Experimental investigation of solute transport across transition 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) in solute movement across material interfaces, indicating the presence of discontinuous concentration that makes solute transport directionally dependent. Transition interfaces are more common in nature than sharp interfaces. To understand solute transport across transition interfaces, well-controlled laboratory experiments were performed. A numerical model was also built to understand mass accumulation and concentration discontinuity through transition as well as sharp interfaces. We conclude that directionally dependent asymmetry of BTCs was found with both sharp and transition interfaces. The asymmetry of BTCs was more pronounced at a transition interface than at a sharp interface. The mobile and immobile (MIM) model can better capture the directionally dependent transport of solutes through a sharp/transition interface than the advection-dispersion equation (ADE). The mobile water partition coefficient (beta) and mass transfer coefficient (omega) in MIM were lager in the direction from fine sand to coarse sand (F-C). The time difference between tracer replace and tracer input is greater in the presence of an interface, especially transition interfaces. Even at small Reynolds numbers (1 x 10(-4) to 0.116), solute transport across a discontinuous interface under reversible flow directions is most likely dominated by convective dispersion rather than an assumed diffusion process.