Groundwater flow patterns in a coastal fen exposed to drainage, rewetting and interaction with the Baltic Sea

Low-lying coastal peatlands are unique ecosystems where water levels and salt input with seawater determine vegetation and greenhouse gas emissions, while the export of nutrients and carbon via submarine groundwater discharge influences biogeochemical processes in shallow coastal sediments. Anthropogenic interferences like drainage and diking strongly impact the water flow. This study aimed at understanding the hydrological system in a rewetted coastal fen typical for the southern Baltic Sea. Groundwater observation wells were installed both in the peat layer and underlying sands, and continuous readings of water levels and specific conductivities were combined with groundwater dating and major ion composition analyses. Apparent tritium-helium groundwater ages between 7 and more than 60 years in the shallow aquifer below the peat revealed a long memory of the system. Recharge through the peat layer was identified as a major source of groundwater from neon and helium isotopes in combination with water levels. Chloride concentrations showed a homogeneous fraction of 30 % sea water throughout the aquifer despite differing groundwater ages. Drainage ditches still affect groundwater flow today depending on the peat and aquifer thickness. Both nutrient-enriched, brackish, deeper groundwater and freshwater from the precipitation-fed dune dike discharge to the sea. We conclude that rewetting of coastal peatlands mainly affects the groundwater in the upper peat, with little impact on the underlying aquifer, and might revive submarine groundwater discharge and associated nutrient fluxes despite low hydraulic gradients.