Here are the two web sites that I referenced for information on the sources of MTBE contamination:
http://toxics.usgs.gov/highlights/mtbe.html
-- "Traces of MTBE in Ground Water May not be from the Air"
-- "Methyl tert-Butyl Ether"
The first site discusses the main sources of MTBE contamination in groundwater after pointing out that contamination from the air is minor. The second site gives great background info. for policy, science and history of MTBE. Both of these USGS sites have numerous reference links for additional information and background.