Researchers with the U.S. Geological Survey and the Oregon Graduate Institute's Department of Environmental Study found about 9,000 of 26,000 wells looked at were within a kilometer of a leaking fuel tank.
Head researcher John Zogorski said it is likely that not all of the 9,000 wells are contaminated with MTBE, or methyl teritary butyl ether.
The study omitted 19 states, including California and Texas, because they lacked needed information on well sites.
The study was posted in the on-line edition of the journal Enviornmental Science & Technology.
EPA announced Tuesday actions by the Clinton Administration to phase out MTBE from gasoline, but it will take at least a decade before the possibly carcinogenic substance no longer poses a threat to the water supply.
It is hardly the only carcinogenic pollutant to be found in leaking underground fuel tanks. But other pollutants, like benzene, "tend to absorb more toward soil and degrade much faster," said Zogorski.
Many variables play a role in determining whether MTBE will make it into a well, researchers said. Key among them is the rate at which wells are pumped.
"Pumping excessively makes the situation worse," said Zogorski. "It draws down contamination into the well. If the pump rate is low, typically the plume of MTBE passes right by."
The study and more than 50 others on MTBE and related issues will be presented this weekend in San Francisco during the national meeting of the American Chemical Society.