Study of Submarine Groundwater Discharge (SGD)
into the northwestern Mediterranean Sea
The objective is to study SGD along the French Mediterranean coastline. This includes sites that been known for a long time (i.e. Vise spring, Thau lagoon ; Fontestramar spring, Salses-Leucate lagoon, …) and potential new sites. We combine methods that allow us to detect SGD into the coastal seas (airborne Thermal Infrared images ; analyses conducted in situ) and methods that allow us to quantify SGD fluxes and to provide information on the functioning of the systems (geochemical tools). Ultimately, our goal is to evaluate the role of SGD as a source of chemical compounds into the northwestern Mediteranean Sea.
Submarine Groundwater Discharge (SGD) is nowadays recognized as an important pathway at the continent-ocean interface for the transfer of chemical elements and species into coastal waters and may thus have a significant impact on their quality and their ecosystems, as well as on geochemical cycles. SGD includes both i) the discharge of fresh groundwater into coastal seas and ii) the recirculation of seawater through “subterranean estuaries” where chemical reactions take place - like in their surface, river-fed counterparts - thus releasing chemical species to the coastal waters. Although both SGD components may transfer chemical compounds to the coastal zone, SGD contributions are not considered to date in oceanic budgets. Yet the magnitude of SGD input, their impact and their relative importance in the budgets of the Mediterranean Sea are not known. It is also worth studying fresh groundwater flows into the sea because questions on water resources have become a major issue under the context of global change. These freshwater fluxes could potentially be used, e.g. for agriculture (irrigation) and also for drinking water supply.
Field work was initiated in year 2009 in the framework of the CYMENT project (RTRA; PI : A. Cazenave). In collaboration with Thomas Stieglitz (Poste Rouge OMP), we performed in situ analyses (S, T, radon) as well as sampling of seawater (radium analyses) in springs that have been well-known for a long time (e.g. submarine spring of Port-Miou in Calanques of Marseille-Cassis ; Vise spring in the Thau Lagoon ; Font Estramar spring in the Salses-Leucate Lagoon). In parallel to these field studies, we conducted an airborne survey in year 2012 (LEGOS/ CNES) along the coastline between Toulon and Banyuls-sur-Mer where we acquired thermal infrared (TIR) images to detect fresh groundwater input.
Fig. 1 : Airborne TIR image obtained in the area of Calanques de Marseille-Cassis (LEGOS/ CNES/ TCC, Beauvais). The black color indicates cold waters.
Fig. 2 : 228Ra activities determined in the Lapalme lagoon. The higher 228Ra activities in the northern part of the lagoon are associated with the discharge of groundwater into the lagoon.
Stieglitz T., van Beek P., Souhaut M., Cook P., 2013. Groundwater discharge Karstic groundwater discharge and seawater recirculation through sediments in shallow coastal Mediterranean lagoons, determined from water, salt and radon budgets, Marine Chemistry 156, 73-84.
Participants: Pieter van Beek, Marc Souhaut, Virginie Sanial, Bruno Lansard, Alexei Kouraev/ Coll. Thomas Stieglitz
LEGOS : Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (Toulouse)
JCU Townsville Australia, Thomas Stieglitz
Fundings: RTRA; CNES