Natural and anthropogenic controls on rare earth elements in groundwater: Indicative significance for distinguishing agricultural and urban-industrial impacts.

Rare Earth Elements (REEs) are emerging contaminants whose behavior in groundwater under anthropogenic influence remains inadequately understood. This study investigates dissolved REE geochemistry in the northern Gan-Fu Plain, an area impacted by distinct agricultural and urban-industrial activities. Groundwater exhibits similar hydrochemical types but differs in pH. Total REE concentrations are significantly higher in agricultural zones (avg. 390.57 ng/L) than in industrial areas (235.73 ng/L), with pronounced Light REE enrichment in both zones in terms of composition. Despite this concentrations contrast, NASC-normalized patterns consistently show relative Heavy REE enrichment. Geochemical modeling identifies carbonate complexes as the dominant species, with pH controlling REE abundance mainly through mineral dissolution, while anion complexation governs fractionation. Anthropogenic signatures are evident: elevated Light REEs link to agricultural inputs, whereas localized Gd anomalies and systematic variations in Eu and Ce anomalies trace urban-industrial and medical discharges. Agriculture attenuates Eu anomalies, whereas industry enhances them and reduces negative Ce anomalies. These findings elucidate the dual natural and anthropogenic controls on REEs in groundwater and establish their geochemistry as a sensitive tracer for discriminating between agricultural and urban-industrial pollution sources. This provides a novel geochemical tool for pollution source identification and groundwater management.