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Abstract

Over the last decade data have been published that demonstrate that natural attenuation of hydrocarbons in the subsurface is dominated by anaerobic processes. Some data have indicated that benzene is recalcitrant; some have shown it degrades but at a slower rate than alkyl benzenes (primarily TEX) under anaerobic conditions. Many natural attenuation studies have pointed to the sequential order of attenuation. This paper evaluated data from four sites in the Midwestern U.S. (Illinois, Indiana, Michigan, and Missouri) that explain and contrast existing impressions. Although the actual attenuation rates varied from site to site, primarily dependent upon the relative availability of electron acceptors, data from these sites indicated that attenuation of BTEX compounds under anaerobic conditions is concurrent. The benzene attenuation rate appeared to be a function of the relative abundance of TEX. The ratios of attenuation rates between the compounds, however, appear to be relatively constant within certain brackets. For example, of the BTEX compounds, at the four sites in this study, toluene attenuated at the highest rate followed by benzene which attenuated at average rates between 70 and 80% of toluene, xylenes which attenuated at average rates between 60 and 70% of toluene (which may be dependent upon which xylenes are most abundant), and lastly ethylbenzene which attenuated at average rates 50 to 60% as high as toluene. These were observed at both natural and sulfate-enhanced attenuation sites.



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