Hydrogeochemical and Structural Controls on Rare Earth Element Distribution in Spring Waters from the Chongqing,...

Rare earth elements (REEs) are effective tracers of groundwater circulation and water-rock interaction in carbonate aquifers. This study investigates hydrochemical parameters and REE compositions of spring waters from Chongqing (eastern Sichuan Basin) to clarify controls on REE mobilization, fractionation, and redox behavior within a fold-controlled karst aquifer. Spring waters exhibit slightly acidic to slightly alkaline pH values (6.2-8.4) and variable redox conditions (Eh from -338 to 148 mV). Hydrochemical facies evolve from Ca-HCO₃ and Ca·Mg-SO₄ types to Na-Cl and Na-SO₄ types, indicating a transition from shallow recharge systems to deeper, structurally controlled groundwater circulation. Total dissolved REE concentrations (ΣREE) range from 0.025 to 1.732 μg/L and show clear spatial variability, with relatively higher values occurring in structurally complex southeastern zones. Correlation analysis indicates that ΣREE concentrations are weakly related to salinity but are influenced by temperature, redox conditions, and bicarbonate availability, reflecting the importance of groundwater circulation depth and water-rock interaction. NASC-normalized REE patterns consistently display LREE depletion and relative MREE-HREE enrichment, typical of carbonate aquifers. Predominantly, Eu anomalies suggest localized reducing or thermally influenced environments associated with deeper groundwater circulation. REE behavior in Chongqing spring waters is indirectly controlled by tectonic structure through its regulation of groundwater circulation and hydrochemical conditions.