Effects of glacial isostatic adjustment on fault reactivation and its consequences on radionuclide migration in crystalline host rock

Deep geological repositories (DGRs) are designed to isolate radioactive waste (RW) from the biosphere over extremely long-time scales (i.e., several hundred thousand years). In order to assess the robustness of a safety case for a DGR, it is therefore necessary to analyse probable, less probable and hypothetical future developments due to, for example, climate change. Climate models predict that several ice sheets will advance and retreat over the next several thousands years. Glacial isostatic adjustment (GIA), resulting from the large moving mass of an ice sheet, can alter the displacement and far-field stress field of a DGR.

Due to their extremely low matrix permeability, crystalline rocks are suitable host rocks for the disposal of RW in DGRs. However, the mechanical properties of crystalline rocks often promote crack growth and faulting, which in turn compromise their barrier function with respect to groundwater flow. In the INFRA project (DFG NA1528/2-1 and MA4450/5-1), we quantify how faults prone to reactivation during glacial events can affect radionuclide migration around a DGR in crystalline rock.

We apply boundary conditions derived from an established GIA model [1,2] to a finite element model [3,4] of coupled fluid flow and radionuclide transport to numerically solve the component transport problem before and after fault reactivation. The Coulomb failure stress criterion is used as an indicator of permeability changes. The simulations show that GIA can increase permeability in the upper 400m of the reactivated faults. There, groundwater flow enhances radionuclide migration along the fault. In contrast, groundwater flow is reduced in the direction perpendicular to the fault plane. Although the proposed numerical workflow has been applied to the case of GIA, it can also be adapted to study hydromechanical processes induced by seismic events or by hydrofracking in enhanced geothermal systems.

 

[1] Argus, D.F., Peltier, W.R., Drummond, R., Moore, A.W.: The Antarctica component of postglacial rebound model ICE-6G C (VM5a) based on GPS positioning, exposure age dating of ice thicknesses, and relative sea level histories. Geophysical Journal International 198(1), 537–563 (2014)
https://doi.org/10.1093/gji/ggu140

[2] Peltier, W.R., Argus, D.F., Drummond, R.: Space geodesy constrains ice age terminal deglaciation: The global model. Journal of Geophysical Research: Solid Earth 120(1), 450–487 (2015)
https://doi.org/10.1002/2014JB011176

[3] Bilke, L., Flemisch, B., Kalbacher, T., Kolditz, O., Helmig, R., Nagel, T.: Development of Open-Source Porous Media Simulators: Principles and Experiences. Transport in Porous Media 130(1), 337–361 (2019)
https://doi.org/10.1007/s11242-019-01310-1

[4] Bilke, L., Fischer, T., Naumov, D., Lehmann, C., Wang, W., Lu, R., Meng, B., Rink, K., Grunwald, N., Buchwald, J., Silbermann, C., Habel, R., Günther, L., Mollaali, M., Meisel, T., Randow, J., Einspänner, S., Shao, H., Kurgyis, K., Kolditz, O., Garibay, J.: OpenGeoSys. Zenodo (2022)
https://doi.org/10.5281/zenodo.7092676