Characterising recharge, storage and flow pathways in karst aquifers in central Slovenia using stable water isotopes, tritium and chlorine-36

Study regions Approximately half of Slovenia is underlain by karstified carbonate rocks that host important groundwater resources. The Rovte region in central Slovenia is characterised by fluviokarst, complicating the understanding of aquifer flow pathways and storage. Study focus Determining recharge, storage and flow pathways in karst aquifers remains a major challenge due to the heterogeneity of permeability; ranging from large conduits to micro-fractures. In this study, stable water isotopes (δ²H and δ¹⁸O), together with tritium and chlorine-36, were measured in precipitation, river water, karst springs and deep artesian borehole waters at a biweekly resolution over one year. The aim was to characterise links between infiltration and subsurface flow, and provide information on aquifer properties. New hydrological insights for the region The karst springs emerging from shallow aquifers exhibited rapid and synchronised isotopic trends related with precipitation, indicating rapid groundwater transport through wide conduits and fractures. However, some water storage in soil or near-surface micro-fractures that mobilised only during the heaviest rainfall events was revealed. In contrast, the artesian borehole waters showed little isotopic variability, consistent with well-mixed groundwater deriving from deeper aquifers that likely recharged under cooler conditions and subsequently mixed with small contributions from local shallow aquifers.