Hydrogeophysical evaluation of the groundwater potential of the central part of Ogun State, Nigeria

Abstract

Vertical Electrical Soundings (VES) and hydrogeological data were used to delineate the subsurface sequence and map the aquifer units with a view to evaluating the groundwater prospect of the central part of Ogun State, Southwestern Nigeria. Thirty Schlumberger VES with maximum current electrode spacing (AB) of 2000 m were carried out and interpreted by partial curve matching and computer assisted 1-D forward modeling using the WIN-RESIST software. The aquifer test data were analyzed using the Cooper and Jacob method and an empirical technique. The results indicated that the subsurface up to 1000 m (1 km) depth consisted of five major layers. The first layer, which was about 25 m thick with resistivity of 69 - 2795 ohm.m, was the residual soil composed of the topsoil, laterite and saprolite. The second layer, about 30 - 380 m thick, was a highly resistive (738 - 8942 ohm-m) partially saturated coarse-grained sandstone; with a saturated portion characterized by representative hydraulic conductivity (K) of 2.44 m/day, constituted the first aquifer unit. A clayey horizon characterized by layer resistivity of 40 ohm-m to 300 ohm-m and thickness of 30 m to 70 m was the third layer. The moderately resistive (331 1812 ohm-m) fourth layer, about 10 m - 900 m thick, was a fine-grained sandstone . that constituted the second aquifer unit with representative K value of 6.34 m/day The fifth layer was the highly resistive  geoelectric basement occurring at depths ranging from 30 - 1078 m. The study concluded that based on the representative K values and aquifer thickness, the sedimentary terrain could be classified as having high groundwater potential while the transition zone can be classified as having low to moderate  groundwater potential.