Mapping and quantifying groundwater fluxes and origin at the land-sea interface using temperature, chloride and stable isotopes: An example from Königshafen, Sylt

Abstract

<p>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 catchment morphology, variable density flow caused by saline sea water lying above and below the fresh terrestrial groundwater. Thus tracing water and nutrients fluxes through the subterranean estuary is not trivial. In this work we use a combination of temperature measurements and heat modelling to estimate water fluxes through the subterranean estuary and pore water stable isotopes and chemistry to infer the origin of this discharge in the Königshafen, on Sylt Island, Northern Germany. The results show that flow paths are complex, with dune morphology influencing the focal point for fresh groundwater discharge. Seaward, saline and brackish discharge occurs into the tidal creek draining the bay. The complexity of heat transport modelling in the coastal zone depends 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 combined with pore water chemistry show potential to understand water and chemical exchange through the subterranean estuary and thus help to understand water and material fluxes at the terrestrial-ocean interface.</p>