Temporal Fluctuations and Poroelasticity Can Generate Chaotic Advection in Natural Groundwater Systems

Abstract

Although steady, isotropic Darcy flows are inherently laminar and nonmixing in the absence of diffusion, it is well understood that transient forcing via engineered pumping schemes can induce rapid, chaotic mixing flows in groundwater. In this study we explore the propensity for such mixing to arise in natural groundwater systems subject to cyclical forcings, for example, tidal or seasonal influences. Using a conventional linear groundwater flow model subject to tidal forcing, we show that under certain conditions these flows generate Lagrangian transport and mixing phenomena (chaotic advection) near the tidal boundary. We show that aquifer heterogeneity, storativity, and forcing magnitude cause reversals in flow direction over the forcing cycle which, in turn, generate coherent Lagrangian structures and chaos. These features significantly augment fluid mixing and transport, leading to anomalous residence time distributions, flow segregation, and the potential for profoundly altered reaction kinetics. We define the dimensionless parameter groups which govern this phenomenon and explore these groups in connection with a set of well‐characterized tidal systems. The potential for Lagrangian chaos to be present near discharge boundaries must be recognized and assessed in field studies.

When Do Complex Transport Dynamics Arise in Natural Groundwater Systems?

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In a recent paper (Trefry et al, 2019, https://doi.org/10.1029/2018wr023864), we showed that the interplay of aquifer heterogeneity and poroelasticity can produce complex transport in tidally forced aquifers, with significant implications for solute transport, mixing, and reaction. However, what was unknown was how broadly these transport dynamics can arise in natural groundwater systems and how these dynamics depend upon the aquifer properties and tidal and regional flow characteristics.

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“…In this paper we will study advective flow in the linear groundwater model over the tidal flow parameter space , identify regions of anomalous transport and chaotic advection, and comment on how and χ together control these topological transitions and the underlying physical mechanisms. Chaotic advection arises when fluid particles undergo chaotic orbits, leading to exponential stretching of fluid elements with time, rapid mixing, and augmented transport (Ottino, ; Trefry et al, ). Table summarizes the various dimensionless dynamical and heterogeneity parameters and provides a brief physical motivation for each.…”

“…The authors wish to thank the anonymous reviewers for their helpful and constructive comments. The figures and all data contained in this manuscript can be generated from the codes stored at https://github.com/ danlester000/Tidal-mixing-code.git and the numerical methods employed by these codes are described in Trefry et al (2019).…”

Mixing Enhancement Mechanisms in Aquifers Affected by Hydropeaking: Insights From Flow‐Through Laboratory Experiments

“…At the Darcy scale, anisotropic (Chiogna et al., 2015; Ye et al., 2015a, 2018) and heterogeneous flow fields (e.g., de Barros et al., 2012; Dentz et al., 2016) enhance plume dilution and reactive mixing. Variability in the hydraulic conductivity fields (Rolle et al., 2009; Werth et al., 2006; Ye et al., 2015c), topographic features (Bandopadhyay et al., 2018), and transient conditions (e.g., de Dreuzy et al., 2012; Kahler & Kabala, 2016; Piscopo et al., 2013; Sposito, 2006; Trefry et al., 2019) lead to spatially variable flow fields.…”

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

“…(1990) and Hopkins and McCarty (1995). Nevertheless, mixing can be enhanced by the heterogeneity and anisotropy of porous media (Bauer et al., 2009; Bennett et al., 2017; Castro‐Alcala et al., 2012; de Barros et al., 2015; Di Dato et al., 2016) and chaotic advection (e.g., Jones, 1991; Lester, Dentz, & Le Borgne, 2016, Lester, Trefry, & Metcalfe, 2016; Ottino, 1990; Trefry et al., 2019).…”