Experimental investigation of solute transport across discontinuous interface of porous media under reversible flow directions

Understanding solute transport through macroscopic interfaces is essential to understand the effects of geological heterogeneity on contaminant transport in porous media. Studies of solute transport in compartmental porous media have noted the asymmetry of breakthroughs (BTCs) as solutes move across material interfaces, indicating the presence of discontinuous concentration that makes solute transport directionally dependent. Transitional interfaces are more common in nature than sharp interfaces. To understand solute transport across the transitional interface, well-controlled laboratory experiments were carried out and a numerical model was also performed to understand the mass accumulation and concentration discontinuity through transitional as well as sharp interfaces. We conclude that directionally dependent asymmetry of BTCs was found with both sharp and transitional interfaces. As in previous studies, BTCs increased in a slower rate with longer tailings when flow is from the coarse sand to fine sand (C-F) direction than that from the fine sand to coarse sand (F-C) direction. The asymmetry of BTCs was more pronounced with a transitional interface than that with a sharp interface. The Darcy-scale numerical simulations could not reveal the asymmetry of BTCs and the Fickian-based advection-dispersion equation (ADE) did not fit well with the results of experimental BTCs in arrival time and long tailing. The pore-scale numerical simulations supported the findings in the experiments and revealed the mass accumulation at the interface. The ratio of coarse/fine particle diameters and the arrangement of particles near the interface were found to influence the transboundary solute transport for both sharp and transitional interfaces.