The role of eddies in solute transport and recovery in rock fractures: Implication for groundwater remediation

Abstract

A better understanding of solute transport and retention mechanism in rock fractures has been challenging due to difficulty in their direct observations in microscale rough‐walled fractures. Six representative troughs in a rough‐walled fracture were selected for microscale observations of eddy formation with increasing flow velocity and its effect on spatiotemporal changes of solute concentration. This experimental study was enabled by a microscale visualization technique of micro particle image velocimetry. With increasing flow velocity (Re ≤ 2.86), no eddies were generated, and solutes along the main streamlines transported rapidly, whereas those near the wall moved slowly. A larger amount of solutes remained trapped at all troughs at Re = 2.86 than Re < 1. For Re = 8.57, weak eddies started to be developed at the troughs on the lee side, which little contributed to overall solute flushing in the fracture. Accordingly, a large of amount of water was needed for solute flushing. The flow condition of 1 < Re < 10, before a full development of eddies, was least favourable in terms of time and amount of remediation fluid required to reach a target concentration. After large eddies were fully developed at troughs on the lee side for Re = 17.13, solutes were substantially reduced by eddies with less amount of water. Fully developed eddies were found to enhance solute transport and recovery, as opposed to a general consensus that eddies trap and delay solutes. Direct inflow into troughs on the stoss side also made a great contribution to solute flushing out of the troughs. This study indicates that fully developed eddies or strong inflows at troughs are highly possible to form for Re > 10 and this flow range could be favourable for efficient remediation.

Geometry‐Based Prediction of Solute Transport Process in Single 3D Rock Fractures Under Laminar Flow Regime

“…Fractures occur ubiquitously in the earth's crust at multiple scales spanning from microscopic to continental, due to tectonic process, lithostatic stress, geothermal loading, or geofluid pressure. These fractures play a crucial role in subsurface substance migration, and their transport properties are thus relevant to many natural geophysical processes and engineering activities, such as mineral precipitation, carbon and nitrogen cycling, induced seismic activity, groundwater contamination remediation, and oil/gas resource and geothermal energy for exploitation (Bhattacharya & Viesca, 2019; Birdsell et al., 2015; Ge & Saar, 2022; Hyman et al., 2019; Jones & Detwiler, 2019; Kang et al., 2015; Lee et al., 2017; Vasseur & Wadsworth, 2019; Wang et al., 2020; Zheng et al., 2019). Due to geometric heterogeneity of rock fracture and complexity of flow regime (Lee et al., 2015; Stoll et al., 2019), it is not easy to characterize solute transport in rock fractures, let alone predict this process.…”

Distributed order Hausdorff derivative diffusion model to characterize non-Fickian diffusion in porous media

“…The time fractional diffusion equation for the propagator u(x,t) [9], i.e., the 6 probability of finding the solute particle at position x at time t,…”

“…Contaminant transport in porous media is one of the most important topics in environmental fluid mechanics [1], such as radionuclide transport through fractured rock [2], seawater invasion in karstic [3], coastal aquifers [4] and contaminant migrates at waste landfill site on fractured porous media [5]. Understanding the solute transport can help to control the contaminant diffusion, remediate polluted water and provide ways to sustainably use the natural resources [6]. It is well known that diffusion of solute particles in free space is Fickian diffusion and satisfies the popular Einstein relationship [7], i.e., 2 ( ) x t t < > , where 2 ( ) x t < > is the mean squared displacement (MSD).…”

Numerical model of the behavior of chlorinated ethenes in a fractured, karstic limestone aquifer

“…Connected fractures act as conduits for groundwater, whereas matrix diffusion and stagnant zones, due to the presence of dead-end channels and irregularities of fracture walls, trap the contaminated mass. This is later released with the result of anomalous discharge behavior characterized by an early arrival and long tailing (Lee et al 2017;Dou et al 2018). Non-Fickian behaviour has been associated with mass diffusion between the fracture and matrix (Zhou et al 2006), the channeling effect due to variable apertures (Tsang and Tsang 1987), and stagnation zones near fracture walls (Cardenas et al 2007).…”