Hydrogeologic and Geophysical Assessment of Groundwater Prospects and Aquifer Vulnerability in Alade Idanre, Southwestern Nigeria

Abstract

Geophysical survey, involving Schlumberger depth soundings, were conducted at Alade, Ondo State, Nigeria. This was aimed at delineating the water supply areas and assessment of the vulnerability of the aquifers to contaminants infiltrating from the surface. The area is underlain by the basement complex rocks of Southwestern Nigeria. A total of 56 Schlumberger Vertical Electrical Soundings were carried out along the study area. The VES curves were preliminarily interpreted by partial curve matching. The interpretation results were refined with the WinRESIST software, and used to generate geo-electric sections and maps. The HA curves constitute the highest percentage of curves in the area. The geo-electric sections show three to four geo-electric layers which include topsoil, weathered layer and fresh basement. The topsoil varies between 0.4 and 3.6 m, and the depths to the bedrock are generally less than 7.0 m. The thickness of the vadose zones across the area ranges between 1.4 and 6.0 m. This shows that unconsolidated materials are generally not significantly thick and hence of apparently low groundwater prospect. The topsoil layer has a resistivity mostly within the range of 1 – 100 ohm-m across the area. Resistivity values within the bracket indicate clay sequence; this suggests that aquifers within the unconsolidated overburden at Alade Idanre are mostly capped by semi - pervious materials, geologically protecting the aquifer from near - surface contamination.