Abstract
<title>Abstract</title> <p>Groundwater is a critical resource for drinking water supply, irrigation, and ecosystem sustainability, yet its quality is increasingly threatened by both natural hydrogeochemical processes and human activities. In Greece, despite the hydrogeological importance of groundwater and the strong spatial heterogeneity of aquifer systems, national-scale comparative assessments across different aquifer types remain limited. This study presents a nationwide evaluation of groundwater quality in Greece using data from the National Water Monitoring Network for the period 2018–2020. A harmonized hydrochemical dataset was analyzed using descriptive statistics, correlation analysis, principal component analysis (PCA), and non-parametric statistical tests to identify dominant hydrochemical processes and assess differences among porous, karst, and fractured aquifers. The results revealed a consistent hydrochemical structure dominated by two principal factors. The first corresponds to salinization and mineralization, with strong associations among electrical conductivity (EC), Na⁺, Cl⁻, Mg²⁺, and SO₄²⁻, indicating the influence of processes namely seawater intrusion, elevated ionic strength, and related geogenic or anthropogenic pressures. The second reflects carbonate dissolution and groundwater evolution, primarily expressed through HCO₃⁻, Ca²⁺, and related hydrochemical conditions indicative of water-rock interaction. Nutrient-related variability, particularly involving NO₃⁻, NH₄⁺, NO₂⁻, and PO₄³⁻, showed greater temporal heterogeneity, suggesting localized agricultural inputs, wastewater influence, and redox-related processes. Trace elements exhibited highly skewed distributions and a substantial proportion of censored observations, limiting their robustness in multivariate modeling and supporting their interpretation mainly at a descriptive level. Comparative analysis among aquifer types showed statistically significant hydrochemical differentiation, especially among the dominant components. Groundwater samples from porous aquifers exhibited greater hydrochemical variability than those from karstic and fractured aquifers, suggesting a stronger influence of shallow recharge, land use, and mixed hydrochemical inputs. Overall, groundwater quality in Greece is shaped by the combined influence of lithology, water-rock interaction, salinization processes, and anthropogenic inputs. The findings provide a robust national-scale baseline for understanding groundwater quality patterns in Greece and support the need for aquifer-specific monitoring and management strategies.</p>