Tert-Butyl Alcohol (TBA) Production from MTBE Plumes Under Sulfate Reducing Conditions Under the Influence of Ethanol

Tert-butyl alcohol (TBA) is an increasingly important contaminant at leaking underground storage tank (LUST) sites. In existing methyl tert-butyl ether (MTBE) plumes, incomplete degradation of MTBE under anaerobic conditions results in the production of TBA. An important question is whether there is significant anaerobic degradation of TBA under sulfate reducing conditions, the electron accepting process driving most degradation in fuel contaminated aquifers. We present results of long term experiments in a confined sulfate-reducing aquifer at Vandenberg Air Force Base (VAFB), where we have investigated the effect of ethanol, a recently introduced fuel additive, hypothesized to stimulate conversion of existing MTBE plumes to TBA.

Deployment of 13C-labeled biotraps at the site suggested the presence of sulfate-reducing, TBA-degrading microbes, yet there was no clear macroscopic evidence from plume monitoring that TBA was biodegraded under sulfate reducing conditions at a practically significant rate. Microcosms of site material amended with TBA and incubated under anaerobic conditions indicated that high concentrations (~1,000 ppm) of TBA were degraded under sulfate reducing and methanogenic conditions; however, results for more environmentally relevant concentrations (~1 ppm) were inconclusive.

Field measurements of changes in population densities of bacteria and archaea following ethanol and MTBE injection indicated increases in numbers of both groups. Bacterial communities, as represented by terminal restriction fragment length polymorphism (TRFLP) fingerprints, remained distinct in the portion of the aquifer downstream of the injection zone. Our results suggest that TBA, MTBE and ethanol-induced methane concentrations are strong determinants of the composition of the indigenous microbial community that develops during MTBE transformation. Some of the changes in microbial communities induced by ethanol may be long lasting, thus potentially altering the natural attenuation capacity of the impacted aquifer.