Saltwater Intrusion in Coastal Aquifers of the Friuli-Venezia Giulia Region: Monitoring and Modelling Methodologies

Saltwater intrusion (SWI) and coastal groundwater salinisation are increasingly relevant issues in low-lying Adriatic plains, where sea-level rise, subsidence, droughts and intensive water management act on aquifers hosted in highly heterogeneous sedimentary successions. In Friuli Venezia Giulia (FVG, NE Italy), salinisation is locally documented, yet its present-day extent and the relative importance of geological controls and external drivers remain difficult to constrain, especially in the shallow phreatic system that is hydraulically connected to lagoons, rivers and reclaimed drainage networks. This thesis follows a progressive approach across scales, combining regional hydrochemistry, process-oriented coastal observations and multi-method hydrogeophysical imaging in order to reduce ambiguity and to provide practical constraints for monitoring and interpretation. The first step establishes a regional hydrochemical reference for the confined multilayer aquifer system of the lower Friulian Plain, integrating more than two decades of ARPA-FVG monitoring with new sampling. Multivariate patterns and long-term trends distinguish three groundwater families linked to redox conditions, residence time and anthropogenic loading. Within this framework, nitrate dynamics emerge as the dominant long-term vulnerability for the confined resource, whereas chloride trends do not support active SWI at the regional scale of the confined aquifers. The analysis then narrows to the coastal boundary, focusing on the Porto Nogaro sector, where the availability of background information allows temporal evolution to be explored rather than a single snapshot. New periodic sampling is integrated with ARPA-FVG archive data and complemented by continuous monitoring of river and groundwater dynamics. The results show that shallow salinity in reclaimed lowlands is intrinsically heterogeneous, shaped by local hydraulic connectivity and stratification. River–groundwater interactions are not uniform: signals can be selectively transmitted, delayed or damped depending on site-specific conditions. Salinisation is also shown to be intertwined with groundwater-quality evolution. Iron enrichment is most pronounced in brackish mixing waters under low-oxygen conditions, consistent with redox-controlled mobilisation, and confirms that salinity changes may coincide with broader geochemical responses relevant to water quality. A final step develops and tests the methodological bridge that enables spatial upscaling. The complementarity between electrical resistivity tomography (ERT) and frequency-domain electromagnetic methods (FDEM) is exploited through an ERT-anchored calibration strategy, improving the robustness and transferability of FDEM interpretation across instruments and sites. The integrated approach is applied to the Belvedere-San Marco paleo-dune system, where multidisciplinary converge reveals a consistent picture of a persistent freshwater lens within the dune body, sharply contrasting with surrounding reclaimed lowlands. The calibrated workflow is then extended to a lagoon-scale survey, providing the first spatially continuous screening of near-surface conductivity patterns across the western Grado Lagoon coastal plain and supporting a pragmatic, screening-level delineation of the inland extent of shallow salinisation within the investigated depth range. Overall, the thesis links regional groundwater geochemistry, coastal monitoring and calibrated hydrogeophysical imaging into a coherent framework that clarifies where and how salinisation develops in the FVG coastal plain, and provides actionable constraints to prioritise monitoring and targeted follow-up investigations in the most vulnerable coastal sectors.