First results of the application of the isotopic ratio 87Sr/86Sr in the characterization of seawater intrusion in the coastal karstic aquifer of Murgia (Southern Italy)

Murgia aquifer, made up by limestone and dolomitic limestone belonging to Cretaceous, represents the most important karstic coastal aquifer ofthe Apulia (Southern Italy). The carbonate sequence reveals a considerable lithological heterogeneity, accompanied by fracturing and karsification. Due to overexploitation, groundwater salinisation is continuously expanding from coast inland. Previous studies outlined that aquifer hosts saline waters having chemical features sensibly deviating from those of present seawater. Water-carbonate rock interaction processes can account for most of the major and minor constituent variations: Ca. Mg, Sr and Li ions have been already used as natural tracers for distinguishing the different saline end-members ofmixing. The 87Sr/86Sr ratìo has been tested as natural tracer as well, basing the methodology both on the knowledge of the Sr isotopic composition of the different carbonate rocks in relation to their geological age and on the fact that salt waters interacting with marine carbonates may inherit Sr isotope composition of the same rocks. The 87Sr/86Sr ratio of saline waters can be diagnostic both of an evolution due to water-rock interaction processes and of the age of the carbonates they interact. Brackish waters of the coastal springs show clear different 87Sr/86Sr ratios, which ultimately can be traced back to the extent to which rock-water interaction has modified the different salt groundwaters during their migration history.