Abstract
Three-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.
Highlights
Submarine groundwater discharge (SGD) is the flow of groundwater from the seafloor to the coastal sea (e.g. Burnett et al 2003; Moore 2010)
Data produced during this study consist of the following—12 km of groundpenetrating radar (GPR) profiles acquired in October 2017; 12 km of shallow reflection seismic profiles from the offshore area acquired in May 2017; 0.9 km of refraction seismic survey along the shoreline acquired in April 2018; and 3 km of a walking survey of electrical conductivity (EC) and temperature measurements of near-bottom water in the shallow area along the shoreline carried out in June 2017
A three-dimensional geological model of the shallow coastal aquifer belonging to the First Salpausselkä ice marginal formation in the northern Baltic Sea was carried out in IsolähdeLappohja area, southern Finland
Summary
Submarine groundwater discharge (SGD) is the flow of groundwater from the seafloor to the coastal sea (e.g. Burnett et al 2003; Moore 2010). Submarine groundwater discharge (SGD) is the flow of groundwater from the seafloor to the coastal sea These groundwater areas are hydrologically complex with heterogeneous sedimentary cover including till, glacioaquatic gravel and sand, glaciolacustrine and post-glacial silt and clay, and in some areas, reworked littoral gravel, sand and clay (Saarnisto and Saarinen 2001; Donner 2010; Räsänen et al 2009). For many locations along the Hanko Peninsula, groundwater flow models imply fresh groundwater discharge from the SSI to the coastal Baltic Sea (Luoma and Okkonen 2014). The significance of SGD and associated fluxes on the overall eutrophic environmental state of the Baltic Sea is poorly understood
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