As much as 35 percent of the nation's underground tanks containing motor fuel may be leaking, posing a serious threat to drinking water for half the U.S. population, according to a study by the Environmental Protection Agency.

The percentage of leaky tanks found in the two-year survey substantially exceeds industry estimates, fanning fears of environmental groups that such carcinogenic constituents of motor fuel as benzene, and such toxic chemicals as xylene, are contaminating ground-water supplies for millions of private and municipal wells.

"If you're a homeowner and your well is involved, it's not great news," Ronald Brand, director of EPA's Office of Underground Storage Tanks, said of the study scheduled for release this week.

EPA's random sampling of the nation's 800,000 underground tanks containing gasoline and diesel fuel reveals for the first time the dimensions of a public health problem potentially more serious than leaking toxic-waste dumps, according to Larry Silverman, executive director of the Environmental Task Force.

"Not everyone lives next door to a toxic-waste site," he said, "but everyone has a fuel tank within a stone's throw."

EPA is in the process of formulating regulations for underground storage of motor fuel in such places as gas stations, airports, marinas, fire departments and car rental agencies.

Congress passed legislation in 1984 requiring that all tanks be made of fiberglass or coated with fiberglass or other materials.

Nine of every 10 buried tanks today are made of steel, which corrodes over time and leaks fuel that eventually trickles into the ground water, Brand said. Another cause of leaks is faulty installation, he said.

The study focused on 433 tanks in every region of the United States. The vessels were filled with fuel, then tested over a two-hour period. Although 35 percent of the tanks lost fuel, it was unclear what caused the leak, according to Joseph Carra, one of the EPA officials who coordinated the survey.

Carra said the leaky tanks would have to be dug up and examined to determine if they were cracked or simply missing a plug. If the problem was not structural, he said, the tanks might not leak unless filled to capacity.

"The problem could range from something relatively easy to fix to replacing the whole tank," he said.

Although the survey sampled a fraction of the nation's undergound vessels, its findings can be extrapolated to the total motor fuel tanks, Carra said. A margin of error could increase or decrease the results by 5 percent.

Major oil companies have estimated that 2 to 5 percent of underground tanks containing motor fuel are leaky.

Brand said the study intensifies concerns of possible contamination of underground water supplies. Although many urban centers tap lakes and rivers for drinking water, half of the U.S. population gets water from wells.

Although fiberglass tanks have been seen by industry and environmental groups as a safer alternative, the study showed that they leaked at the same rate as those made of steel.

Tanks containing motor fuel have been buried since 1905 as a precaution against explosion.

A few states have adopted regulations for the buried vessels. In most states, however, tank owners are not given training and the tanks are not subject to inspection.