Coupled effects of local dispersion, density and chaotic advection on mixing enhancement

In the subsurface, many biogeochemical reactions are characterized by inefficient mixing. Therefore, it is essential to investigate mechanisms that can enhance mixing processes. One example is the case of transient flow conditions. Specifically, Engineered Injection and Extraction (EIE) protocols can generate chaotic advection and are well known in the literature to enhance solute mixing and contaminant degradation. The objective of this work is to provide experimental evidence of the interplay between local dispersion, density-driven flow, and chaotic advection on solute transport and mixing. Density-contrasts are indeed particularly relevant in the context of groundwater remediation and saltwater intrusion. We conducted a series of experiments in a quasi-two-dimensional chamber representing a vertical cross-section of a homogeneous unconfined aquifer. The setup is equipped with four wells which operate sequentially following a prescribed pumping schedule. In our set of experiments, two different grain sizes are used to investigate the role of local dispersion, while different injected solute concentrations are used to study the impact of density contrasts. The effect of chaotic flow generated by the operation of the EIE system is compared to experiments run under no-flow conditions in the surroundings to isolate the contribution of purely density-driven flows to solute mixing. The conservative tracer is injected in the middle of the area delimited by the four wells, and a non-invasive optical method is applied to track the evolution of the solute at a high temporal resolution. Mixing is quantified by computing the plume area. Our results show that local dispersion plays a significant role in density-driven flow as experiments performed in coarse porous media display higher mixing enhancement in comparison to those conducted in the fine porous media. At early times in the experiments, density effects are more pronounced in the experiments performed in the coarse porous material, but they decrease over time when the plume is more diluted and the density-contrasts are less pronounced. Finally, chaotic advection has a major effect on mixing enhancement. However, its impact decreases as the plume area increases, and at later times local dispersion is the dominant process contributing to the enhancement.