Naturally occurring dechlorination reactions in rock matrices: Impacts on TCE fate and flux

• Abiotic dechlorination reactions in rock matrices are simulated.
• Back-diffusion timeframes can be greatly reduced due to abiotic dechlorination.
• Downgradient migration can be mitigated due to dechlorination in the rock matrix.

Chlorinated solvents in rock matrices can serve as a long-term contaminant source in fractured rock aquifers, sustaining groundwater plumes for extended periods of time. The intensity and longevity of the groundwater plume will be impacted by the diffusional flux between the rock matrix and adjacent conductive fractures, as well as the fate of contaminants residing within the rock matrix itself. In this study, 1-dimensional numerical simulations are performed to assess the impacts of slow naturally occurring abiotic dechlorination reactions on TCE fate and transport in rock matrices. Simulation parameters for the rock matrix, including effective diffusion coefficients and trichloroethene (TCE) first order dechlorination rate constants, are derived from experimental data from intact rock cores. Simulations show that a TCE dechlorination rate constant of 1×10−8  s−1 can have a substantial impact on TCE uptake and release from the rock matrix. In addition, varying the simulated matrix porosity indicates that the impacts of matrix reactions are exacerbated in low porosity matrices. Overall, simulation results show that contaminant removal from rock matrices can be dominated by abiotic reaction, and that the back-diffusion timeframes for sustaining bedrock plumes above regulatory levels may be limited to a few decades if these abiotic reactions are occurring within the rock matrix.