| Methyl tertiary butyl ether (MTBE), an oxygen-bearing hydrocarbon, has been added to automobile fuels since the early 1980s to improve combustion efficiency and reduce smog.; Before this work, the existence of native microbial consortia (group of species with varying carbon preferences and capabilities) capable of MTBE oxidation under aquifer conditions was not established, and no controlled tests had been conducted to clearly demonstrate the feasibility of the permeable reactive biobarrier (PRB) treatment approach for MTBE. In microcosm work described herein, microorganisms from within an MTBE plume at Vandenberg Air Force Base, CA, were shown to degrade MTBE and its breakdown product tert-butyl alcohol (TBA) when provided solely with dissolved oxygen (DO) (i.e. without need of additional inorganic nutrients or other co-factors). Repeated MTBE challenges showed that the consortium is capable of degrading MTBE concentrations up to 88 mg/L, far in excess of what is currently found anywhere on site. The average MTBE degradation rate in seven challenges was 0.065 ± 0.017 day−1. TBA was only transiently observed, and it appeared that once the consortium adapted to TBA degradation, the rate was faster than that for MTBE. Furthermore, there was no lag phase observed—degradation commenced immediately after exposure of the sediments and groundwater to oxygen. This suggested that it might be possible to promote the reaction under field conditions, and field experiments were undertaken.; In controlled field tests using these oxygen releasers, it was found that degradation of MTBE was possible under field conditions (highest concentration studied was 2.1 mg/L), and TBA was observed only once. MTBE (and likely TBA) degradation rates were actually faster than that observed in the lab: 5.2 to 6 day−1. This was likely because more efficient mixing of oxygen and MTBE was achieved in the former, due in part to the method of oxygen delivery and partly because the system was constantly flowing. The presence of the polymeric tubing did not appear to adversely affect biodegradation. This microbial consortium was also capable of MTBE degradation concomitant with toluene degradation, suggesting that it may be possible, assuming labile oxygen demand could be met with the oxygen release system, that such a treatment system could be installed in source zone areas.; These results, taken together, suggest that it is possible to promote in situ MTBE biodegradation solely by enhancing dissolved oxygen concentrations at the studied field site. Indeed, on the basis of these results, a full-scale PRB employing passive oxygen emitters is being installed across the plume by contractors to the Air Force to control MTBE migration. This work has highlighted a number of scale-up issues that require assessment prior to full-scale application of aerobic PRBs for MTBE treatment. These issues include the need to establish the presence and capabilities of MTBE-degrading microorganisms, and understand the spatial variability of microbial capabilities, various oxygen demands, and plume mass discharge. (Abstract shortened by UMI.)... |
