REACTIVE TRANSPORT MODELING OF SEDIMENTARY BASINS AFFECTED BY LONG-TERM GLACIATION-DEGLACIATION EVENTS

The migration and melting of ice sheets may lead to the ingress of fresh water into aquifer systems hosted in sedimentary basins. To support the safety assessment of sedimentary rock units for hosting deep geologic repositories (DGR) for used nuclear fuel, the geochemical stability of these formations is investigated using reactive transport modeling. In this work, we will shed light on the effect of long-term perturbations over geologic time scales (e.g. glaciation-deglaciation events) on the hydrogeological and geochemical processes in a hypothetical sedimentary basin. Processes included in the model are density-driven flow and transport, as well as chemical reactions (aqueous complexation, mineral dissolution and precipitation including evaporites, sulfates and carbonates, cation-exchange between major ions, and redox processes involving the decomposition of organic matter, the oxidation of ferrous iron and sulfide). A base case and a series of alternate scenarios are being investigated to assess the sensitivity of the model results to different conceptual models and boundary conditions. The various simulations are designed to evaluate the following aspects: (1) effect of domain size and internal boundary conditions on hydrogeological and geochemical system evolution, (2) the evolution of groundwater signature during density-driven flow and reactive transport, (3) the effect of mineral dissolution on density-driven flow and reactive transport., (4) the effect of permafrost and taliks on flow patterns and reactive transport, (5) the potential effect of fracture formation due to glacial loadings, (6) the effect of the fresh water depth, and (7) the effect of transient boundary conditions caused by the advance and retreat of a continental glacier. Preliminary results suggest that flow processes induced by ice sheet loading are insufficient to displace deep brines, and that the main flow and geochemical processes occur in shallow aquifers focused on the subsurface region around the leading edge of an ice sheet.