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.