The freshwater origin of a brackish submarine spring inferred from a long-term stable isotope survey: the Port-Miou karstic spring (SE France)
Résumé
Coastal karst aquifers represent crucial water resources which are high vulnerable to sea-level rise, climate variability and anthropogenic inuences. Understanding their vulnerability requires to identify their recharge area, aquifer characteristics and the mechanisms of seawater intrusion. The aim of this study is to de_ne the origin of freshwater that feed a regional submarine karst spring, and give insight into the salinization mechanism. To this purpose, we propose an original approach based on a combined salinity and stable isotopes mass balances. The submarine Port-Miou spring (S-E of France), represents a major regional discharge. Its situation is representative of the typical feature of Mediterranean coastal karsts developed at great depth below the current sea level. The catchment area has been evaluated around 400 km2, from the sea up to 1148 m of elevation. It is located close to the Marseille city, where the freshwater demand is very high for drinking, irrigation and industrial supply. This submarine brackish spring discharges through a karst conduit explored by cave-divers, down to 237 meter deep below the sea level, 2 km inland from the seashore. This study is based on a long-term monitoring (over 10 years) of spring water water level (or discharge) salinity, and water isotopic composition (_18O and _2H). This survey is complemented by a database of regional groundwater and precipitation isotope compositions. At the Port-Miou spring site, the discharge may exceed 35 m3/s during ood events, but the salinity never falls under 2 p.s.u., making it continuously brackish. First, a two-endmember mixing equation based on springwater salinity allows the source to be partitionned between continental freshwater and seawater. Second, the isotopic composition of the freshwater endmember is calculated using the seawater isotopic composition and mixing fraction. We discuss the time variations of the reconstructed freshwater isotopes composition with regards to the composition of regional aquifers and precipitation. During long summer recessions, the fresh isotopic signature moves toward a constant mean interannual value. It suggests the existence of a base ow, with an additional contribution of fast infiltration water during flood events characterized by more variable isotopic compositions.