Determining Anaerobic BTEX Decay Rates in a Contaminated Aquifer

Abstract

Intrinsic biodegradation of petroleum hydrocarbons in the subsurface has gained increased acceptance as a remedial alternative where the risk of exposure is within acceptable standards. However, methods to predict reliably the rate and extent of biodegradation at contaminated sites have not been established. Laboratory microcosm and in situ column experiments were conducted in a petroleum-contaminated aquifer undergoing intrinsic biodegradation to (1) document that anaerobic biodegradation of benzene, toluene, ethylbenzene, and xylene (BTEX) isomers occurred, (2) determine the rate and pattern of BTEX biodegradation, and (3) compare measured decay rates in laboratory and in situ microcosm results with contaminant concentrations along the length of the plume. Both methods verified that indigenous microorganisms have the capability to biodegrade anaerobically BTX under ambient conditions. However, microcosms constructed with aquifer material from different areas of the plume showed significant variability in rate and extent of contaminant biodegradation. In experiments conducted in the same area of the plume, the biodegradation rate for the individual compounds measured in both in situ columns and microcosms was over 10 times higher than the rate calculated from field monitoring. Thus, these methods should not be used to infer field-scale decay rates.