Review of Natural Attenuation of BTEX and MTBE in Groundwater

Abstract

Monitored natural attenuation has become accepted as an appropriate “technology” for treating petroleum hydrocarbon-contaminated groundwater. Commonly, the monoaromatic compounds benzene, toluene, ethylbenzene, and xylenes (BTEX) and oxygenated additives, e.g., methyl tert-butyl ether (MTBE), are the major constituents of regulatory importance. Although a variety of naturally occurring attenuation mechanisms may reduce BTEX and MTBE concentrations, intrinsic biodegradation is the primary destructive mechanism. Attenuation by anaerobic biodegradation is important for BTEX compounds because of the relative abundance of anaerobic electron acceptors as compared with dissolved oxygen. Field studies indicate that MTBE is also biodegradable in shallow aquifers, but at a slower rate relative to BTEX compounds; thus, dispersion and dilution may also be important MTBE attenuation mechanisms. To demonstrate that natural attenuation is occurring, it is critical to document the proposed natural attenuation processes in the field. These processes often cause measurable “footprints.” At a large number of petroleum-hydrocarbon release sites, natural attenuation processes have been observed and documented and found to control the extent of migration of BTEX contaminants. Although the presence of MTBE may compromise the use of natural attenuation at some sites due to the greater mobility and persistence of MTBE compared to BTEX, some data indicate that MTBE plumes do eventually stabilize.