Tracing groundwater salinization processes in coastal aquifers: a hydrogeochemical and isotopic approach in Na-Cl brackish waters of north-western Sardinia, Italy

In the Mediterranean area the demand of good quality water is often threatened by salinization, especially in coastal areas. The salinization is the result of concomitant processes due to both marine water intrusion and rock-water interaction, which in some 5 cases are hardly distinguishable. In northwestern Sardinia, in the Nurra area, salinization due to marine water intrusion has been recently evidenced as consequence of bore hole exploitation. However, the geology of the Nurra records a long history from Paleozoic to Quaternary, resulting in relative structural complexity and in a wide variety of lithologies, including Triassic evaporites. To elucidate the origin of the saline 10 component in the Nurra aquifer, may furnish a useful and more general model for the salinization processes in the Mediterranean area, where the occurrence of evaporitic rocks in coastal aquifers is a common feature. In addition, due to intensive human activities and recent climatic changes, the Nurra has become vulnerable to desertification and, similarly to other Mediterranean islands, surface-water resources can periodically 15 suffer from drastic shortage. With this in mind we report new data, regarding brackish waters of Na-Cl type of the Nurra, including major ions and selected trace elements (B, Br, I and Sr) and isotopic data, including 18O, D in water, and 34S and 18O in dissolved sulphate. To better depict the origin of the salinity we also analyzed a set of Nurra Triassic evaporites for 20 mineralogical and isotopic composition. The brackish waters have Cl contents up to 2025mgL−1 and the ratios between dissolved ions and chlorine, with the exception of the Br/Cl ratio, are not those expected on the basis of a simple mixing between rain water and seawater. The 18O and D data indicate that most of the waters are within the Regional 25 Meteoric Water Line and the Global Meteoric Water Line supporting the idea that they are meteoric in origin. A relevant consequence of the meteoric origin of the Nurra Na- Cl type water is that the Br/Cl ratio, extensively used to assess the origin of salinity in fresh water, should be used with care also in near coastal carbonate aquifers. Overall, and consistent with the geology and the lithological features of the study area, 34S and 18O in dissolved sulphate suggest that water-rock interaction is the responsible for the Nurra Na-Cl brackish water composition. Evaporites dissolution also explain the high chlorine contents since halite has been detected in the gypsum levels. Finally, 5 the Nurra Na-Cl brackish water are undersaturated with respect to the more soluble salts involving, in a climate evolving toward semi-arid conditions, that the salinization process could dramatically intensify in the near future.