Beyond the Ca–HCO₃ facies: Multivariate identification of hydrogeochemical subsystems in a complex karst aquifer of northeastern Hungary

Study region The Aggtelek Karst of northeastern Hungary, part of the transboundary Gömör–Torna Karst system, is one of the most important carbonate aquifers in Central Europe. Study focus This study investigates the hydrochemical and stable water isotope characteristics of 27 springs to identify hydrogeochemically coherent groundwater groups and evaluate the factors controlling groundwater variability. Combined Cluster and Discriminant Analysis was applied to major-ion and stable water isotope data and interpreted within the geological framework of the aquifer. New hydrological insights for the region Seven statistically robust hydrogeochemical groups were identified, demonstrating that the Aggtelek Karst is not a hydrochemically uniform carbonate aquifer despite the predominance of Ca–HCO₃ waters. The differences among the groups are associated with variations in carbonate lithology, particularly the relative influence of limestone and dolomitic units, variable contributions from non-carbonate and evaporite-bearing lithologies, and heterogeneity in hydrogeological setting and groundwater-flow conditions. Importantly, hydrochemical similarity is not determined primarily by geographical proximity: springs from different parts of the karst may belong to the same group, whereas geographically close springs may show distinct hydrogeochemical characteristics. The results demonstrate that conventional facies-based classifications may underestimate the internal hydrogeochemical complexity of carbonate aquifers. These findings improve the conceptual understanding of groundwater circulation in the Aggtelek Karst and demonstrate the value of integrating hydrochemical, stable water isotope, and multivariate statistical approaches in regional karst investigations.