Isotope biogeochemical dynamics of pore waters under impact by submarine ground water discharge after removal of coastal protection

Seasonal isotope biogeochemical dynamics of coastal pore waters adjacent to Hütelmoor peatland, Germany were investigated for coastal changes influence. SGD influenced the sediments at maximum discharge zone 2-3 mbsf. Groyne removal may impact the surface sediment transport and pore water dynamics. These influence the sediment-water interface, the biogeochemical processes intensity and kinetics and DIC production. Overall the hydrology of freshwater intrusion below from adjacent peatland area and brackish water intrusion from offshore may be impacted by the hydrographic regime. Collected pore waters from two permanent lances before and after groyne removal were measured for physical in-situ parameters, major, nutrient, trace elements, stable water isotopes and C isotope composition of DOC and DIC, and dissolved sulfate and sulfide S and O isotope composition. Results showed mixture of at least two different water sources, dominant DOC source and DIC sources and sinks. Both brackish and freshwater signatures were indicated in the water stable isotopes. A coupled ẟ¹³C-DIC evaluation show DIC from seawater-derived DIC, carbonate dissolution, DOC mineralization and soil CO₂. ẟ¹³C-DOC evaluation indicates terrestrial and marine organic matter mixture. Metal and major ion concentrations demonstrate system stability and reestablishment though long-term changes may still occur. This study shows anthropogenic activity influences on coastal water biogeochemistry and hydrological water cycle.