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

Understanding solute transport across interfaces between different porous materials is crucial for subsurface applications. Column tracer experiments have suggested solute accumulation at these interfaces. This effect cannot be explained by standard models based on Fickian flux continuity and the advection–dispersion equation. To analyze this phenomenon, we present reactive transport experiments in a 2D intermediate-scale horizontal tank to visualize and evaluate the spatiotemporal evolution of a solute plume crossing a sharp interface between coarse and fine materials. The plume results from the reaction of two fluid solutions entering the tank in parallel through inlet ports. The reaction product is analyzed using mixing and reaction metrics. Results show the reaction product encounters anomalous resistance when the plume crosses the coarse-to-fine (CF) interface. This effect is less pronounced in the fine-to-coarse (FC) transition. This asymmetric resistance does not produce solute accumulation behind the interface, a difference from the results obtained with the one-dimensional model. Instead, results show enhanced transverse spread of the reaction product in the coarse-to-fine transition, with slow release in the fine material. A sudden decrease in the longitudinal concentration profile across the interface is observed. Mixing metrics show that as apparent transverse dispersivity increases closer to the interface in the CF transition, the scalar dissipation rate and total mass reacted increase, indicating that the CF configuration promotes greater solute reactivity near the interface compared to the FC configuration.