Long term redox evolution in granitic rocks: Modelling the redox front propagation in the rock matrix

Redox evolution in groundwaters in granitic rocks is considered to be largely due to reactions between dissolved species in the groundwater and minerals that line the walls of fractures in the bedrock. In a long term perspective, however, the reducing fracture minerals eventually become depleted. In this situation, O2 diffuses out from the fractures and reacts with reducing species present in the porous rock matrix. The rock matrix contains the main reservoir of reducing capacity in the form of reducing minerals such as biotite. The aim of this work is to emphasize the coupled transport and reaction processes deemed important for the redox evolution in recharge groundwaters over long times, i.e. thousands of years. Results indicate that matrix diffusion of O2 becomes limiting for the redox reactions rapidly after the reducing capacity of the fractures is depleted. The numerical model results are verified by simplified cases solved analytically. From the analytical solutions insights also are gained into when different mechanisms dominate the overall reaction, which is more difficult if only numerical results are available.