Integrated geophysical and geochemical methods for environmental assessment of subsurface hydrocarbon contamination

Oil spills have become a major source of environmental pollution in Nigeria, especially in the oil-producing communities. Assessment of hydrocarbon contamination within the subsurface is a complex program and requires both field-based measurement and geochemical analysis, to give a precise definition of the extent of pollution. To ascertain the extent of contamination of the subsurface around an oil spill site which was reportedly contaminated by oil pollution, an integrated geophysical method involving 1D vertical electrical sounding (VES) and 2D, 3D electrical resistivity tomography (ERT) techniques supported with geochemical method was employed. Orthogonal set of 2D resistivity data consisting of six (6) parallel and six (6) perpendicular profiles were acquired in a 100 × 50-m² rectangular grid using the Wenner array. Ten (10) vertical electrical soundings were also acquired in the site to supplement the 2D profiles. 2D resistivity data were processed individually using 2D Earth Imager software. The entire set of 2D resistivity data is collated into a 3D dataset using a 3D collation code and was inverted to produce a 3D resistivity structure of the subsurface, following from which contaminant plumes with high resistivity anomalies due to oil pollution were observed from 2D and 3D resistivity images. 2D resistivity tomography imaged contaminant plume to depth of 17.1 m, while 3D resistivity tomography imaged plumes of hydrocarbon contaminant to depth of 19.8 m within the subsurface. 3D depth slices imaged hydrocarbon contaminants within the third, fourth, and fifth layers at depths of 5.38-8.68 m, 8.68-12.5 m, and 12.5-16.9 m respectively. Thus, hydrocarbon contamination was evidently increasing with depth as observed in VES survey which indicated pollution to depth beyond 24 m, which goes beyond the depth to first and second aquifers in the area, indicating that the first and second aquifers have been invaded by oil pollution. Similarly, geochemical analysis of groundwater samples from three boreholes showed average concentration of total petroleum hydrocarbon (TPH) as 722.75µg/L which is higher than DPR limits indicating high contamination. Poly aromatic hydrocarbons (PAHs) concentration in the samples was summed up to 0.66µg/L in BH₁, 0.68µg/L in BH₂, and 0.30µg/L in BH₃, which are above the DPR target limit. BTEX compounds also showed elevated concentrations in the water samples. These findings validate the result of geophysical investigation; therefore, future research should aim to proffer remediation solutions to oil spill impact on the environment within the study area. The findings of this study can help for better understanding of the subject of electrical resistivity inversion, and particularly on 3D inversion technique for delineating subsurface plumes of hydrocarbon contamination.