Post-impact assessment of oil pollution in Agbada west plain of Niger Delta, Nigeria: field reconnaissance and total extractable hydrocarbon content.

Typical post-impact assessment of an oil-spill-impacted site in Agbada west plain of the Niger Delta basin of Nigeria was carried out 13 months after recorded incidence of spillage, using empirical indices of reconnaissance and extractable hydrocarbon content. Field-reconnaissance surveys revealed lower species' numbers and diversity of surface and subterranean flora and fauna. The presence and absence of such plant species as Elaeis guineensis, Musanga cecropiodes, and Andropogon gayanus, as well as animal species like earthworms and nematodes in sampled plots, corroborated the evidence provided by the total extractable hydrocarbon content (of (2.53+/-0.43)x10(2) mg/kg and (2.00+/-0.76)x10(2) mg/kg at surface and subsurface depths, resp.) on the level of degradation and/or regeneration at the polluted site. The most important evidence that oiling must have been responsible, at least in part, for the reduction in species' number and diversity was provided by the plant cover, Andropogan gayanus, which occurred at a lower density of 0.9 plants/m2 in the oil-impacted plots and 14.00 plants/m2 in the unimpacted reference plots of an adjacent, geographically similar area. The presence of this species on site thus presented a significant difference (P<0.05) of over 85%, and indicated exhaustion or impoverishment of impacted soils. Elaeis guineensis, with a population density of 0.1 stands/m2, provided evidence of past cultivation on site, while species like Musanga cecropioides, with a density of 0.5 stands/m2, at the unimpacted site, confirmed the abandonment of the farmstead over a long period of time. For the fauna, the most sensitive indicator of hydrocarbon toxicity was provided by nematode Xiphinema sp. that were completely absent in the oil-impacted site.