Groundwater potential evaluation in the basement complex terrain of Ekiti East Local Government Area, Southwestern Nigeria

Abstract

Availability of groundwater often constitutes a problem in basement terrain because of its erratic occurrence. Incessant failures of boreholes have been reported in Ekiti East Local Government Area (EELGA), Southwestern Nigeria warranting evaluating its groundwater potential using a multi-criteria model that involved geology, resistivity and thickness (GRT) model of the aquifers across the study area. Thirty-five (35) locations were occupied using vertical electrical sounding (VES) technique employing the Schlumberger electrode array. The VES data were interpreted quantitatively by partial curve matching and 1-D computer iteration to facilitate siting of appropriate boreholes locations. The geophysical survey revealed twelve (12) types of resistivity curve: KH ($\rho 1<\rho 2>\rho 3<\rho 4)$, KQH ($\rho 1<\rho 2>\rho 3>\rho 4<\rho 5)$, KQQ ($\rho 1<\rho 2>\rho 3>\rho 4>\rho 5)$, QH ($\rho 1>\rho 2>\rho 3<\rho 4)$, KQ ($\rho 1<\rho 2>\rho 3>\rho 4)$, HKQ ($\rho 1>\rho 2<\rho 3>\rho 4>\rho 5)$, AK (ρ1 < ρ2 < ρ3 > ρ4), HA ($\rho 1>\rho 2<\rho 3<\rho 4)$, KHA ($\rho 1<\rho 2>\rho 3<\rho 4<\rho 5)$, HAA ($\rho 1>\rho 2<\rho 3<\rho 4<\rho 5)$, QHA ($\rho 1>\rho 2>\rho 3<\rho 4<\rho 5)$ and HKH ($\rho 1>\rho 2<\rho 3>\rho 4<\rho 5)$. The four-layer curve types observed within the area accounted for 57% of the curve types, while the five-layer types were 43%. The maps of aquifer layer resistivity and aquifer layer thickness were generated and synthesized with the geology of the study area in producing the GRT model groundwater potential map. The groundwater potential map revealed the northern part of the study area as an area with high groundwater potential, while the southern part was characterized as low groundwater potential terrain. EELGA is categorized into low, medium and high groundwater potential zones with the northern part more favorable for siting of boreholes. The groundwater potential map will serve as an exploration guide in addition to its usefulness in groundwater sustainability development.