Historical Trend of Polycyclic Aromatic Hydrocarbons Contamination in Recent Dated Sediment Cores from The Imo River, Southeast Nigeria

Inyang O. Oyo-Ita , Orok E. Oyo-Ita

The distribution and historical changes of polycyclic aromatic hydrocarbons (PAHs) contamination in recent sediment cores from the Imo River were evaluated over the last ca 5 decades. The concentrations of total PAHs (TPAHs-sum of parent and alkyl) ranging from 402.37 ng/g dry weight (dw) at the top layer of the Estuary site (ES6; 0-5 cm) to 92,388.59 ng/g dw at the near-top layer of the Afam site (AF5; 5-10 cm) indicated that PAHs contamination was localized not only between sites but also within the same cores. Sediment-depth profiles for the four (Afam-AF, Mangrove-MG, Estuary-ES and illegal Petroleum refinery-PT) cores generally revealed irregular TPAHs patterns, except the fact that the levels became maximized at the near-top layers (5-10 cm; ca.1997-2005). This time-frame coincided with the period of intensive oil bunkering/pipeline vandalism by the Niger Delta militant groups. The general decline in TPAHs levels up-cores to the most recent layers (0-5 cm) was attributed to the recent offer of amnesty to the Niger Delta militants and subsequent effort by the Nigerian government in clamping down the illegal activity of the economic saboteurs. Examination of perylene distributions down cores revealed natural terrestrial biogenic, pyrogenic and non-marine petrogenic origins for the compound at different sites. Thus, in view of the uncertainty surrounding the mechanism of perylene formation, the study highlighted the need for more attention to be paid to the involvement of terrigenous instead of aquatic OC in arriving at the probable mechanism for the compound.