  Although steady, isotropic Darcy flows are inherently laminar and non-mixing,
it is well understood that transient forcing via engineered pumping schemes can
induce rapid, chaotic mixing in groundwater. In this study we explore the
propensity for such mixing to arise in natural groundwater systems subject to
cyclical forcings, e.g. tidal or seasonal influences. Using a conventional
linear groundwater flow model subject to tidal forcing, we show that under
certain conditions these flows generate Lagrangian transport and mixing
phenomena (chaotic advection) near the tidal boundary. We show that aquifer
heterogeneity, storativity, and forcing magnitude cause reversals in flow
direction over the forcing cycle which, in turn, generate coherent Lagrangian
structures and chaos. These features significantly augment fluid mixing and
transport, leading to anomalous residence time distributions, flow segregation,
and the potential for profoundly altered reaction kinetics. We define the
dimensionless parameter groups which govern this phenomenon and explore these
groups in connection with a set of well-characterised coastal systems. The
potential for Lagrangian chaos to be present near discharge boundaries must be
recognized and assessed in field studies.
