Geochemical Behavior of Rare Earth Elements in Saline and Salty Springs From a Carbonate–Evaporite Aquifer in the Northeastern Sichuan Basin, China

ABSTRACT Rare earth elements (REEs) in saline groundwater are effective tracers of hydrogeochemical processes in deep carbonate–evaporite aquifer systems. This study examines the hydrochemistry, distribution, fractionation, and aqueous speciation of REEs in brackish, saline, and salty springs from the northeastern Sichuan Basin, China. Spring waters are dominated by Na‐Cl and Cl·SO 4 ‐Na facies, with TDS up to 82.9 g/L, mainly controlled by Triassic evaporite dissolution and deep brine mixing along faulted anticlines. Dissolved REE concentrations are low (ΣREE = 0.096–0.314 μg/L) and decrease with increasing salinity and pH. NASC‐normalized patterns show consistent MREE‐HREE enrichment and pronounced positive Eu anomalies in deep, reducing thermal waters. Geochemical modeling indicates that REEs are predominantly stabilized as carbonate complexes, with LnCO 3 + and Ln (CO 3 ) 2 − accounting for over 90% of dissolved species. These results demonstrate that tectonically driven deep circulation, redox conditions, and carbonate complexation jointly regulate REE behavior in saline groundwater systems.