Abstract
Abstract. 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 side-scan 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 side-scan 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 modelling and flux studies at the SGD site.
Highlights
Submarine groundwater discharge (SGD) is understood as the flow of groundwater, or quite often a mixture of groundwater and seawater, from the seabed to the coastal sea (Burnett et al, 2003; Moore, 2010)
The denser units SU1–SU3 are identified in the deeper-penetrating reflection seismic profiles (Fig. 3c, d), whereas the units SU4– SU7 of lower acoustic impedance are better visible in the compressed high-intensity radar pulse (CHIRP) profiles (Fig. 3a, b)
Allostratigraphical architecture and aquifer geometry have been established for a SGD site in the First Salpausselkä icemarginal formation on the Hanko Peninsula in Finland
Summary
Submarine groundwater discharge (SGD) is understood as the flow of groundwater, or quite often a mixture of groundwater and seawater, from the seabed to the coastal sea (Burnett et al, 2003; Moore, 2010). Moore, 2010; Szymczycha et al, 2012, 2016). SGD-associated fluxes potentially have considerable effects on the marine ecosystems. Eutrophication is a major concern for large parts of the Baltic Sea (Andersen et al, 2017). The main nutrientsupply routes to the Baltic Sea are well known, detailed studies of SGD and associated fluxes have been carried out only at a couple of locations along the south coast: the Eckernförde Bay in Germany
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