MED-SGD : Submarine groundwater discharge along the French Mediterranean coastline

by TIM — last modified Nov 20, 2020 10:36 AM

Submarine Groundwater Discharge and circulation of seawater through the sediment as sources of chemical elements to the coastal seas

Objectives:
The continents are known to deliver chemical species to the coastal seas. Among the continental sources, the fluxes of chemical elements associated with rivers and aerosols that reach the coastal seas have been widely studied. In contrast, inputs of chemical elements associated with karstic groundwater discharge or circulation of seawater through the sediments have been relatively poorly studied. This assertion is especially true for the French Mediterranean coastline, where almost no information is available on the fluxes of water and chemical elements associated with these processes and on their potential impact on the geochemical cycling and ecosystems of the coastal zones. We built a team of scientists (LEGOS, GET, CEREGE, LOMIC, EPOC, HSM, UAB) to study these unexplored fluxes. We combined
methods that allow us to detect karstic groundwater discharge into the coastal seas (ie.,Thermal InfraRed images acquired with a plane or drone; analyses conducted in situ) and methods that allow us to quantify the water and chemical fluxes associated with the karstic groundwater discharge and circulation of seawater through the sediments. In some cases, we also studied the temporal variability of these fluxes.

Scientific Rationale

Fresh groundwater discharge is nowadays recognized as an important pathway at the continent-ocean interface for the transfer of chemical elements and species into coastal waters. However, groundwater contributions are often not considered in oceanic budgets and almost no information is available on the impact of these fluxes on the quality of the coastal seas and ecosystems. Along the French Mediterranean coastline, several karstic springs are well known, as is the case for the Port Miou and Cassis springs in Calanques de Marseille-Cassis where the coastal aquifers are connected to the coastal seas. In most cases, seawater intrusion takes place in these coastal environments, thus raising the salinity of the groundwater that flows at the outlet. In addition to these flows of fresh/ brackish groundwater, the circulation of seawater through permeable sediments also generates a flux of chemical species into the coastal seas. In the framework of the MED-SGD project, we used radium isotopes (223Ra, 224Ra, 226Ra, 228Ra) to quantify the water and chemical fluxes associated with these two different flow paths. The various half-lives of these isotopes (from 4 days to 1600 years) allow us to study these processes at different time-scales, or to discriminate between these two processes.

Fig.1: left: karstic spring observed in a bay of Côte Bleue coastline (west of the city of Marseille); right: airborne TIR image acquired in Salses-Leucate lagoon (TCC Beauvais; LEGOS/CNES) that allows us to locate a flow of groundwater into the lagoon.

Results and perspectives

TIR images (plane/ drone) were used to locate the areas where groundwater potentially discharges into coastal lagoons and coastal seas (Fig. 1). In parallel, we also conducted various field studies along the French Mediterranean coastline (La Palme lagoon; Salses-Leucate lagoon; Côte Bleue; Calanques de Marseille-Cassis; etc…). The analysis of radon and radium isotopes (223Ra, 224Ra, 226Ra, 228Ra) allowed us to quantify the flux of water fluxes and associated chemical species (Stieglitz et al., 2013; Bejannin et al., 2017; Tamborski et al., 2018; Bejannin et al., in prep.). As an example, we studied the karstic groundwater spring that discharges into La Palme lagoon (Tamborski et al., 2018). These groundwaters are a source of 224Ra, NO3- and DSi to the lagoon (Fig. 2).


Fig. 2: Salinity, 224Ra, NO3- and DSi in La Palme lagoon. The yellow star marks the location of the karstic groundwater spring that discharges into the lagoon.

In the same coastal area, we conducted surface water transects in the coastal seas that revealed near-shore enrichments of Ra and DSi, attributed to wave-setup and water exchange through the permeable beach between the lagoon and the sea (Fig.3). Upscaling over the 9.5 km stretch of sandy beaches results in a marine SGD-driven DSi flux of 2.3 +/- 1.3 x 104 mol d-1, similar in magnitude to the Têt river during November 2016 (3.3 +/- 2.4 x 104 mol d-1), the largest river in the region (Tamborski et al., 2018).


Fig. 3: Surface water transects of salinity, DSi, 224Raex and 228Ra off the coast of La Palme lagoon into the coastal Mediterranean Sea during November 2016.

Participants:

Pieter van Beek, Marc Souhaut, Simon Bejannin, Joseph Tamborski, François Lacan, Catherine Jeandel

Collaborations:

GET, Toulouse; CEREGE, Aix-en-Provence; LOMIC, Banyuls-sur-Mer; EPOC, Bordeaux; HSM, Montpellier; MIO, Marseille; UAB, Barcelone ; Stony Brook University, USA; Univ. Heidelberg
TCC (Beauvais)

Fundings:

ANR MED-SGD (ANR-15-CE01-0004, 2016-19; PI: Pieter van Beek); RTRA CYMENT; CNES

Peer review articles related to this project:

Tamborski, J.J., Bejaninn, S., Garcia-Orellana, J., Souhaut, M., Charbonnier, C., Anschutz, P., Pujo-Pay, M., Conan, P., Crispi, O., Monnin, C., Stieglitz, T., Rodellas, V., Andrisoa, A., Claude, C., van Beek, P., 2018. A comparison between water circulation and terrestrially-driven dissolved silica fluxes to the Mediterranean Sea traced using radium isotopes. Geochimica et Cosmochimica Acta 238: 496-515. 10.1016/j.gca.2018.07.022.

Bejannin S., van Beek P., Stieglitz T., Souhaut M., Tamborski J., 2017. Combining airborne thermal infrared images and radium isotopes to study submarine groundwater discharge along the French Mediterranean coastline, Journal of Hydrology : Regional studies 13, 72-90.

Stieglitz T., van Beek P., Souhaut M., Cook P., 2013. 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.

Bejannin et al., in preparation. Method comparison to estimate Submarine Groundwater Discharge fluxes from karstic coastal aquifers: Application to Côte Bleue, French Mediterranean Sea.

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