Use of carbon-14 labeled trichloroethene to assess degradation potential in rock core microcosms

In fractured rock aquifers contaminated with trichloroethene (TCE), the extent of groundwater plumes is impacted by degradation occurring within the rock matrix. The objective of this study was to evaluate TCE degradation in rock samples from three sites where in situ conditions may favor natural or enhanced attenuation. Intact rock core microcosms (94 total) were used to assess in situ conditions and enhancement by addition of lactate or lactate + sulfate. A key advance for this study was inclusion of carbon-14 (¹⁴C) labeled TCE in the experimental design, which enables monitoring of ¹⁴C-labeled products in addition to more readily detectable compounds associated with TCE degradation (i.e., cis-1,2-dichloroethene (cDCE), vinyl chloride, acetylene, ethene, and ethane). ¹⁴C-labeled products comprised 35-95 % of the total degradation products recovered over 9-21 months of monitoring, indicating that inclusion of ¹⁴C-TCE was essential to capturing the full potential for abiotic and biotic degradation of TCE. Microcosms infused with TCE but not ¹⁴C-TCE exhibited enrichment in δ¹³C-TCE, and enrichment in δ¹³C-cDCE in microcosms that underwent reductive dechlorination of TCE to cDCE. The results demonstrate the advantages of using diffusion-transport microcosms and ¹⁴C-TCE to document degradation of chlorinated ethenes in fractured rock aquifers.