BTEX BIODEGRADATION IN MICROCOSMS CONTAINING WETLAND VERSUS AQUIFER SEDIMENTS FROM A CRUDE-OIL CONTAMINATED SITE, BEMIDJI, MN

Recent development and installation of in situ microcosms at the Bemidji site has provided new insight into BTEX attenuation processes in the subsurface. Interestingly, BTEX biodegradation appeared to be enhanced in a neighboring methanogenic wetland relative to the historically contaminated iron-reducing aquifer. In the aquifer a 200 day lag was observed before biodegradation commenced and degradation of benzene and ethylbenzene did not proceed until toluene was degraded. In the wetland sediments biodegradation began within 2 days. Thus, the objective of this work was to carry out controlled microcosm studies in order to explore potential factors influencing BTEX biodegradation in the wetland versus aquifer sediments. Since it is now widely being implemented as a biofuel, the effect of ethanol on BTEX biodegradation was also investigated.

Wetland and aquifer sediment and corresponding water samples were collected from the Bemidji site in June 2011 and transferred to 70-mL microcosms to compare predominantly iron reducing (amended with FeCl₃) versus methanogenic (no amendment) conditions in the two sediment types. All bottles were spiked with the same initial concentration of BTEX (~1 mg/L) and a subset of each condition was amended with ethanol or potentially bioavailable organic carbon (PBOC) extracted from the wetland sediments. Killed controls (amended with HgCl₂) were also monitored.

BTEX concentrations are currently being monitored in the microcosms with time in order to determine biodegradation rates and identify whether the enhanced biodegradation rates observed in the in situ studies are an inherent property of the sediment material (aquifer versus wetland) or if it is governed by the predominant electron acceptor conditions or presence of alternative organic compounds, such as ethanol or PBOC. Microbial community analysis will also be performed in order to gain insight into the roles of native populations on BTEX biodegradation. The overall results are expected to support improved understanding of processes governing BTEX fate in the subsurface.