Hydrogeological Evaluation and Lithological Characterization of Crystalline Bedrock Terrain of Ilora and Environs, Southwestern Nigeria

Abstract

In order to delineate possible water-bearing zones in area underlain by crystalline basement area of Ilora and its environs; hydrogeological attributes were inferred from the lithological characterization of geo-electric parameters interpreted from seventeen vertical electrical soundings (VES) conducted across the study area. The results of the geo-electric parameters showed that the saprolite resistivity was between 19 – 1677 ?m with the lithologies varying from clayey, sandy gravelly and compacted lateritic clay across the study area. The regolith thickness varies widely from 5 to 99 m while the topsoil thickness was between 0.1 and 6.2m. The bedrock resistivity was between 326 – 22589 ?m with areas underlain by migmatite having more resistive bedrock. The bedrock resistivities of areas underlain by quartzites and schist are less resistive, and are characterized by fractures and weathered basement with higher potential for groundwater occurrence. The hydrogeological inference from the lithological interpretation obtained from the geo-electric results revealed that the lithology of the weathered middle layer (saprolite) at north-center and south-east locations is more of compacted to loose clayey soils while that of the western half is sandy. There are generally have thick regolith development and the center region is underlain by fresh crystalline bedrock, which is not capable of storing and transmitting water. The results established the heterogeneity of the crustal units in areas underlain by typical basement rocks. Notwithstanding, locations with sandy/coarse saprolite and those with clayey overburden underlain by fractured bedrocks in the southwestern parts showed greater potential for groundwater prospecting in the study area