Salt-Freshwater Dynamics in a Subterranean Estuary—Simulating the Impact of Tidal fluctuations and Engineering Controls on Contaminant Mass Flux

Flows in a subterranean estuary are driven by the inland hydraulic gradient, density variations between the fresh groundwater and seawater and oceanic forcing including tidal oscillations. The classic salt-wedge conceptual model accounts only for the density variations and is an idealized model of a dynamic complex phenomenon. The tidal forcing produces oscillating landward- and seaward-directed hydraulic gradients in the near-shore aquifer. This can result in the movement of large quantities of seawater across the aquifer–ocean interface relative to fresh groundwater discharge. Field investigations using depth discrete sampling in estuaries have produced a renewed understanding of these complex patterns. The tidal forcing across a sloping beach leads to the formation of an upper saline plume in the intertidal region in addition to the classic salt water wedge along with a freshwater discharge ‘‘tube’’, which pinches out near the low tide mark.

Numerical models of tidal forcings are relatively recent developments, and fully three-dimensional (3D) models for engineering applications have not been previously presented in literature. We present results of variable-density simulations using USGS’ model SEAWAT and demonstrate the development of upper saline plume, freshwater discharge tube and salt-wedge. Whereas previous modeling efforts have used an indirect approach for landward propagation of tidal fluctuations, we discuss a more efficient method that is also numerically stable. We present the development of a two-dimensional (2D) cross-sectional model, followed by a 3D model. We use particle tracking simulations to show the effect of low permeability barriers on the freshwater discharge tube.