Quantifying dynamic water fluxes and origin at the land-sea interface from days to weeks using temperature and stable isotopes: 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. Mixing between saline seawater and fresh terrestrial groundwater in the subsurface 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, especially when attempting to quantify temporal dynamics on time scales from days to weeks. In this work we use long-term (months) temperature profile measurements and numerical heat modelling to investigate the dynamics of water fluxes through the beach sediments into the Königshafen Bay, Sylt Island, North Germany. Temperature measurements were complemented by stable isotope (δ¹⁸O, δ²H)  and pore water chemical measurements to infer the origin of water discharging into the bay. The results showed that the temporal fluxes vary considerable depending on season, location and catchment characteristics. The freshwater flow paths are complex, with dune morphology influencing the focal point for fresh groundwater discharge. Moreover, it appears that either the isotope signature of the islands fresh groundwater is variable or there are 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 the dynamics in water and element exchange through the subterranean estuary and thus help to understand local water and material fluxes and transformations at the land-ocean interface.