Abstract

Data from dc resistivity depth sounding and hydrogeologic measurements from Odigbo, Nigeria, were incorporated to delineate the groundwater prospect zones in the area. The area is underlain by the basement complex rocks of southwestern Nigeria, and characterised by biotite-gneiss and quartzite lithologies. The hydrogeologic data acquisition involved determination of depths (of wells) and static water levels which enabled determination of thickness of vadose and phreatic zones in 45 existing wells. The dc resistivity measurements involved Schlumberger depth soundings in 61 locations. The interpretation of the sounding data enabled the delineation of geoelectric sequence across the area. Depth to water table (groundwater head) ranges from 2.3 to 9.6 m while the thickness of the phreatic zone, an approximation of water column thickness, varies from 0.1 to 3.0 m, with the thickness in the range of 2.1 to 3 m constituting about 9%, and occurring in the central segment of the study area. The regolith overlying the basement rock constitutes the main water-bearing layer, since the resistivity parameters of the bedrock across the area do not suggest significant fractures or faults. The northwest – southeast and north – south trend of the aquifer unit within the regolith, with resistivity below 210 Ω-m, and constituting about 62% of the area, are considered the groundwater prospect zones. Based on aquifer thickness parameters, the groundwater prospect zones have been classified into low (< 20 m), medium (20 to 40 m) and high (> 40 m) respectively. 46% falls within the medium/high groundwater prospect rating while 54% falls within the low rating. The maps derived from the above are considered reliable prospect guides for groundwater developers in the area.

Key words: Groundwater, unconsolidated regolith, phreatic zone, resistivity sounding, geoelectric layers.