Smog-Reducing Additive May Not Help Environment

Gasoline additives provide a classic example of how unintended consequences can actually turn out to be more critical in environmental science than the intended effects of a remedy.

Take methyl tertiary-butyl ether, or MTBE, for example. MTBE is derived from natural gas and is an oxygenate—a chemical that increases the octane level of gasoline and makes it burn more efficiently, thus reducing carbon monoxide and ozone-causing emissions from automotive exhaust. The 1990 Clean Air Act mandated that oxygenates be added to gasoline in parts of the country plagued by ozone, and by the early 1990s, more than 20 billion pounds of MTBE were being produced annually, eventually accounting for about 80 percent of the oxygenate market in the United States.

But MTBE turned out to be a serious groundwater hazard. Like benzene, toluene, ethylbenzene, and xylene—the so-called BTEX compounds, the most common contaminants associated with gasoline—MTBE creates a plume of contamination when it leaks into the ground from storage tanks or a surface spill. While it is less toxic than benzene—the most hazardous gasoline-related groundwater pollutant in the United States—even trace amounts of MTBE can discolor groundwater and make it taste and smell like turpentine.

As a result, California and 11 other states are phasing out MTBE, and the search for alternative smog reducers has led to a renaissance of sorts for ethanol, the alcohol component of gasohol, which enjoyed widespread publicity after the OPEC oil embargo in the 1970s before fading from the national scene. “The direct impacts of ethanol on groundwater are likely to be less significant and of shorter duration than those associated with MTBE,” says Pedro Alvarez, the George R. Brown Chair of Civil and Environmental Engineering.

But even ethanol, although fairly benign as a contaminant, has its own set of difficulties. Alvarez has compiled a large body of research that indicates ethanol may, in fact, be worse than MTBE in exacerbating groundwater pollution caused by gasoline releases. While MTBE may be a problem contaminant in itself, it does not significantly speed the migration or slow the biodegradation of BTEX compounds. Alvarez’s research shows, however, that ethanol can slow the rate at which BTEX compounds break down in the soil.

And it doesn’t end there. “Ethanol can extend plumes of benzene by more than 50 percent,” Alvarez says, “increasing the risk of exposure.” More research will be needed to determine whether the impact is significant enough to pose an increased risk to public health.

—Jade Boyd