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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/hess-2018-507
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2018-507
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 30 Oct 2018

Research article | 30 Oct 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

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

Joonas J. Virtasalo1, Jan F. Schröder2, Samrit Luoma3, Juha Majaniemi3, Juha Mursu4, and Jan Scholten2 Joonas J. Virtasalo et al.
  • 1Marine Geology, Geological Survey of Finland (GTK), Espoo, FI-02150, Finland
  • 2Institut für Geowissenschaften, Christian-Albrechts-Universität Kiel, Kiel, D-24118, Germany
  • 3Groundwater, Geological Survey of Finland (GTK), Espoo, FI-02150, Finland
  • 4Applied Geophysics, Geological Survey of Finland (GTK), Kuopio, FI-70211, Finland

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 100–250m seaward sloping gently to ca. 4m water depth, and a steep slope to ca. 17m water depth within ca. 50m 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 1m 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.

Joonas J. Virtasalo et al.
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Joonas J. Virtasalo et al.
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Short summary
Submarine groundwater discharge (SGD) is a significant source of nutrients and other potentially harmful substances to coastal sea. We analyse a rich dataset of offshore seismic sub-bottom profiles, multibeam and sidescan sonar images of seafloor, and onshore ground-penetrating radar profiles to establish the geometry of an SGD site in south Finland. The SGD takes place through meter scale pits (pockmarks) on the seafloor, confirmed by elevated radon concentrations in the overlying water.
Submarine groundwater discharge (SGD) is a significant source of nutrients and other potentially...
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