Mapping and quantifying water fluxes at the land-sea interface using temperature, stable isotopes and pore water chemistry: An example from Königshafen, Sylt

Beach faces form the interface between terrestrial and marine systems. They act as a reactive zone between these two compartments, transporting and biogeochemically modifying chemical constituents such as nutrients, pollutants and carbon. 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 and very dynamic boundary conditions across temporal scales (e.g. tides, storms, yearly variations in terrestrial groundwater levels). Thus tracing water and nutrients fluxes through the subterranean estuary is not trivial. In this work we use a combination of point and long-term (7 months) temperature profile measurements and heat modelling to estimate water fluxes through the beach sediments into the Königshafen, on Sylt Island, Northern Germany. Temperature measurements were complemented by stable isotope and pore water chemistry measurements to infer the origin of discharge into the bay. The results showed that flow paths are complex, with dune morphology influencing the focal point for fresh groundwater discharge, with fluxes up to 20 cm d^-1. Moreover it appears that either the islands fresh groundwater isotopic signature is either variable or at least two end-members contribute to the freshwater signature. Seaward, saline and brackish discharge occurs into the tidal creek draining the bay. 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.