Submarine Groundwater Discharge : a hidden source of chemical compounds at the land-sea interface – MED-SGD

The flow of groundwater into the coastal sea constitutes a vector for many chemical elements that may impact the quality of the coastal environment and marine ecosystems. Additionally, groundwater contributions are not considered to date in oceanic budgets. Although submarine groundwater discharge (SGD) has been investigated in many places of the world, very few studies were conducted along the French coastline of the Mediterranean Sea: almost no information is available on the fluxes of water and chemical elements associated with SGD and on their potential impact on the water quality and geochemical cycling of the coastal zone. Pilot studies conducted on the field and a recent survey where we acquired airborne thermal infrared (TIR) images allowed us to locate the “hotspots” that will be studied in this project. The hotspots include the karstic submarine springs that have been known for a long time along the French Mediterranean coastline (eg. submarine spring of Port-Miou located in Calanques of Marseille-Cassis; Vise spring in the Thau Lagoon; Fontestramar spring that is connected to the Salses-Leucate Lagoon; Fleury et al., 2007), but also additional sites where groundwater discharges into coastal waters or coastal lagoons. The MED-SGD project aims at quantifying the water and chemical fluxes associated with the major SGD sites (“hotspots”) along the French Mediterranean coastline. We want to document the springs, lagoons, coastal seas, exchange between the lagoons and the sea, as well as seawater recirculation processes in the sediments/ sands. The SGD fluxes will be derived from the radium, radon and salinity budgets. Using the SGD fluxes thus determined, the flux of many chemical compounds of interest will be calculated based on a detailed chemical characterization of the SGD (i.e. nutrients, dissolved organic carbon, dissolved inorganic carbon, major elements, uranium, rare earth elements REE, trace metals including pollutants like eg. mercury etc..). Because both the water and chemical fluxes associated with SGD likely vary with time, a second objective of the MED-SGD project is to study the temporal variability of the SGD systems. To our knowledge, a study at such a spatial scale has never been performed and is the first study that attempts to quantify the chemical element fluxes associated with SGD along the entire French coastline of the Mediterranean Sea. We will use this latter information to evaluate the impact of SGD i) on the quality of coastal waters/ coastal lagoons and ii) on the chemical budgets of the Gulf of Lions and of the Northwestern Mediterrean Sea. A unique consortium was created to achieve the goals of the MED-SGD project. This consortium federates scientists with recognized expertise in the study of SGD (including geochemists, modellers, biogeochemists, microbiologists, hydrogeologists, oceanographers).