Trace heavy metals associated with crude oil: a case study of Ebocha-8 oil-spill-polluted site in Niger Delta, Nigeria.

The Ebocha-8 oil-spill-polluted site was identified as study site according to field reconnaissance surveys of oil-polluted sites in Obiobi/Obrikom prospect area of Niger Delta. A sampling area was delimited at each of the oil-spill-affected sites by the grid technique, and soils were collected at surface (0-15 cm) and subsurface (15-30 cm) depths from three replicate quadrants. A geographically similar, unaffected area, located in 50-m distance from each contaminated site, was chosen as control site. Trace heavy metals, Ni, V, Cu, Cd, and Pb, which are normal constituents of crude oil, were determined in the soils by atomic-absorption spectrophotometry after pre-extraction of cations with dithionite-citrate carbonate. Ni varied from 0.53 to 18.00 mg/kg, Cu from 0.15 to 0.30 mg/kg, and Pb from 0.32 to 0.80 mg/kg in surface and subsurface soils, respectively; Cd and V were less than 0.20 mg/kg in all sampled plots. Ni, Cu, and Pb were more enhanced (P<0.05) in the oil-spill-polluted soils, especially at surface depth, and this may be attributed to the fact that metal profiles in polluted soils penetrate a little below the 10-cm region, even after many years, thereby making the metal concentration in surface soils usually higher. Whilst the Ebocha-8 oil spillage may be responsible, at least in part, for the enhanced concentrations and paucity of variations in the metal concentrations of sampled plots, other factors such as the physicochemical characteristics of soils (e.g., soil pH and organic-matter content) and relative mobility of these metals, as well as the intense rainfall and flooding that preceded the period of sampling, may also have contributed in part. Enhanced levels of these micronutrients in the soils may result in enhanced absorption by plants, which may bring about possible bioaccumulation by such plants and the animals that depend on them for survival, and all of these may lead to toxic reactions along the food chain.