Assessment of groundwater yield potential in Ekiti East Local Government Area, Southwestern Nigeria

Abstract

This study presents the categorization of Ekiti East Local Government Area into different groundwater potential zones with a view to reducing incessant boreholes failures and providing sufficient water to meet domestic and agricultural requirements of people in the area. To achieve this objective, 35 locations were sounded using the Schlumberger electrode array. Data obtained from the vertical electrical soundings (VES) were interpreted quantitatively by partial curve matching and 1-D computer iteration to facilitate the selection of an appropriate boreholes location. Thirty-five boreholes were subsequently drilled, and their yields were obtained during pumping test. Synthesizing the geo-electric parameters with the yields of the boreholes, the yield potential map was produced using Surfer 12 software. The VES survey revealed six geo-electric layers including the topsoil (44–1024 Ωm, 0.3–1.7 m), clay (26–494 Ωm, 0.9–30.8 m), laterites (353–1782 Ωm, 0.7–7.7 m), weathered zone–clayey soil (8.9–260 Ωm, 1.4–24.8 m), fractured layer (5.2–929 Ωm, 4.6–11.8 m) and fresh basement with resistivity that ranges from 474 to 77,144 Ωm and undeterminable depth. The bulk of groundwater was localized in the weathered and fractured layers due to availability of pores spaces arising from weathering and fracturing. Long-range in resistivity values especially in the topsoil was due to variation in soil’s composition and differential response to lithification. Two main rock units (migmatite gneiss and fine-grained granite) played major role in the groundwater occurrence of the study area. Thick weathered overburden developed more easily on migmatite gneiss than fine-grained granite. Aquifer yield map classified the study area into low (Eda Ile-Ekiti and Isinbode-Ekiti), medium (Omuo Oke-Ekiti) and high (Ilasa-Ekiti, Ikun/Araromi-Ekiti, Omuo-Ekiti and Kota-Ekiti) yield groundwater potential zones. Low to high groundwater zones cut across migmatite gneiss terrain with about 60% representation of high groundwater potential, indicating the mixed character of the rock. Fine-grained granite had only low groundwater potential coverage. Thus, lithology played a major role in the groundwater occurrence of the study area with terrain covered by fine-grained granite mainly in the low groundwater potential zone. The zones with high yield potential have been deciphered to serve as possible area for the future groundwater development and for choosing the drilling sites.