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

A natural gradient tracer test was performed in the shallow, aerobic sand a quifer at Canadian Forces Base (CFB) Borden in 1988. A mixture of groundwat er, spiked with dissolved oxygenate-containing gasoline, was injected below the water table along with chloride (Cl-) as a conservative tracer. The ma ss of BTEX compounds in the plume diminished significantly over 16 months o f monitoring due to intrinsic aerobic biodegradation; MTBE showed only a sm all decrease in mass over the same period. In 1995/96, a comprehensive grou ndwater sampling program was undertaken to define the mass of MTBE still pr esent in the aquifer. Only 3% of the original MTBE mass remained. Sorption, volatilization and abiotic degradation were ruled out as significant atten uation processes for the field conditions. As well, a study on the phytorem ediation potential of the site showed that the plants in the study area wer e unlikely to contribute to the disappearance of the aqueous MTBE mass. The se results indicate that biodegradation may have played a major role in the attenuation of MTBE within the Borden aquifer. In support of this hypothes is, 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 biode gradation. Unlike BTEX, however, MTBE is biodegraded very slowly making bio degradation less likely to be sufficient in protecting aquifers and downgra dient receptors once MTBE is spilled at a site. (C) 1999 Elsevier Science L td. All rights reserved.