Fifty years of monitoring coastal aquifer dynamics: climate and human forcing effects on the transition zone assessed via electrical conductivity depth profiles

• 50-year EC depth profiles reveal long-term freshwater–saltwater dynamics • Total Dissolved Solids profiles show the transition zone widening overtime and upward migration. • Freshwater quality and quantity decline parallel to the transition zone shifts. • The Sharp Interface indicator provides a quantitative salinisation proxy. • The climate-exploitation nexus explains progressive aquifer salinisation. Coastal aquifers face serious risks from overuse, pollution, and climate change, especially in semi-arid areas. Despite advances in modelling, empirical knowledge of long-term changes in the freshwater–saltwater equilibrium in real aquifers remains limited. A comprehensive 50-year dataset (1974–2024) of Electrical Conductivity (EC) depth profiles for the Salento coastal karst aquifer in Puglia, Southern Italy, is presented. This dataset derives from a monitoring well designed to capture density layering under horizontal flow. The data enable reconstruction of long-term changes in density distribution within a 160 m-thick saturated zone including the saltwater layer, as well as analysis of groundwater salinisation dynamics. Cumulative probability plots of Total Dissolved Solids (TDS) profiles, derived from EC-TDS calibration, enabled the delineation of freshwater, transition, and saltwater zone thicknesses. The theoretical Sharp Interface and Equivalent Saltwater Head, calculated through TDS-density calibration, serve as proxies of salinisation evolution. TDS distributions over time highlight the widening and upward migration of the transition zone, along with a decline in freshwater quality and quantity. Today, the system suffers from a persistent freshwater-saltwater disequilibrium after four multi-year droughts in the last four decades. The climate-exploitation nexus mostly explains the evolution of groundwater salinisation, but the individual roles of droughts and exploitation cannot be disentangled. The timescales of these phenomena cannot be precisely determined either. The results provide an innovative perspective on monitoring by focusing on the aquifer’s vertical development rather than just its horizontal development, and by using a sufficiently long monitoring time frame to reveal delayed phenomena.