ABSTRACT
The distributions, source(s), and eco-toxic potential evaluations of sedimentary PAHs at the Okpu and Iyiowa-Odekpe sections of the River Niger was the focus of this study. Composite samples were taken in the months of June, September, December, and February, at five (5) different stations (ST-1, ST-2, ST-3, ST-4, & ST-5), and extracted with dichloromethane (DCM) using soxhlet extractor. After a silica gel clean-up, samples were analysed for the sixteen (16) priority PAHs listed in United State Environmental Protection Agency (US EPA) priority pollutants list with the aid of gas chromatography – mass spectrometry (GC-MS). Total PAHs (∑16PAHs) concentrations ranging from 252.10 – 5,392.00 µg/Kg with a mean of 1,954.76 ±474.48 µg/Kg were obtained for the different stations. The order of mean total PAHs (∑16PAHs) concentrations for the months was: Sep. > Jun. > Dec. > Feb., indicating significantly higher values in the rains than the dry seasons. The mean total PAHs and the total PAHs for all stations except ST-4 were below the 4,022 µg/Kg Effect Range Low (ERL) target value for sediment quality guidelines (SQG) signifying that ecological toxic effects would be rarely observed. However, the total toxic equivalent quotient (∑8TEQs) for the eight carcinogenic PAHs, implicated some stations to have likelihood of posing threat to the environment. Individual PAH concentrations ranged from ND – 872.87µg/Kg with a mean of 122.17±94.52µg/Kg. The dominant PAHs in the sediments were: BaP (14.0%), Acy (12.2%), Acp (11.5%), Flt (10.9%), and BbK (10.7%). The contribution to sediment toxicities was however, mainly due to BaP, accounting for 85.68 and 78.84% of ∑C8TEQs and ∑C8MEQs values respectively. The total toxic (∑C8TEQs) and mutagenic (∑C8MEQs) equivalent quotients ranged from 14.98 – 668.40 and 16.88 – 775.38 µg/Kg dw respectively with a mean of 320.47 and 348.15 µg/Kg dw respectively. The order of toxicity of the stations based on normalized total toxicity equivalence quotients (∑TEQs) values was: ST-5 > ST-4 > ST-3 > ST-2 > ST-1. Source apportionment using diagnostic ratios implicated mixed – petrogenic and pyrogenic – sources of PAHs. The pyrogenic contributions were however, more abundant. Developmental plans and actions necessary to discourage further increased anthropogenic inputs of PAHs in the sediments in order to mitigate negative impacts on aquatic ecosystem and sustain public health is imperative.
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