Gasoline additive methyl tertiary-butyl ether (MTBE) is turning up in fuel supplies in states where it is not required. Its use was mandated by the 1990 Clean Air Act in a number of highly populated U.S. cities and states. Used as a fuel oxygenate to help gasoline burn more cleanly, It moves fairly quickly in ground water, which is particularly problematic because of the national problem with leaking underground storage tanks. The chemical, is likely being spread above ground by tanker trucks, storage facilities and pipes that once held gasoline containing MTBE, oil companies have until the end of 2002 to phase out its use. MTBE also is found in cities where it is not required as a fuel additive because it is a common octane enhancer in premium-grade gasoline. Out of more than 200 samples collected from gasoline stations in Indiana, Illinois and Michigan, none of which are required to use MTBE. More than 70 percent of the samples in all three states contained MTBE, and 25 percent of the total samples contained significant amounts. In Indiana, 40 percent of the gas stations sampled contained significant amounts of MTBE, over 500 parts per million. One ounce of the substance is capable of contaminating 1,000 tons of fresh water.

A study released last year by the US Geological Survey and the Oregon Graduate Institute's Department of Environmental Study said about one-third of drinking water wells in 31 states might be contaminated with MTBE.

Once MTBE is in a water supply it is difficult to remove from the water because it's highly soluble and resistant to biodegradation. Bans on recreational personal watercraft, i.e., jet skis, etc., has largely been driven by the fact that their engines burn fuel very inefficiently, leaking gasoline—and MTBE—into lakes and reservoirs often used as drinking water supplies.