  In a recent paper (Trefry et al., 2019) we showed that the interplay of
aquifer heterogeneity and poroelasticity can produce complex transport in
tidally forced aquifers, with significant implications for solute transport,
mixing and reaction. However, what was unknown was how broadly these transport
dynamics can arise in natural groundwater systems, and how these dynamics
depend upon the aquifer properties, tidal and regional flow characteristics. In
this study we answer these questions through parametric studies of these
governing properties. We uncover the mechanisms that govern complex transport
dynamics and the bifurcations between transport structures with changes in the
governing parameters, and we determine the propensity for complex dynamics to
occur in natural aquifer systems. These results clearly demonstrate that
complex transport structures and dynamics may arise in natural tidally forced
aquifers around the world, producing solute transport and mixing behaviour that
is very different to that of the conventional Darcy flow picture.
  Key Points:
  * Transient Darcy flows can generate complex transport dynamics in
heterogeneous compressible aquifers.
  * This complex transport can trap dispersing solutes for many years.
  * Global tidal maps indicate widespread potential for complex transport
dynamics in coastal zones.
