Groundwater exploration in either a basement or sedimentary environment is often fairly well defined and focuses on delineating weathered/fractured rocks or saturated formations, respectively. Conversely, unique geological structures, the complex coexistence of different rock types and poorly defined basal/lateral contacts between basement and sedimentary rocks make groundwater development in a geological transition environment very challenging. Ijebu-Ode and its environs lie within such a problematic transition zone, between the Precambrian basement rocks and Cretaceous sediments of the Dahomey Basin, in southwestern Nigeria, where associated acute groundwater development challenges require adequate subsurface information to maximize its groundwater resource potential. This study integrated very low frequency electromagnetic (VLF-EM) and electrical resistivity (ER) geophysical prospecting techniques for a detailed terrain study of Ijebu-Ode in order to establish the reasons for the low groundwater resource potential in the area. Thirty five VLF-EM profiles, 140 vertical electrical soundings (VES) and relevant hydrogeological data were acquired along grids and profiles. Data were filtered, inverted and enhanced using appropriate software packages. The current density and geoelectric parameters of the VLF-EM and VES data were employed to generate terrain maps, the conductivity distribution and a subsurface basement model of the study area. Current density plots and geoelectric parameters identified up to three layers in the basement complex terrain which comprised lateritic topsoil, weathered basement and fresh basement rocks. The five layers encountered in the sedimentary terrain were topsoil, a lateritic unit, a dry sandy unit, a saturated sandy unit and fresh basement rocks. The hydraulic conductivity of the thick (3–18 m) lateritic unit was determined to be 1.32 × 10−5 mm s−1, while that of the underlying sandy units ranged from 2.65 × 10−4 to 1.36 × 10−3 mm s−1. The thick but less permeable lateritic unit which overlaid the more permeable rocks constituted a partial impermeable overburden that prevented an adequate groundwater recharge during and immediately after rainfall. Three zones were delineated as low, medium and high groundwater resource potential areas, located in the northern, central and southern part of the study area, respectively. The low groundwater resource potential around Ijebu-Ode is due to the less permeable lateritic overburden, which prevents an adequate recharge during rainfall and the rugged/undulating basement topography that controls the distribution and storage of the limited recharged water. The southern part of the study area can be developed to meet the water needs of the inhabitants.
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