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.