INTEGRATED GEOPHYSICAL METHODS IN THE CHARACTERIZATION OF SUBSURFACE CONDITIONS OF LAPITE DUMPSITE IN IBADAN, SOUTHWESTERN NIGERIA

Résumé

Groundwater contamination within the vicinity of dumpsites has elicited public health concern in recent time. Studies have shown that contaminants may infiltrate into the groundwater through permeable soils, weathered and fractured bedrocks. Lapite village, underlain by Basement Complex rocks, hosts one of the largest and most active dumpsites in Ibadan. The dumpsite came into existence without prior hydrogeophysical evaluation of its suitability as a landfill site, especially its likely impacts on the surrounding groundwater. This study was designed to assess the suitability of the dumpsite for waste disposal by evaluating hydrogeophysical properties of rocks in the area. Geological field mapping was carried out around Lapite dumpsite to determine various rock types. Very Low Frequency-Electromagnetic (VLF-EM) measurements consisting of four transverses and sixty-one Vertical Electrical Soundings (VES, using Schlumberger array) were carried out along the 200.0 and 400.0 m of the dumpsite. Banded and migmatite gneisses were the underlying rocks in the area. The VLFEM pseudo-section showed high conductivity zones indicating the presence of fluid-filled fractures in the rocks. The VES geoelectric section revealed four layers namely, topsoil (33.6-973.7Ωm), lateritic sand (82.0-520.4Ωm), weathered layer (11.9-133.0Ωm) and fractured/fresh bedrock (387.0-3767.4Ωm) with thicknesses (m) of 0.1-2.8, 0.8-4.8, 1.2-49.2 and undeterminable respectively. The presence of fractured basement as indicated by VLF-EM pseudo-section and VES geoelectric section may act as a transmitting medium for leachate that may emanate from waste dump into the surrounding groundwater. Lapite dumpsite is located on weathered and fractured bedrocks and these may permits infiltration of contaminants into the surrounding
groundwater. Thus, the present condition of waste dumpsite is not suitable for an ideal waste dumpsite.