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

•Flow pathways in rewetted coastal peatland still influenced by ditches.•Apparent tritium–helium groundwater ages reveal long memory of shallow aquifer.•30% of groundwater originates from infiltrated seawater, irrespective of age.•Potential for subterranean export of nutrients and dissolved carbon to Baltic Sea.