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Submarine Groundwater Discharge

Submarine groundwater discharge (SGD), i.e. any subsurface flow of water out across the shoreline is now recognized as a significant source of material input to the ocean influencing the coastal environment and the ocean’s chemical budget.

There are mainly three types of SGD (Fig. 1): a) freshwater discharge b) discharge associated with recirculation of seawater; and c) a mixture of both (a) and b)). Our knowledge on SGD is very limited.

Fig. 1 Schematic description of SGD showing the flow of freshwater and recirculated seawater to the sea; because SGD has high dissolved solid content it effects the coastal environment.

Estimates on the fluxes of SGD to the ocean vary considerably between 6 % of the river water flux on a global base and up to 80-160% of the river flux if only the Atlantic Ocean is considered. It’s not the volume which makes SGD important source but the relatively high dissolved matter content (e. g. nutrients, heavy metals, organic pollutants), thus even a small SGD flux may have considerable effects on the coastal environment. For instance, outbreaks of harmful algae blooms have been attributed to the nutrient supply associated with SGD. On the other hand, freshwater submarine groundwater discharges may be an important freshwater source for area where freshwater resources are scarce like e.g. islands and arid regions.

Fig. 2 Locations where submarine groundwater discharges have been reported (based on literature data up to 2007). The release of water occurs as focused flow where submarine springs are visible at the sea surface (see inlet) or as dispersed flow through porous sediments.(M. SCHLUTER - pers. comm.)

Nuclear techniques play a key role in the detection and quantification of SGD because these waters are tagged with the several isotopes (e.g. 222Rn, 223Ra, 224R, 228Ra, 226Ra, d18O, d2H). In a joint UNESCO-IAEA Coordinated Research Project different methods for investigating SGD have been compared and investigated (see IAEA-TECDOC-1595 “Nuclear and isotopic techniques for the characterization of submarine groundwater discharge in coastal zones”). As one result nuclear techniques are now well established in SGD studies. MEL further assists Member States by producing reference material for short-lived radium isotopes and conducting proficiency test radium isotopes. Future MEL activities will concentrate on the ecological consequences of SGD for the coastal environment.