Since 1992, MTBE has been used at higher concentrations in some gasoline to fulfill the oxygenate requirements set by the United States Congress in Clean Air Act amendments; however, since 1999, in California and other locations MTBE has begun to be phased out because of groundwater contamination (California Air Resources Board, 2004). Due to its higher solubility in water MTBE moves more quickly than other fuel components (California Air Resources Board, 2004). The Energy Policy Act of 2005 reduces the federal requirement for oxygen content in reformulated gasoline[2].

In 1995 high levels of MTBE were unexpectedly discovered in the water wells of Santa Monica, California, and the U.S. Geological Survey reported detections.[3] Subsequent U.S. findings indicate tens of thousands of contaminated sites in water wells distributed across the country. As per toxicity alone, MTBE is not classified as a hazard for the environment. The maximum contaminant level of MTBE in drinking water has not yet been established by the EPA. The leakage problem is partially attributed to the lack of effective regulations for underground storage tanks, but spillage from overfilling remains an important upset scenario. As an ingredient in unleaded gasoline, MTBE is the most soluble part. When dissolved in groundwater, MTBE will lead the contaminant plume with the remaining components such as benzene, toluene, etc. to follow. Thus the discovery of MTBE in public groundwater wells indicates that the contaminant source was a gasoline release. Its criticism and subsequent decreased usage, some claim, is more a product of its easy detectability (taste) in extremely low concentrations (ppb) than its toxicity, as benzene is much more toxic but remains a fuel additive. The MTBE concentrations used in the EU (usually 1.0–1.6%) and allowed (maximum 5%) in Europe are lower than in California.[4]