What happens to MBTE in the environment
MTBE evaporates when exposed to air. Most direct releases of MTBE to the environment are to air.  MTBE also evaporates from water and soil exposed to air.  Once in air, it is expected to break down to other chemicals. 

However, the greatest concern is when MTBE enters the groundwater usually from leaking underground storage tanks ay gasoline stations or fueling facilities used by fleet operators. MTBE is highly soluble in water which means it dissolves very easily in water. Unlike other constituents of gasoline such as Benzene which may stick to organic materials in soils, MTBE does not adhere to soil. As a result, when MTBE gets into groundwater it can move very quickly away from the source of the contamination (i.e. the leaking tank).

Under the Safe Drinking Water Act (SDWA), EPA is required to establish a list of contaminants and acceptable exposure levels for those contaminants. EPA has not yet established a drinking water standard for MTBE though the agency has placed MTBE on its drinking water Contaminant Candidate List (CCL) which is a list of contaminates that are being studied to determine if an enforceable drinking water standards is appropriate. In the meantime, EPA has issued a drinking-water advisory of 20 to 40 micrograms per liter (µg/L) for MTBE. This level is not based on any scientific studies and only represents.