TECHNOLOGY UPDATE Field trial for MTBE bioremediation method The crucial test of a new method for remediating methyl tertiary butyl ether (MTBE) is scheduled to occur in August in a contaminated aquifer under California's Port Hueneme Naval Base. It is among the most promising of nearly 20 research efforts underway to ease the task of removing the odorous fuel oxygenate from groundwater, according to several researchers. MTBE is one of a group of oxygenates that helps fuel makers comply with the 1990 Clean Air Act amendments, and it has been used as a fuel enhancement since 1979. EPA's Toxics Release Inventory estimates that 3.3 million pounds of MTBE, a possible carcinogen, were released into the air and water in 1994, and it has been detected in groundwater in at least 27 states. Leaks from underground storage tanks are the primary means by which the compound contaminates groundwater, according to a report released in June by EPA. The method being tested at the California naval base relies on a "bacterial consortium" of 4-6 microbes. It was developed five years ago by Joseph Salanitro, senior staff research microbiologist at Shell Development Company, Houston, Tex. Unlike some other methods under development for bioremediating MTBE, Salanitro's patented microbes have been shown in laboratory groundwater microcosms to require only the addition of oxygen to degrade the fuel additive to half of its initial concen.tF3.tion within one week This degradation rate is on a with the natural attenuation of the most soluble components in petroleunr benzene toluene ethylbenzene and xylene' (BTEX) The end result of the degradation is 10097 CO according to Salanitro Salanitro hopes to spur a similar rate of degradation in a 20-bv-40-
foot section of a 3600-foot-long MTBE plume from a 14-year-old gasoline spill under Port Hueneme. The test section of the plume contains 2-10 parts per million (ppm) of MTBE, and all its BTEX components have naturally attenuated. Salanitro began pumping pure oxygen into the test section in June to create a biological barrier, for the aquifer's 1-ppm oxygen concentration is insufficient to spur biodegradation by aerobic microbes. Once he determines how the introduced oxy-
This electron micrograph shows the group of microbes being field-tested to remediate MTBE at x5100 magnification. (Photo courtesy of Shell Development Company)
gen migrates through the test area, Salanitro plans to saturate the test plot with 6 kilograms of microbes by injecting them at 1-ft. intervals. To evaluate the method's effectiveness, he has also marked off two control plots: The oxygen content will be artificially raised in one plot, while the second will be unaltered. If all goes well, the microbes in the experimental test plot will reduce the level of MTBE contamination by 3 orders of magnitude, to 5-10 parts per billion (ppb), according to Salanitro. This is well below the 20-40 ppb or lower level recommended to alleviate taste and odor effects in the MTBE drinking-water advisory issued by EPA's Office of Water in 1997. A successful field test could allow his method to become a viable means of attacking MTBE inside contamination source areas Salanitro said. He also believes it could
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prove useful in both low-permeability aquifers, where MTBE is relatively immobile due to geological conditions such as clay-filled soils, and high-permeability aquifers where MTBE is more mobile. Some of the other bioremediation research efforts underway use methane- or propane-oxidizing bacteria to degrade MTBE. These methods are inherentiy more complicated because they require additional chemicals, according to John Glaser of EPA's National Risk Management Research Lab in Cincinnati, Ohio. The end result is often "something short of CO,," he said. Because of incomplete degradation, daughter products not consumed by the microbes often formed such as tertiary butyl alcohol and 2-hydroxy isobutyric acid Salanitro says that no daughter products result from his method The research efforts also include a number of projects evaluating the utility of physical-chemical approaches, especially for situations where groundwater has been pumped out of contaminated aquifers for ex situ treatment. And at least five studies are currently evaluating MTBE's potential for natural attenuation; on the basis of laboratory studies, there is evidence that it can naturally biodegrade. According to one study published in June by researchers from the Lawrence Livermore National Laboratory however there is very limited evidence that natural attenuation is occurring in the field Finally, another promising study uses a method similar to Salanitro's: a single microbe that shows potential in the laboratory for degrading MTBE more quickly than Salanitro's bacterial consortium can. The team of researchers from the University of California-Davis that has isolated the microbe believe its approach should be ready for field testing later this year or early next year. KELLYN S. BETTS
SEPT. 1, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 4 0 3 A
