Transient forcing in heterogeneous aquifers drives solute containment and chaotic mixing

Abstract

Transient forcing of flow in compressible porous media due to tidal fluctuations, recharge cycles, and fluid injection–withdrawal processes, is a key driver of solute dispersion and mixing in geological and engineered systems. The combination of periodic forcing, medium heterogeneity, and compressibility leads to intricate spatio-temporal flow, dispersion, and mixing patterns. We analyse these patterns using detailed numerical simulations based on a stochastic representation of the spatial medium heterogeneity. Solute dispersion is characterised by the interface length and width, and mixing by the dilution index and the distribution of concentration point values. Poincaré sections show how the interplay of heterogeneity and compressibility creates stable regions that inhibit the advancement and dispersion of the mixing interface, and stochastic regions of chaotic advection that enhance solute mixing. This means that spatial heterogeneity in combination with temporal forcing leads to the containment of solute, promoting mixing at the same time. Meanwhile, incorporating local dispersion weakens this confinement, whereas further enhancing mixing. These findings have implications for a diverse range of environmental and industrial applications, including seawater intrusion in coastal aquifers, groundwater remediation, and geological storage activities.