Reactive transport modeling of dissolved oxygen migration and consumption in a sedimentary basins affected by a deglaciation event

In intracratonic sedimentary basins, geochemical conditions are currently reducing at depth. Deep groundwater flow systems are driven primarily by salinity differences, topographic gradients and recharge derived from precipitation; these systems are also influenced by the hydrostratigraphy of the basin. However, during periods of glacial melt water production (i.e., deglaciation events), the melting of ice sheets may alter the patterns of freshwater infiltration, potentially resulting in enhanced recharge of glacial melt water containing relatively high concentrations of dissolved oxygen. Reactive transport modeling can be used to understand the evolution of geochemical conditions and redox-buffering capacity of these formations. Dissolved oxygen will interact with reduced mineral phases that are present in the sedimentary units (e.g., chlorite) or with solid organic matter causing oxygen consumption. Processes included in the model are density-driven flow and transport, vertical mechanical deformation, as well as chemical reactions (aqueous complexation, mineral dissolution and precipitation including evaporites, sulfates and carbonates, cation-exchange, redox processes involving the decomposition of organic matter, dissolution of Fe-bearing minerals, biotite and chlorite, and the oxidation of ferrous iron and sulfide). Transient boundary conditions are imposed in the upper part of the model to mimic ice sheet advance and retreat. Simulation results indicate that the presence of dense brines at depth results in low groundwater velocities during glacial meltwater infiltration, restricting the ingress of oxygenated waters in the basin. In addition, due to the abundance of reduced mineral phases and solid organic matter in these formations, geochemical processes causing oxygen consumption are restricted to shallow aquifers, further limiting the ingress of oxygenated waters to the first 100 m in the main aquifers (i.e., sandstones) and 50 m in the carbonates aquifers. Modeling results also suggest that the ingress of oxygen is practically insensitive to oxygen content in the recharge water.