Spatio-temporal dynamics of groundwater biogeochemistry in the deep subsurface of high-energy beaches

Intertidal permeable high energy beach systems represent complex biogeochemical reactors which attract increasing scientific attention. In these environments morphology variations lead to complex and dynamic groundwater flow paths, saltwater-freshwater mixing zones, and changing biogeochemical conditions. The aim of our study was to assess the spatio-temporal dynamics in the hydrobiogeochemistry of the continuum between a deep subterranean estuary (STE) and the surface of a high-energy beach on Spiekeroog Island (Germany). Several permanent wells distributed along a cross-shore transect (supratidal to intertidal zone) allowed for regular groundwater sampling down to 24 m below ground surface (mbgs). Additional direct push sampling helped to obtain a high resolution cross-sectional view on the deep STE groundwater biogeochemistry. We found salinities below 10 near the dunes increasing to a salinity of about 30 towards the intertidal zone. Tide- and wave induced seawater circulation reached down to more than 24 mbgs. Oxygen and NO₃^- penetrated 12-15 mbgs deep, at least in the supratidal to upper intertidal area. Below and towards the low water line, conditions were Fe-(hydr)oxide-reducing and accumulating Fe sulfides indicated active microbial net sulfate reduction. At few sites, the concurrent presence of dissolved NO₃^- and Fe indicated overlapping redox zones. Deep old freshwater from Spiekeroog’s fresh groundwater lens mixed with the saline groundwater in the lower intertidal zone and added nutrients, especially Si, but lowered dissolved Mn and Fe concentrations. Accordingly, these parameters followed the temporally varying location of freshwater discharge at this site. Except for this most seaward well site, biogeochemical conditions were found to be relatively stable in the zone below 12 mbgs of the STE and more variable above. Temporal changes related to seasonally varying input and processing of organic material seemed to be restricted to the top few mbgs around the high-water line, where tide-induced infiltration regularly adds young seawater to the beach system. In the next step, the results will be analyzed by reactive-transport modeling to allow for a further general understanding and extrapolation of flow and reaction dynamics in the deep subsurface below high-energy beaches.