Anthropogenic gadolinium as an emerging contaminant in megacity rivers: Sources, fluxes, and policy implications in North China.

Anthropogenic gadolinium (Gdanth), as an emerging contaminant, poses considerable environmental risks in river systems, particularly those in megacities. However, studies on Gdanth source identification and pollution mitigation remain limited. This study systematically investigated the spatial variations, anomalies, possible origins, and associated health risks of rare earth elements (REEs) in the Yongding and Chaobai Rivers (YDCB), which are two representative urban river systems located in the Beijing-Tianjin-Hebei (BTH) region. A three-dimensional source tracing framework based on Gdanth, NO3-/Na+, and Cl-/Na+ ratios was established to accurately identify mixing pollution sources from medical, agricultural, and domestic wastewater. Positive Matrix Factorization model revealed three major contributors: industrial discharge (48.8 %), bedrock weathering (28.6 %), and medical wastewater (22.6 %). Although health risk assessments indicated that the hazard quotient values were <1, infants and young children were more vulnerable to exposure. Comparing with European regions, the BTH region in China exhibited significantly higher Gdanth fluxes (∼7.4 t/yr in total and ∼34.3 g/km2·yr per unit area), highlighting its role as a potential "super-source." This study also proposed a source-oriented policy framework including smart tracing via clinical management, advanced modular treatment at wastewater plants, developing substitution scheme, and trans-department monitoring system. These measures provided a transferable strategy for managing Gdanth in megacity river systems.