MTBE Munching Microbes


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Within 100 days, naturally occurring microbes break down MTBE, a gasoline additive, in stream beds.

Data from Aerobic Mineralization of MTBE.


Summertime: Take a long slow drink
10 JUNE 1999. caution, well water contaminated by gasoline sign
Water smelling gunky? The culprit could be MTBE, a new gasoline additive used to cut smog in reformulated gasoline. Although it's not yet proven that MTBE causes illness, water tastes foul when it contains just 40 parts per billion of the stuff that's formally known as methyl-tert butyl ether.

Regulators are increasingly concerned about MTBE contamination -- and for good reason. Three billion gallons of the chemical were produced in the United States in 1997 for use in gasoline. Roughly 300,000 underground storage tanks have leaked gasoline. An unknown percentage of those leaks contain MTBE, which dissolves better -- and travels farther -- in water than proven carcinogens from gasoline like benzene. Normally, the gasoline hydrocarbons benzene and toluene break down before they travel more than about 500 feet from a leaking tank.

MTBE, however, is showing up in so much drinking water that California plans to phase it out by 2003, despite the chemical's smog-fighting abilities. (Here's an assessment of MTBE and smog.)

A new and highly optimistic clue to MTBE's fate in the environment comes from U.S. Geological Survey hydrologists who studied biological degradation of the chemical in sediments from stream beds near leaking underground storage tanks in South Carolina. The tanks were selected to be representative of the 3,000 leaking storage tanks in the state, says James Landmeyer, a USGS hydrologist from Columbia, S.C., who worked on the study.

Bugs devour chemicals
Before we tell you what they found, here's what they did. The researchers simply extracted sediment containing an unknown zoo of microbes from the top layer of the stream-bed from the two sites which were downstream and within 300 feet of leaking storage tanks. Then they added MTBE and another gasoline additive, TBA, or ter-butyl alcohol, to the soup.

Both chemicals were tagged with radioactive carbon for tracking purposes. Because MTBE produces carbon dioxide as it breaks down, the amount of radioactive carbon dioxide in the container indicated how much MTBE had been destroyed.

The researchers determined that between 30 percent and 73 percent of the MTBE was destroyed in 100 days if oxygen was present. Because MTBE did not break down in sediments that had been heated to kill microorganisms, the researchers credited biological activity for the degradation. In the stream itself, no MTBE was detected, even though the sampling was done just downstream of the leaking tanks.

microbe munching chart
At this point, it's not clear which organism deserves credit for the breakdown. "It's probably not one microbe, there are probably a community of microorganisms doing this deed," says Landmeyer.

The researchers are now trying to identify the microbes, which could lead to the development of bioreactors -- think of them as artificial stream beds -- that could clean up leaking tanks where the microbes and/or streams are absent.

It's unclear how many leaking underground gasoline storage tanks contain MTBE, says John Zogorski, a USGS specialist in the chemical. But in Kansas, 90 percent of the leaking tanks did contain the additive. Even though federal regulations should prevent most further leaks, existing plumes of pollution will continue to threaten groundwater for decades to come, says Landmeyer, justifying research into the chemical's fate in the environment.

Marvelous microbes
As state and federal regulators decide how to deal with the wave of MTBE pollution, the new study could provide good news -- that nature has the capacity to clean up after some of our mistakes. Whether or not existing studies can prove that MTBE is indeed hazardous to your health, Landmeyer says water with even 40 parts per billion of MTBE smells foul, and "You would not want to drink it."

Far better, naturally, would be to build gasoline tanks that don't leak in the first place, which is the aim of this federal program.

Since MTBE seems to be reducing smog in many metropolitan areas with high ozone concentrations, dumping MTBE -- as California plans to do -- to protect groundwater amounts to a false dilemma between sacrificing our ability to breath and obtaining a clean drink of water. As the Natural Resources Defense Council (Acrobat Reader required) pointed out in a position paper. "The challenge is to preserve the air quality benefits that have resulted from reformulated gasoline ... while taking action to improve our protection of reservoirs, ground water and surface water."

--David Tenenbaum

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The Why Files Aerobic Mineralization of MTBE and tert-Butyl Alcohol by Stream-Bed Sediment Microorganisms, Paul Bradley et al, Environmental Science and Technology, vol. 33, no. 11, pp. 1877-1879.

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