Fluid mixing and solute transport in transient subsurface flow

The transport of solutes in groundwater systems is central to a wide array of physical, chemical, geological and biological processes that impact the quality of this critical water resource. Whilst research efforts over the past century have focused on understanding and quantifying solute transport in steady groundwater flows, transport in transiently forced systems has received significantly less attention. The additional degree of freedom associated with these transient flows admits a much richer set of possible transport dynamics, with significant implications for the above fluid-borne processes. This thesis aims to uncover the complex transport dynamics that arise in transiently forced aquifer systems, the mechanisms that govern these dynamics, and the relationships between the aquifer transport structure, forcing parameters and physical properties of the porous medium.

A combination of numerical modelling and dynamical systems theory was used to investigate this phenomenon, where a conventional linear groundwater flow model subject to tidal forcing was developed, and a group of governing dimensionless parameters were identified to quantify transport dynamics. Simulations were performed over the dimensionless parameter space, and specialized mathematical techniques were used to visualize and classify the Lagrangian transport structure of the flow and elucidate the impacts on solute transport.

It was found that under certain conditions, steady Darcy flows generate complex transport and mixing phenomena near the tidal boundary. Aquifer heterogeneity, storativity, and forcing magnitude cause flow reversals which generate closed flow regions and chaotic mixing. These features significantly augment fluid mixing and transport, leading to anomalous residence time distributions, flow segregation, accelerated mixing and the potential for profoundly altered reaction kinetics. It also found that governing parameters control the complex transport dynamics and the bifurcations between transport structures topologies (i.e. open, closed or chaotic).

This study uncovers a wide range of complex transport dynamics in transiently forced aquifers that has profound implications for the understanding and quantification of solute transport in these important groundwater systems. And the results in this thesis predict that complex transport structures and dynamics arise in natural transiently forced aquifers around the world. When such dynamics do arise, it will lead to mixing and reactive transport patterns that are very different from those predicted by conventional Darcy flow models.