A multi-isotope (O, H, B, Sr) approach for identifying salinity contamination along the coastal sector of Murgia aquifer (Apulia, Southern Italy)

The analysis of pressures and impacts on the Apulian aquifers, as required by the EU WFD, has evidenced physical and chemical parameters related to groundwater salinization, particularly along the coastal strip. Predisposing factors such as lithology, tectonic evolution, aquifer over-exploitation (Passarella et al., 2017), and the ongoing climate change are various. Therefore, distinguishing natural from anthropogenic sources of salinization becomes a fundamental issue for assessing the aquifer's chemical status. In this framework, isotopic measures of O, H, B, and Sr, suitably support distinguishing different sources of salinity, water-rock interaction processes, and the origin of the water molecules (Pennisi et al., 2006). This study focuses on the coastal sector of the Murgia aquifer located on the Adriatic side of the Apulia region (south Italy). It is made up of several hundred-meter-thick Mesozoic calcareous and calcareous-dolomite rocks affected by fracturing and karst phenomena. The aquifer is confined and characterized by irregular geometry. Groundwater flows to the sea where it rises in numerous coastal springs. Paleo-seawater as an additional source of salinity has been suggested by previous studies. 87 Sr/86Sr, 11B/10B, (expressed as ?11B permil), ?18O, ?2H determined with overall chemistry in about 50 samples collected in autumn 2019 in the study area, range from 0.70768 to 0.70884, from +15.3? to +43.0?, from -7.78? to -3.40? and from -49.50? to -23.30?, respectively. The coupled chemical and isotopic approach evidenced that different mixing processes concur in the water, highlighting hydrogeologic zoning and complex groundwater circulation patterns. Some samples reveal Sr and B isotopic compositions typical of the local Cretaceous carbonate rocks implying a prolonged water-rock interaction. Chemistry and isotopes in 6 samples, where chlorine ranges from 3162 to 9684 mg/L indicate a significant contribution from modern seawater. Water>s intermediate compositions are explained by a different mixing degree of known endmembers, such as meteoric, marine, and rock-interacting water. ?11B values, 87 Sr/86Sr ratios, and Cl, B, and Sr contents seem to exclude the contribution of fossil seawater to the studied groundwater. Further studies on high conductivity samples are ongoing to better detail the zoning based on the isotopic and hydrogeological characteristics and to confirm or deny the occurrence of fossil marine waters.