Linking chemical weathering, evolution of preferential flowpaths and transport self-organisation in porous media using non-equilibrium thermodynamics

Abstract. We apply non-equilibrium thermodynamic framework to analyze the reaction-transport interaction of reversible dissolution-precipitation of calcite, characteristic to subsurface chemical weathering, that leads to emergence of preferential flow paths in subsurface geophysical systems. Within this framework, we identify the entropy generation sources, attributed to the dissipative processes inherent to this physical scenario and analyze them to arrive at a correlation between the evolution of the emerging paths and the accompanying dissipative dynamics in artificially generated porous medium fields of varying degree of heterogeneity. The reaction-transport interaction enhances the emerging preferential flow paths, leading to transport channelization, as attested by the Shannon entropy of concentration of chemical species, and a decline in the normalized entropy generation with time due to percolation and chemical reaction, signifying a decrease in reaction intensity and frictional dissipation as the coupled process evolves. The latter is attributed to the intensification of transport channelization, as it leads to the bounding of reaction within preferential flow paths, thus providing conductive channels for the flow which are preferable energetically. The flip side of transport channelization is the decline in the mixing of reactive species, corresponding to intensification of concentration gradients. This allows the interpretation of results as the evolution of a non-equilibrium system towards a stationary state under the applied constraint of influx of reactive species. This asymptotic stationary state corresponds to complete channelization of the medium, thus minimizing the mixing of reactive species, reducing the ensuing chemical reaction and providing energetically preferable conductive paths for the flow.