Hydrochemistry of confined aquifers in Friuli Venezia Giulia, Italy: Regional patterns, trends, and drivers

Confined aquifers are considered to be protected from surface contamination, yet intensive abstraction and long-term land-use pressures may alter their quality, especially in coastal areas. This study characterises the hydrochemistry of the multilayered confined aquifer system of the lower Friulian Plain (NE Italy), where groundwater is extensively exploited and locally affected by anthropogenic stressors. The study combines hydrochemical data from 1996 to 2025 from the monitoring activity of Regional Environmental Protection Agency of Friuli Venezia Giulia (ARPA FVG) and new samples collected in 2024–2025, compiling 1210 samples from 103 wells. The aim is to determine how recharge sources, residence time, and human activities control groundwater quality. Hydrochemical families were identified using major ions, redox-sensitive species, organic micropollutants, stable oxygen isotopes, and multivariate statistics. Three groundwater families were recognised: submodern waters, with hypoxic conditions, low to absent contaminant load and likely associated with recharge dating back to the late Würmian glacial period; modern-pristine waters, which are characterised by oxidising conditions, are SO 4 2− -rich, and show low to absent NO 3 − and pesticide concentrations; and modern-impacted waters, which are similar to the latter but display elevated NO 3 − concentrations, lower pH values, and detectable anthropogenic contaminants. Long-term trends indicate that although NO 3 − concentrations are overall stable or slightly decreasing in shallow aquifers, slowly increasing trends occur in deeper wells and near the margins of the NO 3 − plume, suggesting downward and lateral migration of impacted modern groundwater. Cl − concentrations show no increasing trends in coastal wells, indicating no evidence of active seawater intrusion.