Geophysical and Hydro-Chemical Investigations of Oke Asunle Dumpsite in Ile-Ife, Southwestern Nigeria for Subsoil and Surface Water Pollution

BACKGROUND. Waste deposited in dumpsites immediately becomes part of the hydrological system. Over time, waste components bio-accumulate and/or decompose into contaminant liquid, leading to pollution of soil and water and posing a risk to human health. OBJECTIVES. The present study employed integrated hydro-chemical and geophysical methods to assess surface water and soil/subsoil within the premises of the Obafemi Awolowo University waste dumpsite in Ile-Ife, southwest Nigeria, for possible leachate pollution. METHODS. The electrical resistivity method involving 1D vertical electrical sounding (VES) and 2D dipole-dipole profiling techniques and hydro-chemical analysis were used. Two-dimensional profiling data were gathered along two orthogonal traverses and inverted into 2D resistivity images. Schlumberger VES data were gathered and quantitatively interpreted using partial curve matching and computer assisted 1D forward modeling. Hydro-chemical analysis was carried out on three water samples collected from the Asunle River for water quality testing. Anthropogenic pollution determinant parameters such as pH, conductivity, total dissolved solid, cations (calcium, magnesium, sodium, and potassium), anions (chloride, sulphate, biocarbonate, and nitrate) and the less abundant heavy metals in granitic gneiss-derived soil such as cadmium, copper, iron, manganese and lead were analyzed. RESULTS. Three geologic layers: topsoil, weathered basement and fresh basement were identified. Within the topsoil and weathered layer, two zones with contrasting geoelectrical characteristics were observed. The first zone, outside the dumpsite boundary, was characterized by relatively high resistivities (78–178 Ωm), typical of unimpacted soil. The second zone, within the dumpsite boundary, was characterized by relatively low resistivity values (15–47 Ωm) up to depth levels between 2.5 and > 15 m. The analyzed physico-chemical parameters, except for turbidity, fell within set limits for potable water quality. However, the concentration levels of heavy metals such as cadmium (0.017 - 0.018 mg/l); iron (0.544 - 0.739 mg/l) and lead (0.501 -0.551 mg/l) significantly exceeded standard limits. CONCLUSIONS. The results of the present study indicate that subsoil and surface water within and around the dumpsite can be considered to be polluted. COMPETING INTERESTS. The authors declare no competing financial interests.