Time-lapse geoenvironmental hydrocarbon attenuation in Nigeria: integrated geoelectrical, geochemical and geotechnical site characterization

Hydrocarbon contamination from oil spills presents geoenvironmental and geoengineering challenges, notably in Eleme, Nigeria. This study integrates electrical resistivity tomography (ERT), soil total petroleum hydrocarbon (TPH) analysis, and geotechnical testing for treated spill site monitoring and characterization over six months. Four 100 m ERT lines, L1 to L4, with spacings at 1.5 m, 3 m, 6 m, 9 m, 12 m, and 15 m, were established for the first and second sampling phases. Twenty-one soil samples, 12 TPH, and 9 mechanical analyses, were obtained from 5 boreholes, BH1 to BH4, for the study site and the BH5 control site across the phases at 0.5 m, 3.0 m, and 5.0 m depths along ERT lines. ERT results reveal resistivity reductions averaging 18% in shallow zones of active degradation, correlating with an average 41% TPH-decrease. Specific gravity averaged 2.49 in the spill soils, compared to 2.58 in control samples, reflecting hydrocarbon-induced density reductions of 3.5%. Particle size showed spill soils contained >50% fines, increasing water retention and reducing permeability by 30%. Consolidation tests highlighted increased compressibility, with settlements of 1.89 mm in spill soils versus 1.01 mm in control samples, indicating a 47% increase in settlement from hydrocarbon reduction. Correlation analysis shows slower consolidation at BH3 (−0.62 Cv) with moderate settlement increase (0.25), while BH4 exhibits much higher compressibility (0.95) but minimal Cv impact (0.23), indicating increased structural weakness with higher residual TPH. Spill degradation reduced TPH by 19%–64% in shallow zones, with persistent contamination at deeper layers exceeding the regulatory limits, emphasising the need for ongoing monitoring and targeted remediation for long-term stability and sustainability.

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