Groundwater potential assessment of shallow aquifers in the basement complex underlain Osogbo metropolis, Southwest Nigeria, using geo-electric attributes

The inadequate public water supply in Osogbo metropolis, Southwest Nigeria, necessitates the extraction of groundwater from shallow aquifers via hand dug wells and boreholes as an alternative source of potable water. However, crystalline rock aquifers are localized and discontinuous with limited vertical and lateral extent. This therefore informed the use of electrical resistivity attributes to appraise the groundwater prospect of the area. One hundred and nine (109) depth sounding data were acquired and quantitatively interpreted to generate the subsurface geo-electric layer parameters (resistivities and thicknesses). Weathered basement aquifer resistivity and thickness, bedrock relief, and resistivity anisotropy coefficient thematic maps were created using the Krigging interpolation technique. The thematic maps were integrated to evolve the composite groundwater prospect map of the area using the weighted linear combination overlay technique. Borehole yield data were used to validate the map. Three (3) to six (6) layers were delineated. These included the topsoil (13-4697 ohm m; 0.2-6.5 m), laterite (500-4727 ohm m; 0.3-7.5 m), weathered basement (7.0-641 ohm m; 0.8-57.6 m) (main aquifer), partly weathered/fractured basement aquifer (18-826 ohm m; 1.8-27.9 m), and fresh basement (485-20,728 ohm m). Weathered basement resistivity, weathered basement thickness, bedrock relief, and resistivity anisotropy coefficient thematic maps showed range < 100-641 ohm m; < 10-> 40 m; > 340 m (ridge), 320-340 m (pediment), < 320 m (basement depression), and 1.00-3.80 unit respectively. The composite groundwater prospect map revealed very low, low, moderate, high and very high groundwater prospect zones. The study concluded that the groundwater prospect of Osogbo metropolis was generally very low to moderate.