Mapping and quantifying water fluxes through the subterranean estuary using temperature: an example from a complex high energy beach face (Spiekeroog)

Benjamin Gilfedder1, Hannelore Waske2, Fabian Wismeth1, Sven Frei3

3.4 in Grundwasser-Oberflächenwasser-Interaktionen - vom Flussabschnitt zum Einzugsgebiet

Beach faces form the interface between terrestrial and marine systems. They act as a reactive zone between these two compartments, transporting and biogeochemically modifying terrestrially sourced chemical constituents such as nutrients, pollutants and carbon towards the sea. This interface is referred to as the subterranean estuary. The subterranean estuary is important on global scales for modifying and processing dissolved and particulate matter sourced from the sea as it cycles through the beach sediments. Re-circulation of sea water through beach sediments is largely driven by tidal pumping and pressure gradients caused by tides, wave setup, and storm events that pile sea water up on the beach face. In contrast, terrestrial groundwater systems provide a source of low salinity and often nutrient rich water to the coastal zone. Mixing between these water sources is complicated by variable density flow caused by saline sea water lying above and below the fresh terrestrial groundwater. Tracing water and nutrients fluxes through the subterranean estuary is not trivial however, with complex boundary conditions such as tides, storms and beach morphology influencing water flow paths and patterns in the beach subsurface. In this work we use temperature measurements and heat modelling to estimate water fluxes through the subterranean estuary on the island of Spiekeroog, Northern Germany. The site is highly complex, with high energy surf zone, a runnel system and a large (meso) tidal range. The results show that flow paths are complex, with the runnel system being a focal point for groundwater discharge, while landward of the runnel infiltration dominates. Seaward to the runnel changes between infiltration and exfiltration depending on beach morphology. The complexity of heat transport modelling in the coastal zone depending on the boundary conditions, with very complex boundaries requiring more complex modelling structures (e.g. steady state vs. non steady state). Overall temperature measurements and heat modelling show potential to understand water exchange through the subterranean estuary and thus help to understand water and material fluxes at the terrestrial-ocean interface.