Methyl tert-butyl ether (MTBE) is a volatile organic compound (VOC) derived from natural gas that is added to gasoline either seasonally or year round in many parts of the United States to increase the octane level and to reduce carbon monoxide and ozone levels in the air. In 1993, production of MTBE ranked second among all organic chemicals manufactured in the United States. Currently, the U.S. Environmental Protection Agency (EPA) tentatively classifies MTBE as a possible human carcinogen. Health complaints related to MTBE in the air were first reported in Fairbanks, Alaska in November 1992 when about 200 residents reported problems such as headaches, dizziness, eye irritation, burning of the nose and throat, disorientation, and nausea. Similar health complaints have been registered in Anchorage, Alaska; Missoula, Montana; Milwaukee, Wisconsin; and New Jersey.
As part of the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program, concentrations of 60 VOCs were measured in samples from 211 shallow wells in 8 urban areas and 524 shallow wells in 20 agricultural areas. Chloroform and MTBE were the two most frequently detected VOCs. MTBE was detected in 27 percent of the urban wells and 1.3 percent of the agricultural wells. Concentrations ranged from less than the detection level of 0.2 μg/L (micrograms per liter) to as high as 23,000 μg/L. When detected, the median concentration of MTBE was 0.6 μg/L. MTBE was most frequently detected in shallow ground water in Denver, Colorado and urban areas in New England. In Denver, 79 percent of the samples from shallow urban wells had detectable concentrations of MTBE and in New England, 37 percent of the samples from urban wells had detectable concentrations. Only 3 percent of the wells sampled in urban areas had concentrations of MTBE that exceeded 20 μg/L, which is the estimated lower limit of the EPA draft drinking water health advisory level. Contaminant concentrations below the health advisory are not expected to cause any adverse effects over a lifetime of exposure. MTBE is on the EPA’s Drinking Water Priority List, which means it is a possible candidate for future regulation.