Impact of Sharp Soil Interfaces on Solute Transport: Insights from Reactive Tracer Test in a 2D Intermediate-Scale Experiment

Column tracer experiments reported in the literature have suggested the occurrence of solute accumulation at the interface between different porous materials. This unexpected effect cannot be explained by standard modeling approaches based on Fickian flux continuity and the advection-dispersion equation. In this article, to further analyze this phenomenon, we present reactive transport experiments in a 2D intermediate-scale horizontal tank aimed at visualizing and evaluating the spatiotemporal evolution of a solute plume crossing a sharp interface between coarse and fine material. The solute plume results from the reaction of two fluid solutions that enter the tank in parallel through the tank inlet ports. The reaction product is analyzed through mixing and reaction metrics. Results show that the reaction product also encounters anomalous resistance when crossing the interface between coarse and fine material. This effect is much less pronounced in the fine-to-coarse (FC) transition when the direction of flow is reversed. However, contrary to the reported one-dimensional results (column experiments), this asymmetric anomalous resistance to crossing the interface does not produce solute accumulation behind the interface. Instead, results show an unexpected significant enhancement of the transverse spread of the reaction product in the coarse-to-fine transition (CF) with a slow release in the fine material. As a result, a sudden decrease in the longitudinal resident concentration profile across the heterogeneity interface is observed. Corresponding mixing metrics show that as the apparent transverse dispersivity increases when approaching the interface in the CF transition, the scalar dissipation rate and the total mass reacted also increase, indicating that the CF configuration tends to promote greater solute reactivity near the interface than the FC configuration.