MIN3P-Thcm Reactive Transport Modelling in Low Permeability Media - Mont Terri in-Situ Diffusion Experiments and EBS-TF-C Benchmarks

Abstract:

Reactive transport modelling is one method for assessing long-term geochemical processes influencing radionuclide mobility in geologic formations considered for implementation of the multi-barrier deep geologic repository concept. A key issue revolves around the quantitative prediction and assessment of solute migration and retardation through engineered and enclosing natural barriers.

The reactive transport code MIN3P-THCm, with several code enhancements (i.e., multisite ion exchange, multicomponent diffusion and hybrid multicomponent diffusion models), was used to simulate a series of benchmarking problems, laboratory experiments, and in-situ diffusion experiments in the framework of the Mont Terri and EBS-TF-C projects. For the Mont Terri project, excellent agreement was achieved with other groups for the generic benchmarking simulations. Additional simulations were undertaken to reproduce in-situ borehole diffusion experiments using parameters determined through laboratory experiments. Comparison of the simulated and the in-situ borehole diffusion data showed: good agreement for HTO, anions, weakly sorbing tracers and major ions, but some discrepancies for strongly sorbing tracers. Sensitivity analysis revealed that a scenario in which the borehole filter is assumed to be partially filled with dispersed fine clay particles originating from the host rock might explain the rapid concentration decrease of strongly sorbing tracers in the borehole solution. For the EBS-TF-C project, comparison of simulated and experimental data shows very good agreement for cases involving conservative transport and anion migration. Excellent agreement was also obtained for cation migration in comparison to other established reactive transport codes; however, some discrepancies remain between observed and simulated data. In the future, continued development of conceptual models and numerical implementations could lead to further improvements in mechanistic descriptions of strongly sorbed cation transport.