The study focused on the Ecological Risk Assessment of Heavy Metals and Polycyclic Aromatic Hydrocarbons (PAH) in Ikpoba Reservoir, Benin City, Nigeria, in order to ascertain the impact of these substances on the investigated ecosystem. Samples of environmental matrices (fish, water and sediment), were collected from four (4) Stations within the Reservoir from August 2005 to November 2007. The concentrations of detected heavy metals (Cd, Mn, Cu, Fe, Pb, Ni, Zn and Cr) were determined in fish,
water and sediment with Atomic Absorption Spectrophotometric technique while the concentrations of detected PAH were determined in dominant fish species, Synodontis clarias (Linnaeus, 1758), Hemichromis fasciatus (Peters, 1857) and Marcusenius psittacus (Boulenger, 1897) and sediment with Gas Liquid Chromatographic technique. The mean concentrations of Cd, Mn, Cu, Fe, Pb, Ni, Zn, Cr in water were 0.0132 mg/l, 0.026 mg/l, 0.44 mg/l, 8.05 mg/l, 0.075 mg/l, 0.033 mg/l, 100.75 mg/l and 0.033 mg/l respectively while the mean concentrations of the heavy metals in sediments were 1.03 mg/kg, 43.11 mg/kg, 14.75 mg/kg, 4434.95 mg/kg, 59.09 mg/kg, 11.95 mg/kg, 78.63 mg/kg and 0.32 mg/kg respectively.
The mean concentration of Cd, did not exceed 0.01mg/kg in the investigated fish species while the mean concentrations of Mn ranged from 1.27 mg/kg in H. fasciatus to 4.75 mg/kg in S. clarias. The mean concentrations of Cu ranged from 0.12mg/kg in H.fasciatus to 0.38mg/kg in M. psittacus while the mean concentrations of Fe ranged from 19.98mg/kg in S.clarias to 33.41mg/kg in M.psittacus.The mean concentration of Pb ranged from 0.56mg/kg in H.fasciatus to 2.45mg/kg in M.psittacus while the mean concentrations of Ni ranged from 0.023 mg/kg in H.fasciatus to 0.027 mg/kg in M. psittacus.The mean concentrations of Zn ranged from 4.53mg/kg in H.fasciatus to 18.15mg/kg in S.clarias while the mean concentrations of Cr ranged from 0.016 mg/kg in M. psittacus to 0.022 mg/kg in S. clarias. The mean concentrations of PAH in fish varied, ranging from 0.737µg/g for acenaphthene in M.psittacus to 19.382 µg/g for benzo(a))anthracene in H.fasciatus. The mean concentrations of PAHs recorded in fish did not exceed the Commission Regulation (EC) maximum allowable level of 2.0µg/kg for benzo(a)pyrene in food and fish.The mean concentrations of PAH in sediment ranged from 0.00365 µg/g for benzo(a)anthracene to 1.687 µg/g for chrysene.The mean concentration of Fe in water exceeded the World Health Organisation (WHO) maximum allowable limit for drinking water while the mean concentrations of Mn and Fe in the fishes and Pb in M. psittacus exceeded the WHO maximum allowable limit for food and fish. The Background Concentrations (BC) of Cd, Mn, Cu, Fe, Pb, Ni, Zn and Cr in sediment were, 0.01, 27.03, 10.91, 6348, 13.55, 21.66, 17.83 and 0.99 while for water the BC values were 0.000.14, 0.015, 0.321, 11.34, 0.016, 0.063, 19.56 and 0.117 respectively.
In the fish species, the BC values for the aforementioned heavy metals were, 0.000104, 0.73-2.73, 0.088-0.277, 28.17-47.06, 0.12-0.52, 0.044-0.052, 0.87-3.52 and 0.06-0.08
respectively.The mean concentrations of Cd, Mn, Cu, Pb and Zn exceeded their respective background concentrations in sediment, water and fish.The potential ecological risk (Ri) of heavy metals in the Ikpoba Reservoir was low with a value of 61.42. Cadmium, presented the greatest potential individual risk to fish while potential PAH toxicity to fish was indicated for 1-methylnapthalene, acenapthene, flourene, pyrene and benzo(a)anthracene. The study revealed that the presence of PAH in the Reservoir was chiefly from pyrogenic sources. Recommendations were suggested for an effective water pollution control and management of the Reservoir, based on findings from the study to include, further monitoring of the concentrations of heavy metals and Polycyclic Aromatic Hydrocarbon compounds in order to generate adequate histopathological data on the effects of these pollutants on the ichthyofauna of the Reservoir with a view to mitigate potential health hazards to fish and ultimately to man.