyl-ether) in Drinking Water
MTBE is a member of a group of chemicals commonly known as fuel oxygenates. Oxygenates are added to fuel to increase it oxygen content. MTBE is used in gasoline throughout the United States to reduce carbon monoxide and ozone levels caused by auto emissions.
Releases of MTBE to ground and surface water can occur through leaking underground storage tanks and pipelines, spills, emissions from marine engines into lakes and reservoirs, and to some extent from air deposition.
If MTBE is a concern for you, these resources may help:
Check out the Oregon Health Division's fact sheet on BTEX (pdf): Benzene, Toluene, Ethylbenzene, and Xylenes.
Or, if you are considering testing your well water and need to locate a lab that tests for volatile organic compounds, please look at the Oregon Environmental Laboratory's List of Accredited Laboratories.