Geo-spatial Classification of Vulnerability Zones using Lithological, Elevation and Geoelectric Parameters in a Typical Basement Complex Environment

Abstract

Lithology, elevation and four (4) geoelectric parameters were utilized in assessing the groundwater vulnerability at northwestern part of Akure, southwestern Nigeria. Vertical electrical sounding (VES) technique of electrical resistivity method was adopted for this work. A total of 224 VES data was acquired and interpreted both qualitatively and quantitatively. Three to five geo-electric layers were delineated across the area which corresponds to four geologic layers. The resistivity of the layers varies respectively from 6.9 - 550 Ohm-m, 60 - 2500 Ohm-m, 20 - 650 Ohm-m and 220 - 7900 Ohm-m in the topsoil, weathered layer, partially weathered basement/partially fractured basement and presumed fresh basement. Likewise, the layer thicknesses also vary respectively from 0.4 - 4.0 m, 0.7 - 19.0 m and 4.0 -60 m in the topsoil, weathered layer and partially weathered basement/partially fractured basement. The results were presented as topsoil (resistivity and thickness) and weathered layer (resistivity and thickness) maps. The six parameters consisting of lithology, elevation, topsoil (resistivity and thickness) and weathered layer (resistivity and thickness) were synthesized using an additive model in order to generate the aquifer vulnerability model map. The aquifer vulnerability model map shows that the area is of very low to moderate vulnerability with 5% of the area having very low vulnerability, 30% low vulnerability and 65% moderate vulnerability. This implies that the groundwater resources in the area are moderately safe.