Clinton D. Church

Laboratory evidence of MTBE biodegradation in Borden aquifer material.

Mainly due to intrinsic biodegradation, monitored natural attenuation can be an effective and inexpensive remediation strategy at petroleum release sites. However, gasoline additives such as methyl tert-butyl ether (MTBE) can jeopardize this strategy because these compounds often degrade, if at all, at a slower rate than the collectively benzene, toluene, ethylbenzene and the xylene (BTEX) compounds. Investigation of whether a compound degrades under certain conditions, and at what rate, is therefore important to the assessment of the intrinsic remediation potential of aquifers. A natural gradient experiment with dissolved MTBE-containing gasoline in the shallow, aerobic sand aquifer at Canadian Forces Base (CFB) Borden (Ontario, Canada) from 1988 to 1996 suggested that biodegradation was the main cause of attenuation for MTBE within the aquifer. This laboratory study demonstrates biologically catalyzed MTBE degradation in Borden aquifer-like environments, and so supports the idea that attenuation due to biodegradation may have occurred in the natural gradient experiment. In an experiment with batch microcosms of aquifer material, three of the microcosms ultimately degraded MTBE to below detection, although this required more than 189 days (or >300 days in one case). Failure to detect the daughter product tert-butyl alcohol (TBA) in the field and the batch experiments could be because TBA was more readily degradable than MTBE under Borden conditions.

Evaluation of biodegradation and dispersion as natural attenuation processes of MTBE and benzene at the Borden field site

Abstract A natural gradient tracer test was performed in the shallow, aerobic sand aquifer at Canadian Forces Bases (CFB) Borden in 1988. A mixture of groundwater, spiked with dissolved oxygenate-containing gasoline, was injected below the water table along with chloride (Cl − ) as a conservative tracer. The mass of BTEX compounds in the plume diminished significantly over 16 months of monitoring due to intrinsic aerobic biodegradation; MTBE showed only a small decrease in mass over the same period. In 1995/1996, a comprehensive groundwater sampling program was undertaken to define the mass of MTBE still present in the aquifer. Only 3% of the original MTBE mass remained. Sorption, volatilization and abiotic degradation were ruled out as significant attenuation processes for the field conditions. As well, a study on the phytoremediation potential of the site showed that the plants in the study area were unlikely to contribute to the disappearance of the aqueous MTBE mass. These results indicate that biodegradation may have played a major role in the attenuation of MTBE within the Borden aquifer. In support of this hypothesis, significant MTBE mass losses were observed in aerobic batch experiments that used authentic Borden aquifer material and groundwater. Therefore, it appears that MTBE, like BTEX, can be remediated intrinsically due to biodegradation. Unlike BTEX, however, MTBE is biodegraded very slowly making biodegradation less likely to be sufficient in protecting aquifers and downgradient receptors once MTBE is spilled at a site.