Density–Inertia Coupling Drives Solute Trapping at Vertical Fracture Intersections

Abstract Density‐driven flow and fluid inertia jointly shape solute transport in fracture networks, with implications for hydrogeology and subsurface engineering. While their individual effects are well recognized, their coupled impact remains underexplored. We integrate pore‐to‐network‐scale dye visualization experiments and numerical simulations to investigate solute trapping at fracture intersections. At the network scale, 3D flume experiments show localized tracer retention caused by density‐induced convection and inertia‐driven vortices. Millifluidic experiments and simulations reveal that density contrast, flow imbalance, and fluid inertia govern these dynamics. Maximum trapping occurs when bottom fracture flow is ∼10% greater than the top, maximizing mass entry while minimizing loss. This imbalance promotes vortex‐driven retention and extended solute residence, leading to breakthrough curve tailing. Simulations in smooth and rough‐walled fractures confirm that roughness alters trapping locations but not the underlying mechanisms. These findings highlight the central role of coupled pore‐scale processes in controlling network‐scale transport and subsurface reactivity.