MONITORING-BASED VALIDATION OF GALDIT SEAWATER INTRUSION VULNERABILITY MAPPING: THE MURAVERA COASTAL AQUIFER (SE SARDINIA)

Index–overlay methods such as GALDIT are widely used to screen coastal aquifers for seawater-intrusion vulnerability, but they are rarely benchmarked against long monitoring records. We apply GALDIT to the Muravera coastal aquifer (SE Sardinia) and validate the vulnerability classes using multi-year hydrochemical and hydraulic observations. GALDIT factors (G, A, L, D, I, T) were implemented in QGIS on a 25×25 m grid. The October 2024 survey was selected as a worst-case snapshot for the phreatic domain. Distance (D) was computed to a saline boundary including both the open coastline and the connected coastal lagoons (“foxi”); L derives from the October 2024 piezometric surface; A and T from the hydrogeological model (Porru et al., 2026); I from the October 2024 conductivity field. The weighted index was classified into five classes. Validation uses the 2017–2024 groundwater dataset: a seawater-influence index (SWI) is computed from major ions and chloride-based seawater fraction and compared across GALDIT classes. SWI increases with GALDIT class (Spearman ρ=0.64 for samples), with class-5 areas showing the highest seawater influence. Age-dating (Porru, 2023) indicates that the most saline waters can be centuries old; therefore, GALDIT is interpreted as susceptibility to seawater-influenced salinisation and potential mobilisation of inherited salinity under present stresses. Consistently, in high-vulnerability sectors SWI tends to increase during low piezometric heads, and continuous piezo–conductivity records at a class-5 site show a delayed conductivity rise following drawdown. Overall, the monitoring-based workflow supports GALDIT as a practical screening tool to prioritise intrusion-prone sectors in Muravera and analogous low-gradient coastal plains.