In rare instances, MTBE concentrations can be very high, but such high concentrations are easily traced to large spills or leaking storage tanks. MTBE in water samples is generally detected most frequently and at the highest concentrations in urban areas. Even in shallow urban ground water, however, concentrations are usually low in comparison to the EPA Drinking Water Advisory. The advisory levels primarily reflect taste and odor considerations. Although MTBE has been tentatively classified as a possible human carcinogen, EPA believes that, based on current knowledge, the advisory level provides a wide margin of public health safety.
Studies continue to further define the health risks.
The USGS National Water Quality Assessment Program recently reported results of a ground-water resources study in an area of high MTBE use in central and southern New England in 133 wells sampled in the Housatonic, Connecticut, and Thames River basins. It was found in 25 percent of the wells. Because of the strong correlation with land use, none of the samples from wells in forested areas had MTBE, but 40 percent of wells in urban areas had MTBE at detectable levels. These New England detections may result from surface waters, with MTBE entering the wells through leaky well seals or shallow fractures rather than being an indication of contamination deep within the aquifer. The EPA and USGS are studying MTBE and other VOC contaminants in the drinking water supplies of 12 Northeastern States from Virginia north through New England.