Spatial and seasonal variation, source and risk assessment of sedimentary polycyclic aromatic hydrocarbons from the Red River to the Ba Lat estuary (Vietnam).

The spatial-seasonal variation, source diagnostics, and ecological risks of 16 priority polycyclic aromatic hydrocarbons (PAHs) were examined in surface sediments of the Red River, from the capital city of Hanoi to the Ba Lat Estuary (Northern Vietnam). The average sum concentrations of the 16 PAHs in 21 sediment samples collected in March, June, and September 2023 were 35.0 ± 12.2 ng g-1 dry weight (dw), 56.0 ± 19.7 ng g-1 dw, and 43.3 ± 23.3 ng g-1 dw, respectively. The highest concentrations of PAHs were measured in sampling stations close to city centers and residential areas, where road traffic and domestic activities are prevalent. The PAH levels were significantly higher during the early rainy season (June) compared to the dry season (March). The composition of the 16 PAHs followed the same pattern for the three sets of samples: 4-ring >5-ring ∼3-ring >6-ring >2-ring. Diagnostic ratios suggested that PAHs at the three periods were mainly from pyrogenic sources, coming both from the combustion of biomass and fossil fuels. According to international sediment quality guidelines, the levels of PAHs quantified in the Red River would present minimal ecological risk to living organisms. As far as it concerns, this is the first study on the contamination of PAHs in the Red River, from Hanoi city center to Ba Lat Estuary in the Gulf of Tonkin.