The second most frequently detected volatile organic chemical in groundwater, MTBE, has been found from coast-to-coast. In 1991, EPA approved MTBE as an air-cleaning gas additive against high smog and carbon monoxide in dense urban areas. Today, MTBE is a suspected chemical carcinogen, which the U.S. Geological Survey has found in more than a quarter of the nation's shallow urban wells and in streams, lakes, rain and snow, and even in remote rural areas.

The Association of California Water Agencies reports that California and about a third of the rest of the country now use gasoline with high levels of MTBE; the rest use gas with low levels of the chemical. Even if MTBE were banned today, years would be required to remove it from the nation's water, and the cost could run into billions of dollars.

Although the EPA classifies MTBE as a possible human carcinogen because laboratory rats and mice that breathe or drink it have developed lymphoma, leukemia, testicular tumors, thyroid tumors, and kidney tumors, so far, the EPA has resisted calls to ban MTBE. One of the subtleties of the MTBE scare in tap water across America is that MTBE is considered a fast-leaching compound. The U.S. Geological Survey reported that MTBE doesn't biodegrade; it can affect water supplies for years.

Multi-Pure Drinking Water Systems were the first to be tested and certified by NSF International to reduce MTBE.