Seasonal Bioaccumulation and Sources of Heavy Metals in Water, Sediment, and Commercial Fish Species from Taunsa Reservoir, Indus River: Implications for Human Health Risk.

Heavy metal pollution in freshwater ecosystems poses risks to both aquatic life and human health through bioaccumulation in fish. This study evaluates seasonal bioaccumulation of heavy metals (Pb, Zn, Cu, Cr, Cd) in three fish species (Catla catla, Labeo rohita, Wallago attu), water, and sediment from the Taunsa Reservoir on the Indus River, Pakistan, and assesses associated health risks. A total of 300 fish and 60 water and sediment samples were collected during summer and winter. Heavy metal concentrations were analyzed using atomic absorption spectrophotometry (AAS). Results showed that in water Zn had the highest concentrations (2.01-9.74 mg/kg), while Cr and Cd frequently exceeded permissible limits, with peak levels of 0.56 mg/kg (Cr) and 1.50 mg/kg (Cd) in W. attu during summer. PCA revealed that sediment acted as the dominant driver of metal accumulation, reflecting the ecological mechanism whereby benthic deposition enhances exposure, especially in bottom-associated species. Cd emerged as the most concerning toxic metal, with the highest THQ (0.25 in W. attu during summer), while other metals remained within acceptable non-carcinogenic risk ranges. Summer showed higher accumulation compared to winter. Cd was identified as the most concerning metal, with a target hazard quotient (THQ) of 0.25 in W. attu (summer). Hazard index (HI) values remained below 1, indicating no immediate risk, but elevated Cd and Cr levels pose long-term threats. Carcinogenic risk assessment also revealed Cd as the primary contributor to cancer risk, especially during summer. These findings underscore the urgent need for targeted mitigation of sediment-borne Cd and Cr pollution to reduce long-term ecological and human-health risks. Given its consistently high metal loads, W. attu is recommended as a bioindicator for environmental assessment.