Geological and groundwater flow model of a submarine groundwater discharge site at Hanko (Finland), northern Baltic Sea

AbstractThree-dimensional geological and groundwater flow models of a submarine groundwater discharge (SGD) site at Hanko (Finland), in the northern Baltic Sea, have been developed to provide a geological framework and a tool for the estimation of SGD rates into the coastal sea. The dataset used consists of gravimetric, ground-penetrating radar and shallow seismic surveys, drill logs, groundwater level monitoring data, field observations, and a LiDAR digital elevation model. The geological model is constrained by the local geometry of late Pleistocene and Holocene deposits, including till, glacial coarse-grained and fine-grained sediments, post-glacial mud, and coarse-grained littoral and aeolian deposits. The coarse-grained aquifer sediments form a shallow shore platform that extends approximately 100–250 m offshore, where the unit slopes steeply seawards and becomes covered by glacial and post-glacial muds. Groundwater flow preferentially takes place in channel-fill outwash coarse-grained sediments and sand and gravel interbeds that provide conduits of higher hydraulic conductivity, and have led to the formation of pockmarks on the seafloor in areas of thin or absent mud cover. The groundwater flow model estimated the average SGD rate per square meter of the seafloor at 0.22 cm day−1 in autumn 2017. The average SGD rate increased to 0.28 cm day−1 as a response to an approximately 30% increase in recharge in spring 2020. Sensitivity analysis shows that recharge has a larger influence on SGD rate compared with aquifer hydraulic conductivity and the seafloor conductance. An increase in recharge in this region will cause more SGD into the Baltic Sea.

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Submarine Groundwater Discharge From Non-Tidal Coastal Peatlands Along the Baltic Sea

Frontiers in Earth Science ◽

10.3389/feart.2021.665802 ◽

2021 ◽

Vol 9 ◽

Author(s):

Erwin Don Racasa ◽

Bernd Lennartz ◽

Miriam Toro ◽

Manon Janssen

Keyword(s):

Hydraulic Conductivity ◽

Baltic Sea ◽

Submarine Groundwater Discharge ◽

Groundwater Discharge ◽

Water Levels ◽

Peat Layer ◽

Discharge Region ◽

The Baltic Sea ◽

Submarine Groundwater ◽

The Baltic

Submarine groundwater discharge (SGD) is an important pathway for water and materials within the land-ocean transition zone that can impact coastal environments and marine life. Although research from sandy shorelines has rapidly advanced in recent years, there is very little understanding of coastal areas characterized by a low hydraulic conductivity, such as carbon-rich coastal peatlands. The objective of this study was to determine the magnitude and location of terrestrial SGD to be expected from a non-tidal low-lying coastal peatland located along the Baltic Sea and to understand the controlling factors using numerical modeling. We employed the HYDRUS-2D modeling package to simulate water movement under steady-state conditions in a transect that extends from the dune dike-separated rewetted fen to the shallow sea. Soil physical properties, hydraulic gradients, geological stratifications, and topography were varied to depict the range of properties encountered in coastal peatlands. Our results show that terrestrial SGD occurs at the study site at a flux of 0.080 m2 d−1, with seepage rates of 1.05 cm d−1 (upper discharge region) and 0.16 cm d−1 (lower discharge region above submerged peat layer). These calculated seepage rates compare to observations from other wetland environments and SGD sites in the Baltic Sea. The groundwater originates mainly from the dune dike—recharged by precipitation and infiltration from ponded peatland surface water—and to a lesser extent from the sand aquifer. The scenario simulations yielded a range of potential SGD fluxes of 0.008–0.293 m2 d−1. They revealed that the location of terrestrial SGD is determined by the barrier function of the peat layer extending under the sea. However, it has little impact on volume flux as most SGD occurs near the shoreline. Magnitude of SGD is mainly driven by hydraulic gradient and the hydraulic conductivity of peat and beach/dune sands. Anisotropy in the horizontal direction, aquifer and peat thickness, and peatland elevation have little impacts on SGD. We conclude that SGD is most probable from coastal peatlands with high water levels, large Ks and/or a dune dike or belt, which could be an essential source for carbon and other materials via the SGD pathway.

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Submarine groundwater discharge from coastal peatlands of northeast Germany

10.5194/egusphere-egu21-12071 ◽

2021 ◽

Author(s):

Erwin Don Racasa ◽

Bernd Lennartz ◽

Miriam Ibenthal ◽

Manon Janssen

Keyword(s):

Hydraulic Conductivity ◽

Submarine Groundwater Discharge ◽

Groundwater Discharge ◽

Volume Flux ◽

Site Analysis ◽

Submarine Groundwater ◽

Important Pathway ◽

Mineral Soils ◽

Set Up ◽

The Baltic

Submarine groundwater discharge (SGD) is an important pathway for water and compounds within the land-ocean transition zone that can impact coastal environments and marine life. Although SGD research from sandy shorelines has rapidly advanced in recent years, there is very little understanding of coastal areas dominated by coastal peatlands, where the prevailing soils are characterized by a low hydraulic conductivity. Peatlands, the world&#8217;s most efficient carbon storage, could be a potential source of carbon, nutrients, and trace metals via the SGD pathway. The objective of this study was to determine the magnitude and location of SGD in a coastal peatland in northeast Germany. We wanted to understand the factors controlling terrestrial SGD from coastal peatlands through numerical modelling employing the HYDRUS-2D modeling package. Steady-state scenarios were simulated based on soil physical properties, hydraulic heads, and geological stratifications and structure. In the model set-up, emphasis was laid upon peat layers extending from land into the sea. Our results show that terrestrial SGD occurs at a net discharge volume flux of 0.0803 m3 m-1 d-1 with seepage rates of 1.05 cm d-1 near the shore and 0.16 cm d-1 at a second discharge region above the submerged peat layer. Calculated seepage rates compare to observations from other SGD sites in the Baltic Sea region and other wetland environments. The upscaled SGD estimate for the 3-km coastal peatland is 240 m3 d-1, which is in correspondence to earlier estimates from the same site. Analysis of the model output reveals that magnitude and location of terrestrial SGD are mainly driven by the magnitude of hydraulic gradient and the hydraulic conductivity of both peat and mineral soils. Additional influencing factors are peat anisotropy, thickness of aquifer sands and peat layers, and peat elevation. Submerged peat layers extending into the sea can restrict SGD flow in deeper discharge regions but may be less critical in terms of volume flux as most SGD occurs near the shoreline. We conclude that coastal peatlands could be an essential source of carbon, nutrients, and other compounds via SGD and may influence local geochemistry budgets and marine ecosystems.

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