Detection and Delineation of Soil and Groundwater Contamination Zones using Geophysical and Geochemical Methods around Oke-Eleke Dumpsite, Ilorin, Southwestern Nigeria

Abstract

A combined geophysical and geochemical investigation was carried out at Oke-elele waste dumpsite in Ilorin, Kwara state, Nigeria. The aim is to determine the geoelectrical characteristics and groundwater chemistry to delineate the subsurface lithologies and assess the degree of soil and groundwater contamination. Nine (9) Vertical Electrical Sounding (VES) stations and four (4) Dipole-Dipole profiling (One as control traverse) were occupied for the geophysical investigation, while the hydrochemical investigation involved the chemical and physical analyses of the groundwater samples from the wells within the study area. The geo-electric sections, correlated with borehole data show that the study area is underlain by a maximum of six subsurface layers which include: topsoil (20 - 113 Ωm, 0.4-1.2 m); laterite (67-102 Ωm, 0.5-1.6m); Clay and plume infected layers have resistivity and thicknesses (8 - 20 Ωm, 1.1 – 4.7m); weathered/Partly weathered Basement (11 -211 Ωm, 1.5 – 17.2m); fractured basement (10 - 23 Ωm, 5-10m); and fresh bedrock (375- 99982 Ωm, infinity). Significantly low resistivity values (2.4-11.6 Ωm) were observed within the upper 25 m at regions suspected to be fractures or highly infected within the waste boundaries, showing that pollution plumes from the dumpsite have infiltrated the subsoil. In comparison, the resistivity values of > 20 Ωm were observed beneath the unimpacted subsoil. The hydrochemical analyses of the water samples from wells have shown that the anions (HCO₃^-, Cl^-, SO₄^2-, NO₃^-, and PO₄^3-) and cations (Ca²⁺, Na⁺, and K⁺) of the sampled water fell within the 4 3 4 World Health Organization (2006) and Nigeria Industrial Standard (2007) thresholds for potable water. However, the values of EC (1375-2000 μS/cm), turbidity (8.45-8.90), TDS (625-1025 mg/l), total hardness (238-406 mg/lCaCO₃), pH (6.25-6.33), manganese (6.58-7.82 mg/l), aluminium (5.45-6.46 mg/l), lead (0.02 mg/l), and cadmium were higher than the standard thresholds, an indication of groundwater pollution. Due to the increase in urbanization, it is recommended that erosion channels be improved upon, and the dumpsite and its environs should be regularly assessed.