Geophysical and hydro-chemical investigation of the area around a waste dump site in Ile-Ife, southwest Nigeria

Abstract

An integrated geophysical and hydro-chemical investigation was carried out in the area around the Stadium-Apollo Market waste dump located in Igboya area of Ile-Ife. It was with the aim of determining the geoelectric characteristics and groundwater chemistry which were used in delineating the subsurface sequence and evaluating the possible pollution of groundwater in the area. Eight Vertical Electrical Sounding (VES) stations were occupied along two traverse lines trending North-South and East-West directions. Dipole-Dipole profilings were also carried out along same traverses. Eight water samples were collected from hand dug wells and analyzed for some chemical elements such as total acidity, major ions (Na+, K+, Cl-, Mg2+, Ca2+), NO3- and heavy metals (Cd, Cu, Fe, Hg, and Pb). Results of the investigation indicate that a maximum of four subsurface layers- the top soil, clay/sandy clay unit, the weathered layer and the basement bedrock exist in the area. The weathered layer constitutes the main aquifer unit with thicknesses of between 7 and 38 m. From the 2-D resistivity structures, contaminant plumes characterized by relatively low resistivity values (< 150 ohm-m) were identified down to depth of about 40 m in some places. Hydro-chemical results showed that the total acidity and concentration of Na+ Cl- and heavy metals (Cd, Cu, Fe, Hg and Pb) in wells closest to and down slope of the waste dump (total acidity 5.1-10.0 mg/l; Na+ 8.92-11.07 mg/l; Cl- 127.62-297.78 mg/l; Cd, 0.006-0.009 mg/l; Cu, 0.330-1.840 mg/l; Fe, 2.300-4.600 mg/l; Hg, 1.247 mg/l and Pb, 0.260 Mg/l) are higher than concentrations in distant wells, and most of them (Cl-, Cd, Cu, Fe, Hg, and Pb) exceeded national and international drinking water quality standards. It can be concluded that the groundwater in the area had been contaminated and the plume had migrated to a significant depth.

KEYWORDS: Geophysical, Hydro-chemical, Waste Dump, Groundwater Quality, Pollution