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.