Mixing along the salt-freshwater interface is critical for geochemical reactions, transport and transformation of nutrients and contaminants in coastal ecosystems. However, the mechanisms and controls of mixing are not well understood. We develop an analytical model, based on the coupling between flow deformation and dispersion, that predicts the mixing dynamics along the interface for steady state flow in coastal aquifers. The analytical predictions are compared with the results of detailed numerical simulations, which show that non-uniform flow fields, inherent to seawater intrusion in coastal aquifer, result in a non-monotonic evolution of mixing width and mixing rates along the interface. The analytical model accurately captures these dynamics over a range of freshwater flow rates and dispersivities. It predicts the evolution of the mixing width and mixing rates along the interface, offering a new framework for understanding and modeling mixing and reaction processes in coastal aquifers.

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 analyse 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 on observed dissolution and mixing patterns, which sheds some new light on Karst propagation in coastal aquifers.