Effect of Local Dispersion on Mixing Enhancement when Applying an Engineered Injection and Extraction System: Laboratory and Model-based Evidence

Engineered Injection and Extraction (EIE) systems represent a promising groundwater remediation technique. This technology generates transient groundwater flow fields by periodically operating a system of pumping wells, with the goal of enhancing contaminant degradation through mixing with a treatment solution. The objective of this work is to provide experimental evidence of the effect of an EIE system on plume mixing and to investigate the effect of local dispersion on mixing enhancement. We perform laboratory experiments in a quasi-two-dimensional setup representing a vertical cross-section of an unconfined homogeneous aquifer. The setup is equipped with four wells, connected to a peristaltic pump, which are activated one at a time following an injection-extraction sequence. The wells operation establishes transient flows within the setup and introduces fluctuations in the groundwater table. A conservative tracer is injected in the middle of the area delimited by the wells, and a high-resolution image analysis technique is applied to track the evolution of the tracer concentration. We perform the experiments considering two different grain sizes and investigate the effect of the application of the EIE system on the plume mixing and spreading in contrast to two benchmark experiments where the wells are not operating and, therefore, only diffusion affects the tracer plume. Additionally, for one porous material, we permute the injection-extraction sequence to study the effect of different transient flow conditions and groundwater table fluctuations on plume deformation and mixing. We also provide a model-based interpretation of the experimental results using Richards equation and the conservative advection-dispersion equation to describe flow and transport processes, respectively. Plume spreading is quantified by computing the second central spatial moments, while the degree of mixing is estimated by calculating the plume area. We use the Okubo-Weiss metric computed over the plume area to link the mixing enhancement to the change in the flow topology. Our results show that EIE effectively enhances mixing and spreading at the laboratory scale, especially when the flow field leads to high values of the Okubo-Weiss metric. Moreover, local dispersion is shown to be a key factor for mixing enhancement in engineered injection and extraction systems.