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MTBE and other volatile organic compounds—New findings and implications on the quality of source waters used for drinking-water supplies

How frequently are VOCs detected in ground water?

Volatile organic compounds (VOCs), which are widely used in the manufacture of many products including refrigerants, plastics, adhesives, paints, and petroleum products, have been detected in about one-third of the wells sampled by the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS).1 Chloroform and other trihalomethanes, the most commonly detected compounds, were found in about 9 percent of the sampled wells. Solvents, particularly chlorinated solvents, were found in about 8 percent of sampled wells. VOCs predominantly occur in urban areas, often in mixtures; specifically, 2 or more compounds were found in about one quarter of the sampled urban wells.

A commonly detected VOC is methyl tert-butyl ether (MTBE), which is a gasoline oxygenate designed to add oxygen to decrease vehicular carbon monoxide emissions and ozone levels in the atmosphere. MTBE has the highest production volume of all fuel oxygenates.

Graphic showing the most commonly detected VOCs in ambient ground water
Trihalomethanes, solvents, and gasoline oxygenates are the most commonly detected volatile organic compounds in ambient ground water.

How often is MTBE detected in ground water?

MTBE was detected in about 5 percent of ground-water samples collected by NAWQA across the Nation. The concentrations typically were low, well below the U.S. Environmental Protection Agency (EPA) drinking-water consumer advisory concentration of 20 to 40 micrograms per liter, which is based on taste and odor thresholds. In fact, less than 1 percent of samples included in the USGS studies exceeded the EPA consumer advisory concentration of 20 micrograms per liter. Still uncertain, however, are possible human health effects. EPA has tentatively classified MTBE as a possible human carcinogen, but because of insufficient toxicity studies has not instituted a drinking-water health advisory or standard. EPA is using NAWQA findings, along with other investigations and research, to investigate possible drinking-water and human-exposure issues.

Where is MTBE most likely found?

NAWQA findings indicate that MTBE is most frequently detected in ground water underlying urban areas in comparison to agricultural and mixed land-use settings. MTBE was detected in about 14 percent of wells sampled in urban areas. In addition, incidence of MTBE increases in high-use areas. Most of MTBE’s high use occurs in Reformulated Gasoline (RFG) areas, where gasoline contains 11-percent MTBE by volume. MTBE has been detected about 4 to 6 times more frequently in the high-use areas than elsewhere.

Graphic showing detection frequency of MTBE in ambient ground water
The detection frequency of MTBE in ambient ground water is highest in urban land use.

Is MTBE in drinking water?

In addition to its studies of ambient ground water, USGS is conducting focused studies to assess MTBE concentrations associated with drinking-water supplies. In cooperation with EPA’s Office of Ground Water and Drinking Water, USGS examined data on finished drinking water from selected communities in 12 states in the Northeast and Mid-Atlantic Regions. The data show that MTBE was detected in 9 percent of the community water systems. Consistent with USGS studies of ambient ground water, concentrations were low; less than 1 percent exceeded the EPA consumer advisory concentration.

USGS presently is conducting a special study of source waters used for community drinking-water systems. This 4-year study, currently in its last year, is being performed in cooperation with the Metropolitan Water District of Southern California, the Oregon Graduate Institute of Science and Environmental Engineering, and the American Water Works Association Research Foundation (AWWARF), and includes about 1,000 water utilities across the Nation. Findings are consistent with previous NAWQA studies. Specifically, MTBE has been detected in low concentrations, and less than 1 percent of the samples exceed the EPA consumer advisory concentration. This study, which includes samples of both surface-water and ground-water sources for drinking water, shows that MTBE was detected more frequently in surface-water samples (14 percent) than in ground-water samples (5 percent). This finding can be explained, in part, by the inclusion of many samples from large rivers and reservoirs that are associated with substantial use of recreational watercraft. Older models of watercraft motors are known to release a fraction of non-combusted gasoline to water.

Increased occurrence of MTBE in large community water systems is also reported in this study. Specifically, MTBE was detected in about 4 percent of community water systems serving less than 10,000 people, and in nearly 15 percent of systems serving greater than 50,000 people. Wells providing drinking water for the larger community water supplies often are co-located in urbanized settings, which are associated with a higher incidence of MTBE. In addition, rates and amounts of pumping likely play a role because increased pumping rates draw water from extended areas, which thereby increases the likelihood of intercepting a source of MTBE.

Significance of findings

Future directions and needs


1 Detection does not necessarily translate to risk. The NAWQA Program measures chemicals at very low concentrations, often 10 to100 times lower than EPA standards and health advisories. For example, the detection frequency for MTBE is based on an assessment level of 0.2 micrograms per liter, which is 100 times lower than EPA’s drinking-water consumer advisory. Low-concentration sampling is used to detect and evaluate contaminants of concern, to track contaminant concentrations over time, and to determine natural and human factors related to chemical occurrence.

By John S. Zogorski, Michael J. Moran, and Pixie A. Hamilton

USGS Fact Sheet 105-01

October, 2001

Contacts for additional information or questions:

Dr. John Zogorski, U.S. Geological Survey
National Water-Quality Assessment Program
1608 Mountain View Road
Rapid City, SD 57702
(605) 355-4560, ext. 214
(605) 355-4523 (fax)

Technical information supporting this fact sheet, as well as access to other NAWQA publications, data, and maps, are provided on the Internet at:


Information on the objectives and scope of the NAWQA Program is provided on the Internet at:


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