It was supposed to clean the air. Instead, it now threatens the nation's drinking water supply.

Methyl tertiary butyl ether -- a gasoline additive that may be carcinogenic --has been detected in so many wells, lakes, and underground aquifers across the country that MTBE contamination is likely to become one of the major environmental quagmires of the next decade.

Cities have been forced to cap tainted wells. Water districts have filed multimillion-dollar lawsuits against oil companies. And, new surveys have even found high concentrations of the additive in rural states like Montana.

In California -- home to 27 million vehicles and more than 9,500 gas stations -- MTBE has contaminated 10,000 shallow groundwater sites, including 1,000 in the Bay Area. It has also been found in dozens of the state's lakes and reservoirs, including Shasta, Tahoe, and Donner in the north and Castaic, Pyramid, and Perrisin the south.

"When you find MTBE in so many drinking water sources throughout the state,
it clearly indicates this is an issue of statewide and nationwide concern," said John Reuter, a University of California at Davis aquatic ecologist and one of the authors of a recent MTBE study conducted for the California Legislature.

The Environmental Protection Agency cleared the way for the use of MTBE as a gasoline additive in high smog and carbon monoxide areas in 1991. Today, the price of that decision is becoming increasingly clear.

The U.S. Geological Survey has found the controversial additive in more than a quarter of the nation's shallow urban wells, as well as in streams, lakes, rain, and snow.

In the past decade, researchers have found that MTBE can cause cancer in animals, and they believe it is a potential carcinogen in human beings.

From leaking underground gasoline storage tanks, it migrates readily to water supplies. In unburned fuel from two-stroke motorboats and jet skis, MTBE directly pollutes reservoirs and lakes. A recent study in Maine found that even small gas spills can cause widespread contamination.

The UC study released last month detailed MTBE's threat to the nation's drinking water. The report further concluded that oil companies can produce cleaner- burning gasoline that meets federal smog standards without oxygenates like MTBE.

Even if MTBE were banned today, years would be required to remove it from the nation's drinking water, experts say. The cost of removing it could run into billions
of dollars.

"The longer MTBE stays in the system, the more contamination sites there will be; and it will very soon outstrip our ability to cope with the problem," said Steve Hall, who heads the Association of California Water Agencies. California, and about a third of the rest of the country now use gasoline with high amounts of MTBE.
Most other parts of the nation use gasoline with smaller amounts of the additive.

Chemical industry officials continue to insist that MTBE helps reduce smog and that the additive is being unfairly demonized.

"If you decide the way to solve the problem of contamination of water is to take out one component of gasoline, then all you are doing is taking away a symptom;
and you're not solving the basic problem of leaking underground storage tanks," said Eric Bolton, spokesman for the Oxygenated Fuels Association, which represents MTBE producers.

By the first of the year, all underground tanks in the nation must meet new safety standards; but so far, the EPA has resisted calls to ban MTBE.

"It's outrageous to jeopardize the public's health," said Brooke Coleman of the Earth Island Institute in San Francisco. "The EPA -- the agency charged with protecting the American people -- is standing behind a cancer-causing chemical that is poisoning drinking water."

Evidence of the threat MTBE poses to drinking water supplies has been seen throughout California.

In Santa Monica, city officials were forced to shut down seven wells, losing more than half their water supply. The cost of the cleanup, which is not scheduled to begin until late next year, could reach $100 million over the next decade.

In the Kern County community of Glennville, the state is spending about $5,000 a month to provide fresh water to nine homes and a business, after tests found MTBE levels as high as 20,000 parts per billion (ppb) in one well. The state health limit is currently 35 ppb.

"I put this MTBE in my body and my grandchildren's bodies," said Freda Kubas, whose well was contaminated. Kubas says she has had rashes, seizures, and gastrointestinal problems. "I gave my grandchildren water to drink, made them iced tea and Kool-Aid, gave them bubble baths. I didn't know I was giving them poisoned water."

At South Lake Tahoe, leaks at underground gas station tanks have caused the water district to close 12 of 34 wells. "These multimillion-dollar gas station spills are devastating for a small community like ours," said Bob Baer, executive director of the South Lake Tahoe Public Utility District.

Some critics complain that the EPA views MTBE contamination primarily as a California problem, even as the additive is being discovered in water supplies from coast to coast.

-- In Maine, a statewide survey found that 16 percent of drinking wells had detectable amounts of MTBE; and as many as 5,200 domestic wells may contain MTBE above that state's drinking water standard of 35 ppb.

At the same time, scientists there determined that minute amounts of spilled gas can pollute wells because the compound moves so easily through soil into the groundwater.

Gas spilled from automobile accidents caused the closure of an elementary school's drinking water supply in Whitefield and the contamination of 25 private wells in Standish. Michael Millett of Standish said his well water had a strong odor and strange taste after a car accident near his home. Testing revealed extremely high levels of MTBE (6,500 ppb). "No level of MTBE is acceptable," Millett said. "I don't want to open up a newspaper 10 years from now and read that what they thought was an acceptable level, is not."

-- New Jersey regulators have found high levels of MTBE near 1,900 of the state's 2,400 leaking underground storage tanks. Sixty-five public drinking water supplies have been contaminated with MTBE, and health officials have warned residents in a half dozen towns not to drink the tap water.

-- In rural areas of New York, MTBE has contaminated the well water for more than 200 homes. Diane Atkins, 47, of Liberty blames an MTBE leak from a nearby gas station for her husband's death from cancer last year. She said several neighbors have died of brain cancer, leukemia, and lymphoma. "Is the government in cahoots with the oil companies?" she asked. "They're still telling us everything is fine. They're going to wait until nobody is left alive."

-- In Pennsylvania, a leak from a gas station storage tank near the town of Blue Bell caused the closure of 15 homes' private wells. High levels of MTBE (1,500 ppb) were detected in tap water. Residents complained of headaches and ulcers. Animals also became ill. Inspectors showed up to hunt for the source of the contamination. "It's been a war zone here all summer, between streets being dug up and people crawling through our house to test," said resident Christine Fisher. "This is a nightmare."

-- Texas only recently began testing for MTBE, but the substance has been found in a dozen public water supplies. With more than 21,223 leaking underground fuel tanks, the state is expected to find significant MTBE contamination.

-- In Kansas, health officials were surprised to find so many instances of MTBE contamination near hundreds of leaking underground storage tanks. "We've had some pretty startling results," said Greg Hatten of the Kansas Department of Health and Environment. "We have a couple of towns that have pretty widespread MTBE contamination. We've had several water supplies contaminated in western Kansas. A couple have been shut down."

As MTBE findings increase, water agencies have grown increasingly concerned about potential health risks and long-term environmental damage.

"This is a health issue and a natural resource issue," said Richard McDonald, production superintendent for a small water district that draws its water from Whiskeytown Lake outside Redding. "What do you want to do -- make our water unfit for human consumption?"

Hall, head of the Association of California Water Agencies, says his group is calling for an aggressive phaseout of MTBE. "We have only one groundwater supply," Hall said. "If we screw it up, there is no replacement."