RESULTS OF LABORATORY MICROCOSM STUDIES TO INVESTIGATE INTRINSIC BIODEGRADATION OF CARBON TETRACHLORIDE IN A FRACTURED BASALT AQUIFER

摘要

Groundwater contamination beneath a manufacturing facility has been the subject of an extensive remediation investigation over the past four years. Beneath this manufacturing facility exists a highly fractured basalt geologic formation containing groundwater contaminated with carbon tetrachloride (CT). Results of recent field investigations have indicated that dissolved-phase contamination appears to be attenuating downgradient from the manufacturing facility. Examination of historical groundwater data for footprints of natural attenuation suggests that biodegradation may play an important role in the fate and transport of CT at this site. A laboratory study was initiated to evaluate the potential for intrinsic bioremediation of CT and related chlorinated methanes. Laboratory microcosms were prepared with crushed basalt and groundwater to simulate actual field conditions, and were spiked with 5 mg/L of radiolabeled [~(14)C]CT. Results show that approximately 85% of the parent compound was degraded after 109 days. Compared to the observed 15% loss of CT in sterile (control) microcosms, this difference (70%) can be attributed to biological degradation of CT to non-chlorinated endproducts. Transient metabolites, namely chloroform and methylene chloride, were occasionally observed over the course of the study. However, complete dechlorination of CT to CO_2 was observed in most microcosms at the end of the one-year study. The results of this study indicate that intrinsic biodegradation of CT is a significant mechanism for natural attenuation of chlorinated methanes in this fractured basalt aquifer system. As a result, monitored natural attenuation may be employed as a component of the remedy at this chlorinated solvent-contaminated site.