Abstract:

Rapid urbanization and indiscriminate waste disposal have intensified soil and groundwater contamination in many Nigerian cities. This study presents an integrated geophysical and hydrogeochemical assessment of subsurface contamination associated with major urban waste dumpsites within Abakaliki Metropolis, southeastern Nigeria, underlain predominantly by fractured and weathered shale of the Asu River Group. Vertical Electrical Sounding (VES) employing the Schlumberger electrode configuration and two-dimensional (2-D) electrical resistivity imaging using the dipole–dipole array were conducted at three major dumpsites: Rice Mill, Umuoghara, and Mechanic Village. The acquired resistivity data were processed using Advanced Geosciences Incorporated (AGI) inversion software and Res2Dinv software to delineate subsurface lithology, fracture systems, aquifer characteristics, and leachate-contaminated zones. The geophysical results revealed low resistivity values ranging from 7–19 Ωm within the topsoil and weathered shale units, indicating highly conductive zones associated with moisture accumulation and leachate infiltration. Moderately to highly resistive layers (118–24,201 Ωm), interpreted as sandstone lenses, silty sand, and siltstone units, were identified at intermediate depths, while deep conductive zones with very low resistivity values (0.18–3.04 Ωm) were interpreted as fractured and saturated shale aquifers vulnerable to contaminant migration. To validate the geophysical interpretation, physicochemical and heavy metal analyses were carried out on soil and surface water samples collected around the investigated dumpsites. Elevated concentrations of heavy metals, including Pb, Cd, Hg, Cr, and Ni, were detected in several samples, with some values exceeding World Health Organization (WHO) permissible limits. The integration of geophysical, physicochemical, and geochemical data confirms the occurrence of leachate-induced subsurface contamination and demonstrates the vulnerability of fracture-controlled aquifer systems within the study area. The study demonstrates the effectiveness of integrating electrical resistivity techniques with hydrogeochemical analyses for environmental assessment and groundwater vulnerability studies in shale-dominated terrains affected by uncontrolled waste disposal activities.