Tracing Recharge Pathways and Drought Stress in Mediterranean Karst Aquifers Using Isotopic Signatures

This study advances the understanding of hydrological and hydrogeological processes in fractured and karstified carbonate massifs by systematically characterizing the isotopic composition of precipitation, surface water, and groundwater in the headwaters of the Llobregat River (Spain). A total of 115 water samples, collected between April 2024 and February 2025, were analyzed to assess temporal and spatial isotopic variability, examine the relationships between stable isotopes and meteorological variables, reconstruct backward air-mass trajectories of moisture sources, and delineate groundwater recharge zones.

Results reveal an isotopic gradient linked to moisture conditions, and show that thermodynamic processes and air-mass origin exert a primary control on d-excess values. 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 a strong link between moisture provenance and isotopic signatures; however, accumulated precipitation samples represent integrated mixtures of multiple moisture sources.

Groundwater and surface water isotopic signatures suggest dominant winter recharge occurring above 1,800 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.

These findings provide valuable insights for water-resource management and highlight the sensitivity of alpine karst systems to climatic variability, underscoring the need for continued long-term isotopic monitoring and expanded future hydrogeological studies.