ALBERT

Description: 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.

Description: Sea surface radon-222 measurements were carried out onboard the research boat Gridi. The survey was run at 1.4 knots, with the boat position recorded by DGPS. Two pumps that continuously supplied water for the measurements were fixed at ca. 50 cm water depth with a Sea & Sun Technology CTD90M multiprobe that was recording pressure (depth), temperature and conductivity (salinity). The radon measurements were carried out using two identical systems in parallel, each equipped with a 3M MiniModule gas contractor that separates the dissolved gas from the continuously pumped water. The gas was dried with a Drierite gas-drying unit, and analysed with a Durridge RAD7 radon detector. The radon-in-water activity concentrations were calculated using the salinity and temperature dependent fractionation of radon-222 between air and water.

Description: Sediment samples were collected using a box corer, and cut in 1-cm sub-sample slices. The Cs-137 activity content of sub-samples was measured for 60 min using a BrightSpec bMCA-USB pulse height analyser coupled to a well-type NaI(Tl) detector. The same sub-samples were then analysed for water content and weight loss on ignition (LOI) by weighing sub-samples fresh, weighing after drying at 105 °C for 16 h, and weighing again after ignition at 550 °C for 2 h.

Description: Water column profiles were measured at the locations of pockmarks B, D and E, using a Sea & Sun CTD90M multiprobe. The sensors for pressure, temperature, conductivity and optical dissolved oxygen in the multiprobe were manufactured by Sea & Sun Technology GmbH. The turbidity sensor was manufactured by Seapoint Sensors Inc., and the ISM-2001C inductive 2D current meter with compass by HS Engineers GmbH.