Electrical Resistivity Method in Delineating Vadose and Saturated Zone in some Selected Dumpsites in Ibadan Part of South-Western, Nigeria

Abstract

The impact of solid waste disposal on the groundwater within the vadose and saturated zone of two dumpsites namely Aba-Eku and Ajakannga in Ibadan Metropolis was investigated using Vertical Electrical Sounding (VES) to determine the depth and thickness of subsurface layers. Aba-Eku Landfill site is located at the extreme south-western area of Ibadan, along Ijebu road covering an area of approximately 3.5 acres, while that of Ajakannga landfill site is located at the south-eastern area of Ibadan, along Old Ijebu, covering an area of 3 acres, The areas under investigation, Ajakannga and Aba-Eku landfill sites are located within the Oluyole Local Government Area and Ona Ara Local Government in Ibadan, Oyo State respectively. Ajakannga is located between latitude 3 0 50 187 and 3 0 50 696 and longitude 7 0 18 021 and 07 0 18 997, Aba-Eku in Ona Ara Local Government is at latitude 3 0 59 009 and 3 0 59 973, longitude 7 0 19 270 and 7 0 19 843. Both qualitative and quantitative interpretations of the field data were carried out from Vertical Electrical Sounding (VES). The qualitative interpretation reveals points of anomalous resistivity along the traverses of 10m station separation. Partial curve matching and computer assisted iteration techniques were used for the interpretation of the VES data. The approximated topography of the dumpsite was plotted using GPS readings. The pictorial pattern produced gives an elevation model of the two dumpsites. An integrated analysis of qualitative and quantitative interpretations reveals the lithologies of the subsurface for VES with topsoil (0.5m-1.0m) with resistivity ranges from (9.2-68.3)Ohm-m, clayey soil (2.0-11.3m) with resistivity ranges from (2.2-12.30)Ohm-m, overlying the fresh basement of unknown thickness of the respective dumpsites. The result from the VES data gives a qualitative lithology of topsoil and clayey soil (aquitard) which has a low resistivity zone < 100 Ohm-m, underlain by higher resistive basement. The combination of both interpretations obtained from geophysical survey and the digital elevation map was used for the establishment of the environmental and health hazards associated with groundwater movement within the subsurface (vadose and saturated zones) along the two dumpsites.