Submarine groundwater discharge site in the First Salpausselkä ice-marginal formation, south Finland

Abstract

Abstract. Submarine groundwater discharge (SGD) has been implicated as a significant source of nutrients and other potentially harmful substances to 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 reconstruct 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 an ice-contact subaqueous fan foreset. It is interpreted that gravel and coarse sand interbeds and lenses in the distal foreset, and, potentially, coarse 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 with fine sand surface sediment, 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.

Bedforms and sedimentary structures related to supercritical flows in glacigenic settings

“…Sand-rich fan deposits form gently (<15°) downflow dipping beds (Fig. 3) (Gorrell & Shaw, 1991;Hornung et al, 2007;Winsemann et al, 2007aWinsemann et al, , 2009Lang et al, 2017b;Virtasalo et al, 2019). Slopes of subaqueous ice-contact fans may become steeper in basins with high topographic gradients (Lønne, 1995) or where subaqueous ice-contact fans prograde in confined settings (Deschamps et al, 2013).…”

Impact of Over-exploitation in Coastal Groundwater on the Variations in Submarine Groundwater Discharge Rate in a Complex Two Aquifer System by Finite Element Modelling: A Case Study From South India

“…These models include the aquifer's nearshore terrestrial portion as well as the marine part below the seafloor, where the effects of density are significant. Detailed understanding of geometry and composition of the aquifer is necessary for successful characterization of an aquifer for SGD modelling (Virtasalo 2019). Fresh submarine groundwater was simulated by using variable-density flow and transport models for understanding complex groundwater flow processes in coastal environments (Langevin 2003) In coastal groundwater systems, Luijendijk et al (2020) simulated submarine and terrestrial groundwater discharge using a numerical model of combined density-driven groundwater flow and solvent transport that solved the equations of fluid flow and solute transport and the equations of state for fluid density and viscosity in a two-dimensional subsurface cross-section.…”