Heavy metal contamination and ecological risk assessment in sediments of the middle Gangetic Plain, at the Patna region, India

Metal pollution in the sediment of the River Ganga emerged as a severe concern over decades, where the middle stretch at Patna has been poorly studied. The study aims to document the pollution status of Pb, Cd, and Hg and associated ecological risk in the sediment of the River Ganga at Patna. A total of 144 sediment samples were collected from four sampling locations during the pre-monsoon and post-monsoon seasons over two years. The collected sediment samples were air-dried and analyzed using an Atomic Absorption Spectrophotometer and a Mercury Analyzer following standard digestion procedures. The result revealed that the studied sediment samples were contaminated with 5.4–77.95 mg/kg of Pb, 0.27–3.95 mg/kg of Cd, and 0.19–1.75 mg/kg of Hg. Meanwhile, Cd and Pb experienced a significant seasonal decrease during the post-monsoon period. The concentrations of Pb, Cd, and Hg have surpassed the Threshold Effective Concentration (TEC) in 27.80%, 77.09%, and 100% of the samples, respectively. In addition, various pollution indices have classified the sediment of the study area as unpolluted to severely polluted, with Cd posing considerable to very high ecological risk during the study. Additionally, the study found high metal loads near sites receiving direct discharges of domestic waste, confluence inputs, and agricultural fertilizer runoff, highlighting the influence of point-source pollution. The findings of this study provide comprehensive data that can support river management strategies and assist policymakers in mitigating metal pollution in the River Ganga.