Assessment of Crude Oil Contamination of Farmland Soils in Ogoni Land, Rivers State

Crude oil contamination of agricultural soils in Ogoni land, Rivers State, South South Nigeria, represents one of the most extensively documented and longest-standing environmental crises on the African continent. This study assessed the extent of crude oil contamination in farmland soils across three communities in Ogoni land, using chemical and biological indicators of contamination. Surface and sub-surface soil samples were collected from contaminated farmlands and adjacent uncontaminated reference sites. Total petroleum hydrocarbon concentrations were determined by the infrared spectrophotometric method following solvent extraction. Polycyclic aromatic hydrocarbon concentrations for sixteen EPA-priority compounds were measured by gas chromatography-mass spectrometry. Soil pH, organic matter, total nitrogen, and microbial biomass carbon were determined to assess degradation of soil ecosystem function. Total petroleum hydrocarbon concentrations in contaminated soils ranged from 12,400 to 48,600 mg/kg, far exceeding Nigerian regulatory standards. Total polycyclic aromatic hydrocarbon concentrations were 124 to 687 mg/kg, with potentially carcinogenic four- to six-ring compounds predominating. Contaminated soils showed dramatically lower pH, nitrogen, and microbial biomass compared to reference soils. Organic matter values were paradoxically elevated due to petroleum carbon input. The severity of contamination, particularly the presence of carcinogenic polycyclic aromatic hydrocarbons at high concentrations, underscores the public health emergency in Ogoni land and reinforces the need for the comprehensive remediation programme recommended by the United Nations Environment Programme 2011 assessment report.

Keywords: crude oil, soil contamination, polycyclic aromatic hydrocarbons, Ogoni land, Rivers State