MTBE (methyl tertiary butyl ether) is an oxygenate compound added to gasoline to increase the oxygen content. It's function is to promote more complete combustion, which results in lower emissions. Diisopropyl ether (DIPE) is a similar compound used in some parts of the country.
MTBE (and DIPE) is very soluble in water. It also has a very high diffusion constant, which means it will move much more rapidly once dissolved in groundwater. But the real problem with these chemicals is that they enhance the solubility of the other chemicals that make up gasoline. The result is that contamination resulting from a discharge will show higher dissolved concentrations and move faster once in the groundwater.
As far as the last info I saw, MTBE has not yet been classified as a known carcinogen. The current regulatory limit for MTBE in groundwater is based on a taste threshold, and not on any exposure/risk assessment.
MTBE in groundwater is tough to remediate. It can be removed from water by air-stripping, but since MTBE likes to be dissolved in water, you need a bigger air-stripper than normal. Granular activated carbon (charcoal) will work but not efficiently, so carbon treatment is quite expensive. Bio-remediation has limited effectiveness.
Fortunately, the use of MTBE is on the decline in the U.S.
