Heterogeneity-induced mixing and reaction hotspots facilitate Karst propagation in coastal aquifers

Abstract

The freshwater-seawater mixing zone is a critical region for chemical activity. Yet little is known about the influence of ever present spatial heterogeneity on the dynamics of mixing and calcite dissolution, which play a key role in the understanding of karst development. We analyze the impact of different heterogeneity structures and strengths on the local and global response of mixing and dissolution rates across the saltwater freshwater mixing zone. We find that the initial heterogeneity structure significantly impacts observed dissolution and mixing patterns, which sheds some new light on karst propagation in coastal aquifers.

Dispersion Versus Diffusion in Mixing Fronts

“…Mixing fronts formed by miscible fluids influence a range of hydrological and biogeochemical processes (Dentz et al., 2011; Rolle & Le Borgne, 2019; Valocchi et al., 2019) including river‐groundwater exchanges (Bandopadhyay et al., 2018; Hester et al., 2017; Ziliotto et al., 2021), freshwater‐saltwater mixing in coastal areas (Abarca et al., 2007; De Vriendt, 2021), subsurface microbial processes (Bochet et al., 2020) and mixing in river confluences and estuaries (Bouchez et al., 2010; Prandle, 2009; Yuan et al., 2022). They are also present in many engineering applications, such as soil and groundwater remediation (Fu et al., 2014; Karadimitriou & Hassanizadeh, 2012; Wang et al., 2022), geological carbon sequestration (Szulczewski et al., 2012; Zoback & Gorelick, 2012), hydrogen storage (Lysyy et al., 2022; Tarkowski, 2019) and geothermal systems (Burté et al., 2019).…”

“…Such flow leads to the appearance of a stagnation point (a point of null velocity) and frequently arise in hydrological systems (Bresciani et al., 2019). This includes hyporheic zones where groundwater upwelling locally competes with flow recirculation produced by variations in the river bathymetry (Hester et al., 2017), fresh‐salt water interfaces (De Vriendt, 2021), density‐driven flows (Hidalgo et al., 2015). The velocity field close to a stagnation point is non‐uniform, with a constant velocity gradient magnitude close to the stagnation point.…”