|MTBE - The UST
Programs Next Great Challenge
MTBE may represent the next great challenge to regulators nationwide in the effort to contain the damage from our use of fossil fuels. Amendments to the Clean Air Act have resulted in the required use of reformulated fuels. Reformulated fuels contain oxygenated blending agents which aid in attaining more complete combustion, resulting in less air pollution while increasing octane ratings. Methyl-tert-butyl ether (MTBE) is the primary oxygenate added to fuels across the country.
The widespread use of this chemical coupled with an increase in requests for this analyte in environmental samples has resulted in the frequent discovery of MTBE. Most detections occur in shallow groundwater from urban areas throughout the United States. For example, Herb Meade from the Maryland Department of the Environment - Underground Storage Tank Program, recently reported that Maryland currently has 30 public drinking water wells that are impacted with MTBE. Eighty percent of these wells have an unknown source. MTBE has also been found in Maryland lakes that have no known contamination source, causing additional concern regarding the ubiquitous nature of this compound. MTBE has a disagreeable taste and odor at extremely low concentrations of 40 ppb, and is considered to be a possible human carcinogen by the U.S. EPA.
Accurate identification and quantification of MTBE and other oxygenates is difficult since they are often accompanied by other gasoline components. Two, in particular, 2-methylpentane and 3-methylpentane, have caused identification errors and false positive results. Consequently, EPA Method 8021B represents a very conservative monitoring tool due to its tendency for over-estimating oxygenate concentrations and false-positive misidentifications. The best method for positively identifying and resolving MTBE is to utilize EPA method 8260, volatile organic compounds by gas chromatography in conjunction with a mass spectrometer (GC/MS).
MTBE is considerably more water soluble than any of the BTEX compounds. This solubility results in great concerns regarding MTBE mobility and degradation rate. MTBE plumes are more mobile than BTEX plumes and the primary attenuation mechanism is dispersion. MTBE concentrations in soil and groundwater do not appear to significantly degrade. Consequently, a mass of MTBE would not become depleted and would travel significantly longer distances, requiring a longer time to attenuate than a similar plume of BTEX. MTBEs recalcitrant nature and mobility may result in a progressive contamination problem. A number of laboratory-cultured microorganisms isolated from various environments have been found to degrade MTBE. There is however no convincing evidence that presently exists suggesting that this destructive process occurs quickly and or actually takes place in field conditions.
It can be expected that increasing regulatory attention will be paid to the MTBE contamination problem. The EPA recently held a meeting of the National Committee of Geologists to discuss the problem of MTBE occurrence in groundwater. They have concluded that additional investigations should be performed.
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