Statistical insights from long-term time-series of hydrogeochemical signatures and radon in groundwater of the Matese area, Central-Southern Italy

Abstract A key challenge in earthquake hydrogeology is separating hydrogeochemical changes induced by seismic activity from those driven by natural recharge dynamics. This study provides a comprehensive long-term multiparameter analysis of hydrogeochemical parameters and dissolved radon concentrations in groundwater, monitored at four springs in the Matese area (central-southern Italy), fed by carbonate aquifers and located within an active extensional tectonic setting. Discrete monthly hydrogeochemical sampling has been performed at the four sites since December 2017, while continuous radon monitoring has been carried out at Rio Freddo spring (since July 2018) and Grassano spring (since July 2020) using gas-monitoring stations. Principal Component Analysis (PCA) was applied to the discrete hydrogeochemical dataset to identify anomalous sampling campaigns and the dominant geochemical processes controlling groundwater composition at each spring. Seasonal Trend decomposition using LOESS (STL) was applied to the continuous radon measurements, discerning long-term trend, seasonal, and residual components. PCA results highlight discrete sampling campaigns with anomalous enrichment in major ions and/or trace elements, tentatively associated with seismic events or stress-induced permeability changes. STL radon decomposition reveals site-specific trend evolutions and seasonal patterns, with residual variability showing a clear dependence on meteorological forcing. Moreover, residual anomalies exceeding ± 3σ were evaluated against regional seismicity. These findings demonstrate the potential of combined PCA and STL approaches for discriminating between deep-seated fluid contributions to carbonate fractured aquifer and shallow meteorological factors in active extensional settings.