Source, Transport, and Fractionation of Rare Earth Elements in Fluvial Sediments from a Typical Small Urban Basin (East Tiaoxi River, Eastern China)

As emerging contaminants, rare earth elements (REEs) have undergone significant anthropogenic enrichment in aquatic systems. This study investigates the REE concentrations, major metal elements, and grain size in surface sediments from the East Tiaoxi (ETX) River in eastern China, a small urban river subjected to substantial anthropogenic influences. Total REE concentrations of surface sediments ranged from 133.62 to 222.92 mg/kg with MREE enrichment and HREE depletion. REE concentration and fractionation were strongly correlated with Ca, Fe, Mg, and Mn, which may reflect the control of clay minerals, Fe-Mn oxides, and specific heavy minerals, and differences in REE behavior between riparian sediments and riverbed sediments highlighted the impact of hydrodynamic sorting and chemical weathering on REE distribution. Anthropogenic activities, particularly urbanization, were found to increase REE concentrations, especially at urban-adjacent sites (e.g., RBS2 and RS2), while natural processes such as soil transport and chemical weathering primarily contributed to REE variation at other sites. The enrichment factor and ecological risk assessment revealed that the enrichment and moderate risks associated with REEs occurred in river sediments adjacent to urbanized areas, though agricultural impacts were less pronounced. The findings emphasize the combined influence of urbanization and natural processes on REE distribution and ecological risks in the ETX River basin and underscore the need to prioritize urban-derived REE contamination in environmental management strategies.