Impact of immobile porosity architecture on reactive transport in mobile/immobile models

Diffusive porosity structures in aquifers can display a large diversity of topologies, from intergranular porosity to the 'dead-ends' of fracture networks. Here we study numerically the influence of this topology on solute transport parameters, such as dispersion coefficient, and on a simple equilibrium reaction. We build a model where diffusive structures of variable topology (with junctions, loops…) exchange with a fast, advective zone. We show that the internal organization of volumes of the diffusive structure has a strong and non-trivial influence on transport and reactivity. We also show, through Multi-Rate Mass Transfer framework, that the signature of this topology on residence times is often sufficient to reproduce the initial reaction rates. We thus propose new ways to determine the appropriate MRMT models for a wide range of porosity types. However, more complex chemical / physical processes (kinetic-limited reactions, gravity effects…) may diminish this relevance of MRMT models to reproduce reaction rates, and additional parameters linked to the topology of diffusive structures may be required.