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

Le résumé fourni par la source

The Upper Cretaceous and Jurassic karst aquifers of Sierra de Segura (southern Spain) constitute a major groundwater resource within the “Sierras de Cazorla, Segura y Las Villas,” the largest Natural Park of Spain and the second largest in Europe. Both car-bonate aquifer systems are separated by the low-permeability Utrillas Formation (UT), composed mainly of sands, sandstones, and clay-rich sediments, and its hydrogeologi-cal role remains poorly understood. To investigate the hydraulic relationship between the aquifers and assess the existence of groundwater transfer through the Utrillas Formation, major ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, NO₃⁻, SO₄²⁻, and HCO₃⁻) and stable wa-ter isotopes (δ¹⁸O and δ²H) were analyzed in 465 groundwater samples from 20 springs between May 2020 and October 2023. Hydrochemical results revealed a progressive evolution from the Upper Cretaceous aquifers, characterized by lower electrical con-ductivity and temperature values (520 µS/cm and 12.2 °C), to the Jurassic aquifer, where these parameters increased to 611 µS/cm and 14.9 °C. Higher concentrations of Mg²⁺, Na⁺, K⁺, Cl⁻, and SO₄²⁻ in the Jurassic system indicate a greater degree of hydro-chemical evolution, consistent with enhanced water–rock interaction and more pro-longed groundwater circulation. Principal Component Analysis identified mineraliza-tion as the dominant control on groundwater variability (48.5% of the total variance) and revealed a hydrochemical gradient from relatively homogeneous Upper Creta-ceous groundwater toward more mineralized Jurassic groundwater, while several Ju-rassic springs overlapped with the Upper Cretaceous group. The convergence of hy-drochemical, isotopic, multivariate, and temporal evidence is consistent with hydrau-lic connectivity between the two aquifer systems. Collectively, these observations support the conceptual “Shower Effect” model, whereby groundwater is inferred to be transferred from the hydraulically elevated Upper Cretaceous aquifers to the under-lying Jurassic aquifer through the low-permeability Utrillas Formation.

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