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.