The Freshwater Origin of a Brackish Submarine Spring Inferred from a Long-Term Stable Isotope Survey: The Port-Miou Karstic Spring (S-E France)
Résumé
The submarine Port-Miou spring (S-E of France), represents a major regional discharge, draining a catchment area of around 400 km² with pronounced relief up to 1148 m a.s.l., close to highly urbanized coastal areas (Marseille). The spring has a brackish salinity, rising up to 14 psu during low flow and diluted until 2 psu during floods. The aim of this study is to better understand the origin of freshwater and the salinization mechanism, using the results of a long-term stable geochemical survey of the brackish Port-Miou spring and of the surrounding aquifers. Since 2012, water salinity and temperature are measured in situ and water sampling was performed unevenly in springs and boreholes for stable isotopes analysis (δ18O and δ2H). Based on a twoendmembers mixing between continental freshwater and seawater, we propose a method for calculating the isotopic composition of the freshwater feeding the Port-Miou spring. Results show δ 18 O values varying between -7.7 and -6.5 ‰ for the reconstructed composition of freshwater. These variations are discussed with regards to the compositions of regional aquifers. A relative stability is evidenced for the isotopic composition of the freshwater endmember during low flow, consistent with the regional deep aquifers. However, during high water events the freshwater endmember shows a more variable isotopic composition, which suggests the contribution of different water sources.
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