A Multi Tracer Approach Reveals Groundwater Residence Times and Dynamic Endmember Mixing in the Subterranean Estuary of a High Energy Beach

Abstract Subterranean estuaries (STEs) under high‐energy conditions function as effective biogeochemical reactors, where waterbodies of varying physico‐chemical properties mix. Chemical reactions within the STE modulate the solute composition of groundwater discharging to the sea providing for example nutrients for plankton growth to coastal ecosystems. Groundwater residence times are fundamental to understanding groundwater flow and transport regimes, and thus to interpreting (bio) geochemical data. The present study aims to exemplarily show for the barrier island Spiekeroog exposed to high energy conditions, how morphological changes of the beach surface imprint on subsurface flow, groundwater age and mixing patterns. To this end, we have combined multiple environmental tracers, including electrical conductivity, apparent tritium‐helium groundwater ages, temperature ( T ) and dissolved silica (Si), obtained over a period of 1.5 years from different depths and locations along a 200 m long cross‐shore transect. The results of apparent groundwater ages, travel times and seawater mixing fractions allowed delineating zones where the different endmembers were prevailing. Recirculating seawater of the upper saline plume (USP), infiltrating near the high water line, was youngest with travel times of days to weeks. Near the dune base, freshwater entering the beach from the islands interior was several years old, subsequently forming the freshwater discharge tube below the USP, and directed toward the discharge zone where decade old brackish groundwater was encountered at depths below 18 m. The results support the interpretation of geochemical data as well as the underlying processes and advance the understanding of the functioning of the biogeochemical reactor in high energy beach systems.