The compound methyl tertiary butyl ether (MTBE) was blended in gasoline originally in 1979 to serve as an oxygenate. This would theoretically cause the combustion process in internally combustion engines to be more complete. This effort to improve air quality by producing a fuel that was more readily oxidized and decrease the resulting troposheric ozone production was somewhat effective. A problem arises, however, in the fact that MTBE moves extremely well in water (it may actually move upstream because of its high affinity for water molecules), causes fish kills due to its toxicity to fish, and even at low concentrations, it can render large surface or groundwater sources to be useless. Its use has since been banned in California.
Source: Atmospheric Chemistry Glossary