Dynamic model of salinity, temperature and redox zoning in a subterranean estuary (STE)

This is a two-dimensional cross-sectional model that simulates the density-dependent groundwater flow and reactive transport processes along a cross-shore transect below a high-energy, meso-tidal beach on Spiekeroog Island, North Germany. The hydrological and geomorphological conditions are characterized by periodic stormfloods and strong morphodynamics.

The simulation results suggest that these highly transient boundary conditions control the flow and salt transport processes in the so-called subterranean estuary (STE) down to more than 30 meters below the beach surface (upper panel). Fresh terrestrial groundwater (blue) from the island's freshwater lens enters the subterranean estuary from the left. Seawater (red) recirculates due to tides, storm floods and density gradients inside the STE. The entire system appears pretty dynamic. More details on the flow and salt transport model is given in Greskowiak and Massmann (2021, Hydrological Processes, https://doi.org/10.1002/hyp.14050 ).

The superposition of high seawater infiltration rates and strong seasonal variations of the sea water's temperature seem to have strong affect on the subsurface water temperatures (middle panel). In turn those may control the reaction kinetics of microbially-mediated redox-reactions, such that the oxic (aerobic respiration), suboxic (nitrate reduction), anoxic (iron reduction) and sulfidic (sulfate reduction) zones constantly change their locations and spatial extents in space and time (lower panel).

This is only a generic model with no verification from real data. The model will be extended and calibarated within the DFG-research unit DynaDeep aiming to explore the deep dynamic subsurface of high energy beaches.