Submarine groundwater discharge site in the First Salpausselkä ice-marginal formation, south 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 100–250 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.

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

10.5194/hess-2018-507 ◽

2018 ◽

Author(s):

Joonas J. Virtasalo ◽

Jan F. Schröder ◽

Samrit Luoma ◽

Juha Majaniemi ◽

Juha Mursu ◽

...

Keyword(s):

Water Depth ◽

Submarine Groundwater Discharge ◽

Groundwater Discharge ◽

Sidescan Sonar ◽

Seismic Profiles ◽

Shore Platform ◽

Sand Surface ◽

Refraction Seismic ◽

Submarine Groundwater ◽

Sonar Images

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.

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Cost-effective recreational-grade single beam echosounder with side scan sonar system in imaging bubbly coastal submarine groundwater discharge

10.5194/egusphere-egu2020-407 ◽

2020 ◽

Author(s):

Mary Rose Gabuyo ◽

Fernando Siringan ◽

Keanu Jershon Sarmiento ◽

Paul Caesar Flores

Keyword(s):

Submarine Groundwater Discharge ◽

Groundwater Discharge ◽

Terrestrial Ecosystems ◽

Good Alternative ◽

Point Sources ◽

Side Scan Sonar ◽

Single Beam ◽

Sonar System ◽

Submarine Groundwater ◽

Research Grade

<p>Submarine groundwater discharge (SGD) is any direct flow of fluid across the seafloor, which forms bubbly or leaky springs and seeps from the intertidal zone to the deep sea. SGDs can significantly alter physico-chemical conditions of seepage zones. Identifying and mapping SGD is crucial to further recognize its influence in both marine and terrestrial ecosystems. However, mapping this phenomenon has been a continuing challenge, mainly due to the difficulty in its detection and quantification. This study explores the capability and applicability of an inexpensive, commercially available, recreational-grade combination of depth meter and side scan sonar system to image different types and identify point sources of coastal SGDs. Standard and systematic methodologies for efficient imaging and processing were established. The utility of the recreational-grade system was assessed and validated using a research-grade side scan sonar. SCUBA diving and CTD casting were conducted for ground-truthing and further characterization. Lower frequency sonars (83/200 kHz) showed more distinct acoustic signatures of discrete and dispersed bubbly SGDs, than the higher frequency system (455 kHz and 780 kHz research-grade unit). Sonar images showed that SGD plumes can be indicated by near seafloor to midwater cloud-like features. Spring-type SGDs tend to form cloud features with a funnel-shaped morphology. In sites where SGDs are dispersed, the acoustic signature is a curtain-like cloud, with higher bubble density in the upper water column. This is consistent with diver-based observation of increasing bubble sizes (<1 mm to ~30 mm) from point source to water surface. CTD casts indicate that the SGDs have recirculated seawater, with increasing temperature and salinity with depth. In the assessment of system and data processing requirements, and costing, a recreational-grade unit provides a good alternative for coastal SGD works.</p>

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