MAPPING SUBMARINE GROUNDWATER INPUTS WITH SHORT-LIVED RADIONUCLIDES ALONG THE FRENCH MEDITERRANEAN COAST

Short-lived radionuclides have been extensively used these last decades to locate and estimate submarine groundwater discharges (SGD) in coastal areas. These studies were generally conducted with the same objective: evaluating the radionuclide budgets within a defined area in order to convert their fluxes into groundwater fluxes. To achieve this, sampling of the investigated area has to be done thoroughly, and all kinds of inputs and outputs have to be considered. Partly due to such difficulties, most of the studies were performed at a local scale (few km, ²²²Rn, ^223,224Ra) or, rather, at a global scale (²²⁸Ra for oceanic scale).

Along the French Mediterranean coast, few SGD spots are known when they are related to karstic areas while SGD from coastal aquifers within quaternary sediments are less easy to map. Two cruises of 8 days were conducted in 2013 and 2014 with the objective of measuring ²²²Rn and ^223,224Ra in surface waters, as close as possible to the 500 km-long coastline, in order to locate SGD inputs. ²²²Rn activities were measured using a continuous monitoring system (RAD-7) with a 10 mn running time, providing one measurement each 1-1.5 km. About 60 points only were sampled for ^223,224Ra, analyzed with Radecc system from 30-60L of surface seawater passed through Mn-fiber.

The ²²²Rn sampling especially allows to locate precisely unknown karstic springs, while the SGD signal from quaternary aquifers is overwhelmed by the river inputs already enriched in ²²²Rn. However, the ²²²Rn signal can be modified by wind events, and coupling these values with ^223,224Ra surveys provides a good comparison, even if the sampling resolution is lower in this case. We will present and discuss here these first short-lived Ra-Rn maps along the French Mediterranean coast.