Biodegradation of TCE in fractured shale and saprolite

A series of investigations were conducted to examine biodegradation of trichloroethylene (TCE) contamination in fractured shale and in saprolite (formed from weathered sedimentary rocks). A plume of groundwater contaminated with trichloroethylene (TCE) was detected at the Oak Ridge Reservation (ORR) in eastern Tennessee adjacent to shallow waste trenches in fractured shale. Monitoring wells at the site indicated a downgradient decline in concentration of TCE and the appearance of its daughter products (cw-dichloroethylene (cDCE) and vinyl chloride (VC)), which suggests the likelihood that anaerobic biodegradation of TCE was occurring. This hypothesis is further supported by the existence of redox conditions, including iron reduction, sulfate reduction, and possibly methanogenesis, which are favorable for anaerobic biodegradation. Microbial community analysis using conventional enrichment methods and molecular methods also support this hypothesis by showing the presence of bacteria previously implicated in the anaerobic biodegradation of chlorinated solvents. This is believed to be the first study to show strong evidence of biodegradation of TCE in shale bedrock.

Additional investigations were performed using large undisturbed columns of fractured saprolite from an uncontaminated site about 1 km from the waste trenches. The experiment involved continuous pumping of groundwater containing dissolved phase TCE through one column containing the natural microbial communities (the biotic column), and through a second column in which the microorganisms had been inhibited. In effluent from the biotic column evidence of anaerobic biodegradation TCE appeared within a few months. This included decreasing concentration of TCE in the effluent, appearance of daughter products (cDCE and VC), development of iron and sulfate reducing conditions, and appearance of iron and sulfate reducing bacteria. In the inhibited column there were no indicators of TCE degradation. It appears that TCE biodegradation processes in the shale and saprolite are very similar, and that they can occur spontaneously and rapidly without amendments to enhance biodegradation. Current Air Force Center for Environmental Excellence (AFCEE) protocols for determination of natural attenuation, which are based on monitoring of geochemical parameters, are appropriate for assessing the potential for TCE attenuation in the shale and shale saprolite found at the Oak Ridge Reservation.