Whole lot more to MTBE than just an additive
What makes MTBE so special?
The controversial fuel additive has helped bring national attention to the issues of leaking fuel tanks and highly pollutant watercraft. But if the chemical oxygenate wasn’t there, the gasoline still contains dangerous substances.
“MTBE is only one component of gasoline,” said Chuck Curtis, water resource control engineer for the Lahontan Regional Water Quality Control Board. “In the past, the primary concern has been with benzene, toluene, ethylbenzene and xylene, which either are toxic in some respect, have represented health effects or have taste and odor that are objectionable.”
Unlike MTBE, where researchers still don’t know a whole lot about it, the dangers of the BTEX compounds are well documented.
Benzene is known to cause cancer, and toluene can cause birth defects. While research is ongoing on MTBE, it is currently classified by the U.S. Environmental Protection Agency as a possible human carcinogen.
So if the health risks aren’t necessarily worse than the others, why the big controversy over MTBE?
Answer: Its physical properties.
“It’s a different breed of cat than the BTEX compounds,” said Carol Boughton, director of research for the Nevada District of the U.S. Geological Survey.
MTBE is much more soluble in water than the BTEX compounds, which means benzene and the others adhere to the soil while MTBE travels with the groundwater. MTBE has been called a tracer, because it is usually the first contaminant to be found. Regulators then know the BTEX compounds also are in the soil.
Before MTBE was in gasoline, wells within a few hundred feet of a gasoline leak still were usually safe, Curtis said. Contaminant plumes rarely extended farther than a few hundred feet.
“That pales in comparison to MTBE,” Curtis said. “We have plumes with 1,500 feet of movement or more in Tahoe.”
Curtis said no public wells in the basin have been contaminated by the BTEX compounds.
The other quality unique to MTBE is the fact that it doesn’t break down like the BTEX compounds.
“Most compounds have some kind of microbe that will eventually munch them,” Boughton said. “They will degrade them: benzene, toluene. They will not degrade MTBE. Because of the shape of the MTBE molecule, the microbe can’t get access to the bonds. And the bonds have such a high energy level, the microbe would have to use a lot more energy (than for breaking down other compounds). The bottom line is MTBE is stable in groundwater and doesn’t break down.”
However, there is a location where MTBE does act like other contaminants – surface water.
In turbulent water, such as a stream, MTBE volatilizes and moves into the atmosphere, where it starts to degrade.
In Lake Tahoe itself, BTEX contamination is of more concern to researchers, Boughton said.
Deep in the lake, where the water is not turbulent, however, MTBE may not volatilize as well as on the surface, Boughton said.
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