By Ben Harder
The recent political debates on the use of two common gasoline additives, methyl tert-butyl ether (MTBE) and ethanol, suggest that refiners may have to rely more on alternative chemicals for oxygenating gas and reducing smog.
A University of California, Los Angeles (UCLA) research group has compared the in-ground behavior of MTBE, four alternatives, and basic gasoline constituents, such as benzene. The researchers analyzed data on more than 850 leaking underground fuel tanks in the Los Angeles area, as well as measurements of those contaminants in groundwater at various distances from the tanks. They conclude that at their present concentrations in gasoline, the alternative additives aren’t as environmentally prevalent as MTBE, but that reformulations with higher concentrations could create problems comparable to those already caused by MTBE.
To meet air-quality standards implemented in the 1990s, many petroleum processors began adding compounds called oxygenates–for the most part MTBE, with ethanol as a distant second–to gasoline. The additives reduce emissions from burning fuel, but they also have environmental drawbacks.
MTBE, which smells like turpentine, often escapes from leaking gas-storage tanks and spreads in underground plumes. The contaminant persists in groundwater for years and is difficult to remove (SN: 4/8/00, p. 229: Available to subscribers at Gasoline additive’s going, but far from gone). Some states now prohibit manufacturers from adding MTBE to gasoline to minimize pollution.
Ethanol is less persistent than MTBE in the environment, but it appears to enhance the diffusion of benzene, a carcinogen and a major ingredient in gasoline, when ethanol-enriched gas leaks from tanks.
Several oxygenates other than ethanol could supplant MTBE as the gasoline additive of choice. They are frequent by-products of MTBE production and therefore already present at low concentrations in MTBE-enriched gasoline.
UCLA’s Tom Shih and his colleagues detected MTBE in the ground at 83 percent of the leaking tanks, making it nearly as prevalent as benzene. The oxygenate tert-butyl alcohol occurred at 61 percent of the sites, and the other three oxygenates–all ethers similar in structure to MTBE–turned up 9 percent to 24 percent of the time.
Half of the MTBE plumes exceeded 84 meters in length, while tert-butyl alcohol plumes were 61 m in median length. Typical plumes of the other ethers were 35 m to 58 m long, Shih and his colleagues report in an upcoming Environmental Science and Technology.
“All indications [nevertheless] suggest that the alternative ethers would pose groundwater contamination threats similar to MTBE if their scale of usage were expanded,” Shih says.
The concentrations of alternative oxygenates found around leaking tanks in the new study are already surprisingly high, says environmental engineer Susan E. Powers of Clarkson University in Potsdam, N.Y.
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