The state agencies that implement the Underground Storage Tank program rely heavily on Monitored Natural Attenuation (MNA) to clean up contaminants such as benzene and methyl tertiary butyl ether (MTBE) at gasoline spill sites. This is possible because the contaminants are biologically degraded in anaerobic ground water at the site. Tertiary Butyl Alcohol (TBA) is generally considered to be more readily degradable than MTBE, and there is a danger that the state agencies will consider contamination from TBA a good prospect for MNA. A close examination of the available information indicates that a default presumption that TBA is readily degraded in anaerobic ground water is not justified. Anaerobic biodegradation of TBA will require a supply of an electron acceptor such as sulfate or biologically available Iron(III) or Manganese(IV). The available survey data indicate that ground water in the source area of the majority of known plumes is devoid of sulfate. Although a procedure to estimate biologically available Iron(III) is commercially available, it is not routinely applied to gasoline spill sites. There is no established procedure to estimate biologically available Manganese(IV). To date, the performance of available approaches to document anaerobic biodegradation of TBA at specific field sites has been disappointing. These include field monitoring to show a statistically significant attenuation in concentration with distance along the flow path, microcosm studies conducted with sediment from the site, and analysis of stable isotope ratios in TBA in the plume.
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