Hydrochemical Evidence of Groundwater Transfer Between Karst Aquifers in Southern Spain: Implications for Climate Resilience and Groundwater-Dependent Ecosystems

The Upper Cretaceous and Jurassic karst aquifers of Sierra de Segura (southern Spain) represent an important groundwater resource within Sierras de Cazorla, Segura, and Las Villas Natural Park (the largest protected area in Spain). The two carbonate aquifer systems are separated by the low-permeability Utrillas Formation, composed mainly of sands, sandstones, and clay-rich sediments. Hydraulic connectivity between these aquifers remains poorly constrained in the literature. This study evaluated the hydrochemical and isotopic evidence of connectivity between the Upper Cretaceous and Jurassic aquifers using major ions (Na+, K+, Ca2+, Mg2+, Cl−, NO3−, SO42−, and HCO3−) and stable water isotopes (δ18O and δ2H) from 464 groundwater samples collected from 20 springs between May 2020 and October 2023. The hydrochemical results show a progressive increase in electrical conductivity and temperature from the Upper Cretaceous to the Jurassic aquifer, together with higher Mg2+, Na+, K+, Cl−, and SO42− concentrations in the Jurassic system, indicating greater hydrochemical evolution. Principal Component Analysis (PCA) identified mineralization as the dominant controller of groundwater variability and revealed a hydrochemical gradient from relatively homogeneous Upper Cretaceous groundwater toward more mineralized Jurassic groundwater, with partial overlap between both groups. The convergence of hydrochemical, isotopic, multivariate, and temporal evidence is consistent with the hydraulic connectivity between the two aquifer systems. We propose the “SHOWER EFFECT” as a conceptual hypothesis involving potential groundwater transfer through the Utrillas Formation. However, alternative explanations cannot be excluded, and further tracer, groundwater age, and numerical modeling studies are required to test the proposed mechanism.