Hydrogeological conceptual model for the Guarani Aquifer System outcrop area in Brazil: Insights from stable isotopes and rare earth elements

Abstract This study refines the hydrogeological conceptual model of the Guarani Aquifer System (GAS) outcrop area in São Paulo state in Brazil and highlights its complexity, particularly the coexistence of semi-confined and unconfined aquifers. Diabase sills act as aquitards, separating a Na⁺-HCO₃⁻ type deep groundwater from shallower Na⁺-Cl⁻-NO₃⁻ and mixed-HCO₃⁻/Cl⁻-NO₃⁻ type groundwater. Stable isotopes suggest that deep GAS groundwater was recharged under climatic conditions distinct from present-day conditions, whereas springs and surface waters reflect modern meteoric recharge. The study integrated major ion geochemistry, stable isotopes (δ 2 H and δ 18 O), and rare earth elements and yttrium (REY) concentrations of water samples collected from springs, surface waters, and shallow and deep aquifers. REY data revealed hydrogeochemical facies not previously detected with conventional hydrogeochemical tools, underscoring the potential of REY as tracers for aquifer connectivity. The results emphasize the need for a discussion about groundwater management, as the semi-confined aquifer is currently being tapped by water-supply wells in the GAS outcrop area, which is not accounted for in previous conceptual models.