Isotopic characterization and recharge dynamics of Karst aquifers in a mediterranean basin

Study region: Headwater of the Llobregat River, Spain. Study focus: This study advances the understanding of hydrological and hydrogeological processes in fractured and karstified carbonate massifs by characterizing the isotopic composition of precipitation, surface water, and groundwater. A total of 115 water samples collected between April 2024 and February 2025 were analysed to assess temporal and spatial isotopic variability, examine relationships between stable isotopes and meteorological variables, reconstruct backward trajectories of moisture sources, and delineate recharge zones. New hydrological insight: (1) An isotopic gradient linked to moisture conditions was identified, indicating that thermodynamic processes and air-mass origin exert primary control on d-excess. (2) Moisture sources contributing to precipitation were identified as the Atlantic Ocean (44 %), the Mediterranean Sea (24 %), France (18 %), and the Cantabrian Sea (14 %). Backward trajectory analysis highlights the relationship between moisture provenance and isotopic signatures; however, accumulated precipitation samples represent integrated mixtures of multiple sources. (3) Groundwater and surface water isotopic signatures suggest dominant winter recharge occurring above 1800 m a.s.l., consistent with regional topography and the highly karstified structure of the Moixeró massif. Seasonal precipitation signals preserved in groundwater further suggest short residence times and rapid recharge responses. (4) These findings support improved water-resource management and highlight the sensitivity of alpine karst systems to climatic variability, underscoring the need for continued isotopic monitoring.