Geoelectrical Assessment of Subsurface Lithology and Potential Hydrocarbon Contamination: A Case Study of Evbareke Automobile Market, Benin City, Nigeria

Abstract

Automobile spare-parts markets are potential sources of subsurface contamination due to prolonged hydrocarbon spills and improper waste handling, posing risks to shallow aquifers in urban environments. This study aims to characterize subsurface lithology and evaluate aquifer vulnerability to possible hydrocarbon contamination, addressing the lack of spatially continuous subsurface evidence in existing point-based geochemical studies. Vertical Electrical Sounding (VES) using the Schlumberger array was employed at four stations within Evbareke Automobile Market, Egor Local Government Area, Benin City, Nigeria, underlain by the Benin Formation of the Niger Delta Basin. Field data were acquired using a PASI Earth Resistivity Meter and inverted to generate one-dimensional geoelectric models with root-mean-square (RMS) errors generally less than 5%. The interpreted results resolved three to four geoelectric layers, comprising topsoil (85–120 Ωm; 0.6–1.2 m), clayey or sandy-clay units (20–60 Ωm; 3–7 m), and thick sand horizons (180–750 Ωm) extending beyond depths of 15–25 m. VES 1 and VES 4 delineated productive aquifer zones characterized by thick, moderately resistive sand units, indicating high groundwater potential. However, abnormally high resistivity values ranging from >900 to 1600 Ωm were observed at VES 3 and parts of VES 4, interpreted as possible hydrocarbon-impacted zones, considering the long-term automobile-related activities at the site. These resistivity signatures are consistent with previously reported geochemical and physicochemical contamination indicators in Benin City, thereby providing independent geophysical validation. The novelty of this study lies in integrating 1-D resistivity imaging to spatially delineate subsurface lithology and contamination-prone zones beneath an active automobile market, bridging the gap between localized geochemical measurements and subsurface hydrogeological interpretation. The findings highlight the dual role of automobile markets as both groundwater resource zones and contamination hotspots, underscoring the need for integrated geophysical geochemical monitoring and sustainable environmental management strategies.