Effects of ethanol versus MTBE on benzene, toluene, ethylbenzene, and xylene natural attenuation in aquifer columns

The increased use of ethanol as a replacement for the gasoline oxygenate, methyl tert-butyl ether (MTBE), may lead to indirect impacts related to natural attenuation of benzene, toluene, ethylbenzene, and the three isomers of xylene (BTEX compounds). Ethanol could enhance dissolved BTEX mobility by exerting a cosolvent effect that decreases sorption-related retardation, This effect, however, is concentration dependent and was not observed when ethanol was added continuously (at 1%) with BTEX to sterile aquifer columns. Nevertheless, a significant decrease in BTEX retardation was observed with 50% ethanol, suggesting that neat ethanol spills in bulk terminals could facilitate the migration of pre-existing contamination. MTBE (25 mg/L influent) was not degraded in biologically active columns, and it did not affect BTEX degradation. Ethanol (2 g/L influent), on the other hand, was degraded rapidly and exerted a high demand for nutrients and electron acceptors that could otherwise have been used for BTEX degradation. Ethanol also increased the microbial concentration near the column inlet by one order of magnitude relative to columns fed BTEX alone or with MTBE. However, 16S-ribosomal ribonucleic acid sequence analyses of dominant denaturing gradient gel electrophoresis bands identified fewer species that are known to degrade BTEX when ethanol was present. Overall, the preferential degradation of ethanol and the accompanying depletion of oxygen and other electron acceptors hindered BTEX biodegradation, which suggests that ethanol could increase the length of BTEX plumes.