A decade ago, MTBE was the silver bullet that would make cars run cleaner. Today, the silver looks tarnished.
Researchers dont agree how useful the gasoline additive really is. Worse, its starting to show up in drinking wells and public lakes. The controversy serves as a cautionary tale about the dangers of mandating technologies without fully understanding their long-term impact.
Politically, once you put [the additive] in, theres inertia for people to change, says Phillip Myers, emeritus research professor at the University of Wisconsin, Madison.
Since the Clean Air Act of 1990, some 17 states and Washington, D.C., have turned to methyl tertiary butyl ether (MTBE) to help them stay within clean-air standards. When blended with gasoline, it adds oxygen to the fuel. About a third of the gasoline in the United States is reformulated with some kind of oxygenate. MTBE accounts for three-quarters of the total, edging out competing substances, such as ethanol.
The problem is that using oxygenates may not help cut auto emissions anymore, several researchers say. Thats because American cars run a lot cleaner than they used to. More than a decade ago, cars used carburetors to mix air and fuel. As the cars aged, they tended to mix more fuel and less oxygen, so the extra oxygen from MTBE-blended fuels helped reduce emissions.
But todays models use catalytic converters, which automatically adjust the fuel-air mixture. So an MTBE-blend simply forces the system to squirt in more fuel. As older cars head for the junkyard, the fuels overall impact lessens.
Worse, all consumers are paying to correct a problem mostly caused by a small minority of people who drive out-of-tune clunkers, says Douglas Lawson, an air-pollution researcher at Colorado State University. You get hit by a double-whammy. Not only do you get hit with decreased fuel economy, you pay more at the pump.
Typically, an oxygenated fuel costs 2 to 3 cents a gallon more even though it reduces a cars fuel economy by 2 to 3 percent.
But MTBE has plenty of backers who dispute the notion that its outdated. The automobile is certainly making enormous strides, says a spokesman for the Oxygenated Fuels Association, which backs MTBE. But if youre going to have air quality in this country, youre going to have the car, the gasoline, and the driver working as a triumvirate.
The air-quality benefit far outweighs the reduction in fuel economy, adds Lori Stewart, group manager for fuels implementation at the US Environmental Protection Agency. While the direct impact may be small, the substance allows refiners to avoid using other, more toxic substances such as benzene.
The latest challenge to MTBE comes from an unexpected source: ground water. Underground storage tanks are leaking gasoline all over the country. California alone has some 32,400 tanks known to be discharging fuel into the soil. While most of the toxins in the fuel appear to dissipate over time, MTBE presents a special problem. It moves faster and biodegrades more slowly. Thus the additive is showing up in alarming concentrations in shallow ground water. Already, Santa Monica, Calif., has had to close three public drinking wells, which supplied nearly half the communitys water, because of MTBE contamination. Even if low levels of the additive pose no health threat, it can make water taste something like turpentine.
MTBE supporters point out that the problem is leaking gasoline, not just MTBE. People should be upset if anything is leaking from those underground storage tanks, says Martha Casey, an EPA spokeswoman. The agency has set Dec. 22 as the deadline for replacing leaking storage tanks in the country.