Hydrographic data, surface seawater radon-222 concentration, and loss on ignition and caesium-137 values of sediment samples from the Hanko submarine groundwater discharge site, northern Baltic Sea, Finland

Submarine groundwater discharge (SGD) has been implicated as a significant source of nutrients and potentially harmful substances to the coastal sea. Although the number of reported SGD sites has increased recently, their stratigraphical architecture and aquifer geometry are rarely investigated in detail. This study analyses a multifaceted dataset of offshore seismic sub-bottom profiles, multibeam and sidescan sonar images of the seafloor, radon measurements of seawater and groundwater, and onshore ground-penetrating radar and refraction seismic profiles in order to establish the detailed stratigraphical architecture of a high-latitude SGD site, which is connected to the Late-Pleistocene First Salpausselkä ice-marginal formation on the Hanko Peninsula in Finland. The studied location is characterized by a sandy beach, a sandy shore platform that extends 100-250 m seaward sloping gently to ca. 4 m water depth, and a steep slope to ca. 17 m water depth within ca. 50 m distance. The onshore radar and offshore seismic profiles are correlated based on unconformities, following the allostratigraphical approach. The aquifer is hosted in the distal sand-dominated part of a subaqueous ice-contact fan. It is interpreted that coarse sand interbeds and lenses in the distal fan deposits, and, potentially, sandy couplet layers in the overlying glaciolacustrine rhythmite, provide conduits for localized groundwater flow. The SGD takes place predominantly through pockmarks on the seafloor, which are documented on the shore platform slope by multibeam and sidescan sonar images. Elevated radon-222 activity concentrations measured 1 m above seafloor confirm SGD from two pockmarks in fine sand sediments, whereas there was no discharge from a third pockmark that was covered with a thin organic-rich mud layer. The thorough understanding of the local stratigraphy and the geometry and composition of the aquifer that have been acquired in this study are crucial for successful hydrogeological modeling and flux studies at the SGD site