Abstract
Terrestrial and marine environments merge at the land-sea transition zone. This zone is important as ~38% of the world’s population live by and depend on the coastal regions, and oceans are considerably affected by it. Furthermore, terrestrial and marine groundwater and seawater mix in the subterranean estuary (STE), where submarine groundwater discharge (SGD), i.e., discharging fresh groundwater and recirculated seawater, results in significant solute fluxes to the sea. With this article, we focus on advances of geochemical, microbiological, and technological aspects related to fresh groundwater, SGD, and STE in sandy coastal areas, using the barrier island Spiekeroog as a case study area. Previous studies showed that the fresh groundwater composition in sandy coastal aquifers is governed by calcareous shell dissolution, cation exchange, and organic matter degradation. Biogeochemical reactions in the STE further modify the water composition of SGD, with a dependence on residence time. Microbial communities, which are present in coastal sediments and usually follow salinity and redox gradients, are the driver for the degradation of organic matter. Regarding organic matter sources in the STE, it is evident that dissolved organic matter is primarily of marine origin and that SGD delivers degraded dissolved organic matter back into the ocean. Furthermore, recent studies used radiotracers, such as radium and radon, and seepage meters as reliable tools to quantify rates and fluxes associated with SGD. We conclude that, despite the advances being made, the complexity and interactions of the different processes at land-sea transition zones require multidisciplinary scientific approaches.
Highlights
The coastal area is the interface between terrestrial and marine environments. From both environmental andeconomic perspectives (Barbier et al 2011), these are areas of enormous relevance, since ~38% of the world’s population lives within 100 km distance from the coastline (UNEP 2014). The importance of this landsea transition zone arises from a number of processes: (i) Land-sea interactions significantly affect global ocean material inventories due to export of terrestrially derived compounds to the ocean (Jeandel and Oelkers 2015); (ii) extensive land use and growing populations in coastal areas have a high impact on coastal ecosystems and aquifers, for example, high population densities in coastal areas present a risk for fresh
It is expected that rising sea levels will increase land loss, storm intensities, floodings, and saltwater intrusion in coastal areas (e.g., McGranahan et al 2007; Werner and Simmons 2009; Nicholls and Cazenave 2010)
The great importance of submarine groundwater discharge (SGD) has become widely accepted with emerging applications of radiotracer balances, and it could be demonstrated that SGD contributes significant fluxes of nutrients (Hays and Ullman 2007; Tait et al 2014) and metals (Basu et al 2001; Windom et al 2006) to coastal oceans
Summary
The coastal area is the interface between terrestrial and marine environments. From both environmental and (socio-)economic perspectives (Barbier et al 2011), these are areas of enormous relevance, since ~38% of the world’s population lives within 100 km distance from the coastline (UNEP 2014). Spiekeroog Island presents an excellent study site for the investigation of land-sea transition zones, SGD, and chemical reactions in sandy coastal STE. As the freshwater composition is critical for drinking water supply in coastal areas and for the chemical processes happening in the STE, the evolution of coastal freshwater aquifers is reviewed, using freshwater lenses below (barrier) islands as an example. As biological processes often are kinetically driven, water infiltration rates and residence times are of great relevance for the chemical composition of pore waters and constituent transport in the STE and, for the efficiency of the biogeochemical reactor (Anschutz et al 2009; Tamborski et al 2017). New methods based on the state-ofthe-art sensor technologies can produce continuous datasets, which allow for detailed insights into physicochemical processes at the land-sea transition zone and provide a basis for modeling approaches, as examined in Sect.
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