Micro-Organisms Help Protect Drinking Water From MTBE Contamination

Micro-organisms that live at the bottom of lakes and streams may offer a solution to the problem of drinking water contamination from the gasoline additive MTBE, according to a new study by government scientists.

In use for more than 20 years as a way to help lower carbon monoxide emissions from gasoline-powered vehicles, MTBE (methyl tert-butyl ether) has come under fire in recent years because of growing concern about its potential harmful health effects in people.

Writing in the June 1 print issue of the research journal Environmental Science & Technology, scientists with the U.S. Geological Survey in Columbia, S. C., say their study indicates that naturally occurring micro-organisms may act as a "significant barrier" to the entry of MTBE into surface water systems. The journal is published by the American Chemical Society, the world's largest scientific society. The study initially appeared April 21 on the journal's Web site.

"Our results indicate that these micro-organisms are able to degrade MTBE to nontoxic byproducts if oxygen is present in the microbial environment," says Paul Bradley, Ph.D., lead author for the study. "This finding indicates that such microbial communities serve to protect surface drinking water supplies from MTBE contamination."

Rain is the primary way that MTBE gets into surface waters. Entering the atmosphere through evaporation when people fill their gas tanks, MTBE is subsequently dissolved by rainfall and washes into streams and lakes. MTBE also can be found in ground water, but surface water is more of a concern to many people since it accounts for approximately 60 percent of municipal drinking water in the United States, says Bradley.

"This is the first time that MTBE biodegradation by naturally occurring surface water micro-organisms has been shown," notes Bradley. Found in the bed sediments of lakes and streams, the micro-organisms also were effective in degrading another gasoline additive, TBA (tert-butyl alcohol). "Up to 73 percent of MTBE and 84 percent of TBA were degraded," according to the research article.

Because the additives are constantly replenished in surface waters through rainfall, some small measurable amounts of MTBE and TBA will still be present, says Bradley. However, he emphasizes, "Imagine how much more tainted surface water would be if microbes were not degrading it."

The additives are converted primarily to harmless carbon dioxide through a process called mineralization, in which the microbes essentially consume the MTBE and TBA. Approximately 30 percent of the gasoline sold in the United States contain additives that have been included as oxygenates to help lower pollution levels, according to the researchers.

MTBE has been tentatively classified as a possible human carcinogen by the U.S. Environmental Protection Agency, the researchers point out. EPA has established a drinking water standard of 20 - 40 micrograms per liter for MTBE, equivalent to less than a millionth of an ounce of the additive in a quart of water.

The study, done at two gasoline spill sites in Laurens, S.C., and Charleston, S.C., has "important implications," claim the researchers, since the "widespread use of these compounds (MTBE and TBA) as fuel oxygenates essentially guarantees that they will remain important environmental contaminants for the foreseeable future."