Groundwater Vulnerability Assessment using Electrical Resistivity Method in the Northern part of Ado-Ekiti, Southwestern Nigeria

Abstract

Over the years, the demand for clean and safe water has skyrocketed globally in response to rising population. Since groundwater is a more reliable source of potable water, assessment of aquifer vulnerability becomes a necessity. This research therefore aimed at delineating geo-electric parameters inform of layer resistivity and thickness, towards generating a second order geo-electric section. This study employed the use of electrical resistivity method to evaluate the vulnerability of aquifers to contamination in the northern part of Ado-Ekiti, southwestern Nigeria. Forty Vertical Electrical Sounding (VES) locations were studied using the Schlumberger array with current electrodes spacing (AB/2) of 150m. ABEM SAS 300 Resistivity Meter was useful for collecting the VES data which were presented as VES curves and quantitatively interpreted through partial curve matching and Computer Assisted 1-D forward modeling using the WinResist software. Geo-electric section along five (5) traverses, iso-resistivity map of the weathered layer and an isopach map showing the depth to aquifer were generated. Seven (7) different types of VES curves were identified, namely A, H, K, KA, HA, QA and KH types. The four geo-electric layers delineated include the topsoil (resistivity values between 29 – 1176 Ωm; thickness from 0.5 – 4.4 m), lateritic layer (resistivity values between 138– 3787 Ωm; thickness between 1.0 – 63.6m), weathered layer (resistivity values between 6 – 298 Ωm; thickness between 0.7 – 17.8 m), and fresh basement (resistivity values between 331 – 7479 Ωm). The total longitudinal unit conductance (S) values obtained range from 0.009 to 2.972 mhos. This suggests that most parts (northern, central, western and eastern portions of the area) have medium to high protective capacity rating; except the eastern portion. Therefore, most parts (80%) of the study area appear to be less vulnerable to contamination.