Deciphering Solute and Reactive Transport in Triple Porosity Systems: Etched Rock Core Experiments and Numerical Modeling

Understanding solute and reactive transport in triple porosity systems─comprising conduits, fractures, and a porous rock matrix─is essential for a wide range of subsurface processes and applications. The inherent heterogeneity of these systems, along with solute exchange between high- and low-permeability regions, results in anomalous transport, including early solute arrivals and prolonged residence times. A key challenge in studying these processes is the inability to directly observe the subsurface transport mechanisms. To overcome this, we employed etched dolostone cores with controlled geometry in reactive transport experiments and applied positron emission tomography (PET) imaging to directly visualize solute transport. Our findings demonstrate that flow rate significantly influences solute exchange between the conduit and the fracture/matrix. PET imaging and reactive transport experiments reveal distinct solute and mineral precipitate distributions under low- and high-inertia flow conditions. Numerical simulations further highlight the role of fracture aperture in modulating solute exchange and show how higher flow rates induce 3D recirculation zones that enhance local reactivity. Tracer breakthrough curve inversions indicate that increased exchange can produce multipeaked breakthrough curves. This multimethodological approach─integrating etched core experiments, PET imaging, and numerical simulations─advances our understanding of solute and reactive transport processes in triple porosity media.