Hydrological seasonality reshapes sources and health risks of potentially toxic elements in a megacity river system of the Beijing-Tianjin-Hebei urban agglomeration.

Reclaimed water has been increasingly used to sustain urban river flows, whereas the influence of seasonal hydrological variations on the behavior of potentially toxic elements (PTEs) remains insufficiently characterized. In this study, the PTEs characteristics in China's most typical reclaimed water-influenced river system were investigated during the whole hydrological year. PTEs in wet season was accumulated in mainstream under stronger hydrological connectivity, whereas that in dry season shifted toward tributaries and local hotspots under reduced discharge and weaker dilution. PTEs sources during wet season were dominated by geological background coupled with Fe-mediated particle transport (37%), integrated industrial/urban inputs (39%), and urban non-point pollution (23%), while dry season was mainly associated with industrial activities, reclaimed water inputs, and internal remobilization. Estuary remained at the lowest pollution level throughout the hydrological year and was not the principal PTEs pollution accumulation zone. Although integrated pollution indices indicated more severe pollution during the dry season (Pn: 2.55-3.34), Monte Carlo simulations identified As as the dominant risk contributor, with higher mean non-carcinogenic and carcinogenic risks during the wet season. Wet-season mean hazard quotients (HQs) for As were 0.64 in adults and 1.44 in children; corresponding mean carcinogenic risks were 1.26×10-4 and 5.83×10-5, respectively. To address the seasonal mismatch among PTEs pollution, this study proposed a differentiated management framework for megacity river system that integrates seasonal diagnosis, source prioritized mitigation, reclaimed water upgrading, ecological buffer interception, and early warning of probabilistic risk, providing an innovative strategy for precise PTE control in reclaimed water-replenished urban rivers.