Controls on heavy metal variability in a coastal mountainous river: Impacts of tides, typhoons, and anthropogenic activities

Understanding the ecological risks of heavy metals is critical for effective environmental management. Yet, the combined influence of tidal processes and tropical storms on heavy-metal dynamics in rivers remains poorly understood. This study examines the variations of heavy metals in sediments from a typical mountainous river in Southeast China, aiming to assess the combined influence of tides and typhoons on metal enrichment and ecological risks. The results show that from 2016 to 2019, there were no significant temporal changes in the ecological risks of heavy metals. Nevertheless, a distinct spatial pattern was observed, with higher levels in the tidal reach than in the non-tidal reach. This spatial variation is due to changes in sediment composition, such as shifts in source areas and grain size, which are driven by tidal processes. The clay fraction had significantly higher concentrations of heavy metals, enrichment, and ecological risks because metals are preferentially adsorbed onto fine-grained particles. Under normal conditions, tidal activity and anthropogenic inputs mainly affect the distribution of heavy metals. However, during typhoons, sediment mobilization intensifies, leading to a 2–4-fold increase in heavy metal concentrations and a higher risk of environmental contamination. The results of this study suggest that effective management of heavy-metal contamination in dynamic river systems requires a holistic approach that considers sediment composition, hydrodynamic conditions, and anthropogenic activities.