Occurrence, distribution, and pollution indices of potentially toxic elements within the bed sediments of the riverine system in Pakistan

Potentially toxic elements (PTEs) are a major source of pollution due to their toxicity, persistence, and bio-accumulating nature in riverine bed sediments. The sediment, as the largest storage and source of PTEs, plays an important role in transformation of mercury (Hg), lead (Pb), nickel (Ni), chromium (Cr), copper (Cu), zinc (Zn), and other toxic PTEs. Several important industrial hubs that contain a large population along the banks of different rivers, such as Kabul, Sutlej, Ravi, Jhelum, and Chenab in Pakistan, are acting as major sources of PTEs. In this study, 150 bed sediment samples (n=30 from each river) were collected from different sites. Total (acid extracted) PTE (Hg, Cu, Cr, Ni, Zn, and Pb) concentrations in bed sediments were determined using inductively coupled plasma mass spectrometry (ICP-MS). Sediment pollution indices were calculated in the major rivers of Pakistan. The results demonstrated high levels of Hg and Ni concentrations which exceeded the guideline standards of river authorities in the world. The contamination factor (CF) and contamination degree (CD) indices for Hg, Ni, and Pb showed a moderate to high (CF >= 6 and CD >= 24) contamination level in all the selected rivers. The values of geo-accumulation index (I-geo) were also high (I-geo >= 5) for Hg and Pb and heavily polluted for Ni, while Cr, Cu, and Zn showed low to unpolluted (I-geo) values. Similarly, the enrichment factor (EF) values were moderately severe (5 <= EF <= 10) for Hg, Pb, and Ni in Sutlej, Ravi, and Jhelum, and severe (10 <= EF <= 25) in Kabul and Jhelum. Moreover, Hg and Ni showed severe to very severe enrichment in all the sampling sites. The ecological risk index (ERI) values represented considerable, moderate, and low risks, respectively, for Hg (The ERI value should not be bold. Please unbold the ERI in the whole paper. It should be same like RI, CD and EF. 80 >= ERI >= 160), Pb and Ni (40 <= ERI <= 80), and Cr, Cu, and Zn (ERI <= 40). Similarly, potential ecological risk index (PERI) values posed considerable (300 <= RI <= 600) risk in Ravi and moderate (150 <= RI <= 300) in Kabul and Jhelum, but low (RI <= 150) risk in Ravi and Chenab. On the basis of the abovementioned results, it is concluded that bed sediment pollution can be dangerous for both ecological resources and human beings. Therefore, PTE contamination should be regularly monitored and a cost-effective and environmentally friendly wastewater treatment plant should be installed to ensure removal of PTEs before the discharge of effluents into the freshwater ecosystems.