Uncovering asymmetrical mass transfer in layered porous media: Insights from pore-scale analysis

Abstract

Natural sediments or aquifers frequently exhibit layering and structural contrasts, which give rise to anomalous mass transfer behavior. Moreover, the effects of the interface on mass transfer and mixing in layered porous media (LPM) may be different when the flow direction is reversed, resulting in asymmetrical dispersive transport and a significant disparity between predictions and actual observations. Despite its importance, the underlying mechanism remains an open question. To address this gap, laboratory experiments and numerical simulations were conducted. The results demonstrated that the effective dispersion coefficient is larger when the solute migrates from the coarse grain layer to the fine grain layer (C-F direction) compared to the F-C direction. The history of the solute across the interface and the flow adjacent to the interface affect this deviation. Pore-scale simulations revealed a more tortuous flow field with a higher proportion of low and high velocities in the fine grain layer, although the velocity characteristics of the entire LPM were insensitive to flow direction. Measuring the velocity characteristics of LPM alone is insufficient to predict the asymmetrical dispersive transport behavior. To quantitatively evaluate mass transfer between layers and residual contaminants in LPM, a mass transfer index ε was introduced. The results of ε, along with the pore-scale concentration, directly indicated that the interface limited the mass transfer. The effect was more pronounced in the C-F direction and particularly evident in the heterogeneous LPM, which ultimately enhanced the solute mixing. In the heterogenous LPM, the maximum dilution index for the C-F direction is 1.31 times larger than that for the F-C direction. The findings contribute to a better understanding of anomalous solute transport and develop models for contaminant transport in layered sediments.