Seasonal variation, and ecological risks assessment of heavy metals pollution in an upstream tropical freshwater river, Bangladesh

Heavy metal (HM) pollution in the Kushiyara River poses a significant environmental threat due to its toxicity and bioaccumulation. This study investigated seasonal HMs fluctuation in water and sediment by using Atomic Absorption Spectrophotometry. The vital physicochemical properties of water were significantly higher (<i>p</i> &lt; 0.05) in summer, except for dissolved oxygen and NH₃. HM concentrations in both water and sediment peaked in summer, followed by winter and monsoon. In water, Cd, Fe, Mn, and Ni were highest in summer, whereas Cu, Pb, Cr, and Zn were prominent in winter. Furthermore, Cu, Cr, Cd, Zn, and Mn were amplified in winter, while Fe, Pb, and Ni were elevated in summer sediment samples. Water quality indices—contamination index and HM evaluation index exceeded threshold limits in summer and winter, while HM pollution index remained high across all seasons. Sediment quality assessment indices <i>i.e</i>., geo-accumulation index indicated unpolluted to moderate pollution by Cd, while pollution load index demonstrated an overall unpolluted status. Cd was extremely enriched, with low to high ecological risk. Health risk analysis indicated that children are more vulnerable to non-carcinogenic and carcinogenic effects. A comprehensive framework requires seasonal monitoring, pollution control, and regional cooperation to sustainably reduce HMs pollution.