Use of carbon-14 trichloroethene to determine degradation rate constants in rock core microcosms

It is challenging to obtain degradation rate constants for chlorinated solvents (e.g., trichloroethene, TCE) that reside within low permeability formations such as fractured bedrock. Degradation rates are needed to evaluate remediation strategies. The overall objective of this research was to determine rate constants for degradation of TCE in rock core samples from fractured bedrock aquifers at three Department of Defense sites. In a prior study (Wang et al., 2024), core samples were used in novel intact rock microcosms that were infused with TCE and carbon-14 (¹⁴C-) labeled TCE. Uncontaminated groundwater was circulated through the headspace of the microcosms (i.e., the simulated fracture space) to induce diffusion out of the core that simulated back diffusion from a low permeability rock matrix. Four incubation conditions were evaluated based on the composition of the groundwater circulated through the simulated fracture space: unamended (i.e., no electron donor or sulfate added); lactate amended; lactate + sulfate amended; and mercuric chloride amended. In addition to monitoring the formation of cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC), data were collected on non-chlorinated degradation products that accumulated in the microcosms, including dissolved gases (i.e., acetylene, ethene, ethane) and ¹⁴C-labeled soluble compounds. The rates at which the non-chlorinated degradation products accumulated were used to estimate pseudo-first-order rate constants by inverting numerical simulations. Rate constants for unamended treatments that best represent in situ conditions ranged from 0.013 to 0.049 yr^-1 and are statistically equivalent to the constants characterizing treatments that were amended with mercuric chloride to inhibit microbial activity. ¹⁴C-labeled degradation products accounted for 75-90 % of the rate constants; not including these products would underestimate TCE degradation potential. Lactate and lactate + sulfate amended microcosms had statistically higher rate constants for TCE degradation (ranging from 0.034 to 0.13 yr^-1) compared to unamended microcosms. Although numerical simulation of intact rock core microcosms infused with ¹⁴C-labeled compounds is more time-consuming and complex than other methods to assess TCE degradation potential in low permeability media, the resulting rate constants provide a robust estimate of the potential for TCE degradation in situ.