Improving the Efficiency of 3‐D Hydrogeological Mixers: Dilution Enhancement Via Coupled Engineering‐Induced Transient Flows and Spatial Heterogeneity

Abstract

Natural attenuation and in situ oxidation are commonly considered as low‐cost alternatives to ex situ remediation. The efficiency of such remediation techniques is hindered by difficulties in obtaining good dilution and mixing of the contaminant, in particular if the plume deformation is physically constrained by an array of wells, which serves as a containment system. In that case, dilution may be enhanced by inducing an engineered sequence of injections and extractions from such pumping system, which also works as a hydraulic barrier. This way, the aquifer acts as a natural mixer, in a manner similar to the industrialized engineered mixers. Improving the efficiency of hydrogeological mixers is a challenging task, owing to the need to use a 3‐D setup while relieving the computational burden. Analytical solutions, though approximated, are a suitable and efficient tool to seek the optimum solution among all possible flow configurations. Here we develop a novel physically based model to demonstrate how the combined spatiotemporal fluctuations of the water fluxes control solute trajectories and residence time distributions and therefore, the effectiveness of contaminant plume dilution and mixing. Our results show how external forcing configurations are capable of inducing distinct time‐varying groundwater flow patterns which will yield different solute dilution rates.

Mixing-Limited Reactions in Porous Media

“…For example, temporal oscillations in the mean flow direction will enhance mixing (Cirpka et al 2015;Lester et al 2010). Such fluctuations can occur in undeveloped groundwater aquifers, since there are often variations in flow direction driven by seasonal hydrologic events (e.g., rainfall, snow) (Rehfeldt and Gelhar 1992;Goode and Konikow 1990;Pool and Dentz 2018;Di Dato et al 2018). From an engineering and remediation perspective, complex spatial and temporal variations in flow can be imposed by fluid injection/extraction (e.g., through pumping wells in a groundwater system), thereby dramatically enhancing mixing among reactants initially segregated in different fluid regions (Piscopo et al 2015;Libera et al 2017).…”

Dispersion in doublet-type flows through highly anisotropic porous formations

“…In this case, one has to maximize the dispersion to enhance reactions (Di Dato et al. 2018), and therefore the design accounting for the lower bound certainly provides another conservative estimate of the efficiency of the procedure.…”

Effective Chemical Delivery Through Multi‐Screen Wells to Enhance Mixing and Reaction of Solute Plumes in Porous Media

“…This result can also be realized by the heterogeneity and anisotropy of the porous media but as a passive process. In groundwater in‐situ remediation, there are several active techniques that are proposed to enhance mixing, which relies on chaotic advection and its resulting spreading (e.g., Di Dato et al., 2018; Trefry et al., 2012; Zhang et al., 2009). One prominent technique is the application of recirculation wells, such as the pulsed dipole method, which utilizing one or more pairs of discharging and recharging wells (e.g., Burbery et al., 2013; Gandhi et al., 2002; Luo & Kitanidis, 2004; Luo et al., 2006, 2007).…”

“…However, the practical application of the circulation wells receives its limitations due to recharging contaminated water into the aquifer and the mixing and reaction of treatment solutions and contaminated water in pumps and wells (e.g., Li et al, 2010Li et al, , 2009Williams et al, 2011). To overcome these restrictions, an engineered injection and extraction strategy of clean water surrounding the contaminated plume and the treatment solution was proposed, and mixing and reaction enhancement has been evidenced by generating more extensive spreading of the plumes (Di Dato et al, 2018;Mays & Neupauer, 2012;Neupauer et al, 2014;Piscopo et al, 2013Piscopo et al, , 2015. This technique also requires extra energy (i.e., pumping), a pumping plan design, and manual operation to guarantee successful groundwater remediation.…”