Polycyclic aromatic hydrocarbons in surface waters and riverine sediments of the Hooghly and Brahmaputra Rivers in the Eastern and Northeastern India

Highlights: • Mean concentration of total toxic PAHs were nearly ten folds higher in the surface riverine sediment of Hooghly compared with Brahmaputra. • Industrial effluents and port activities were the suspected sources of heavier PAHs in Hooghly riverine sediment. • Lighter PAHs in surface water are posing risk to fish species in both the rivers. Sixteen priority polycyclic aromatic hydrocarbons (PAHs) regulated by the United States Environmental Protection Agency (USEPA) were analyzed in surface waters and riverine sediments of Brahmaputra and Hooghly Rivers, along urban-suburban-rural transects. ∑₁₆ PAHs concentrations were higher in Hooghly riverine sediment (HRS) (Avg, 445 ng g⁻¹) than Brahmaputra riverine sediment (BRS) (Avg, 169 ng g⁻¹) dominated by 4-ring PAHs. In contrast, PAHs concentrations in surface water of Brahmaputra River (BRW) (Avg, 4.04 μg L⁻¹) were comparable with Hooghly River (HRW) (Avg, 4.8 μg L⁻¹), with dominance by 3-ring PAHs. Toxic PAHs (BaA, Chr, BbF, BkF, BaP, InP and DBA) were dominant in sub-urban transect of HRS (Avg, 387 ng g⁻¹) and BRS (Avg, 14 ng g⁻¹). Diagnostic ratios, principal component analysis (PCA) and ring wise composition suggested combustion as the main PAHs source in these riverine belts. In BRS, higher PAHs in suburban and rural transects were attributed to incomplete combustion of fossil fuel and biomass burning. In HRS, >85% of high molecular weight PAHs were found in the industrial areas of the suburban transect possibly associated with the discharge of industrial effluents. Harbor and port activities were other major contributors of HMW-PAHs in Hooghly riverine system. Carcinogenic potency estimated in terms of toxic equivalent (TEQ) was several folds higher in HRS (Avg, 106 ng TEQ g⁻¹) compared with BRS (Avg, 2.5 ng TEQ g⁻¹). Mostly low molecular weight PAHs are likely posing a risk to fishes in both the rivers. Risk on edible fish species may be a matter of concern considering the regular consumption of fishes in this region.