Evaluation of groundwater potential zones using integrated geophysical approach in Obun-Ewi, Ondo East local government, Southwestern Nigeria

Abstract

Obun-Ewi area is underlain by the Precambrian Basement Complex rocks of Southwestern Nigeria. The area suffers from a shortage of freshwater and the main source of water is rainwater. Groundwater provides an alternative solution to face the gap between water demand and available water in this area. Groundwater study involving the application of a very low frequency electromagnetic (VLF-EM) survey and vertical electrical sounding (VES) was carried out in parts of Obun-Ewi area Ondo State, Nigeria to evaluate the groundwater potential of the study area. ABEM WADI equipment was used to establish six (6) VLF-EM traverses with a station interval of 10 m. The inversion of the real component data of VLF-EM into 2D pseudo-sections was generated using Karous-Hjelt software. Linear features assumed to be minor fractures/conductors inferred from the filtered real pseudo-sections helped in choosing twenty-three (23) vertical electrical sounding points that were further probed using DDR-3 resistivity equipment. The Schlumberger electrode configuration was employed with half-current electrode separation (AB/2) ranging from 1 to 100 m. The partial curve matching technique was used to interpret the VES data quantitatively. The geoelectric parameters (depths and resistivities) were used to produce the geoelectric sections, isoresistivity map, isopach maps of the weathered layer and overburden, longitudinal unit conductance map and bedrock relief map of Obun-Ewi, Ondo town. The results obtained from the geoelectric sections revealed three subsurface geologic layers, namely: the topsoil, weathered layer (comprises of clay/sandy clay and laterite), and fractured basement/fresh bedrock. The topsoil resistivity varied from 56 to 1222 Ωm, and depth ranged from 0.5 to 1.8 m. The weathered layer resistivity values ranged from 16 to 1168 Ωm; while the thickness varied from 0.7 to 6.2 m. The overburden thickness varied between 1.6 and 7.2 m. The delineated weathered and fractured basement columns constituted the aquifer units. The composite groundwater potential map generated demarcated the study area into good, moderate and poor groundwater potential zones. The groundwater potential map revealed that about 69.6% of the study area fell within the poor groundwater potential rating, about 21.7% constituted the moderate groundwater potential rating while the remaining 8.7% constituted high groundwater potential rating. The study concluded that the groundwater potential of Obun-Ewi, Ondo town is generally poor, with areas around central South (around VES 9) and far Northern part of the study area (around VES 21) has good groundwater potential, suitable for siting boreholes.