GEOPHYSICAL AND HYDROLOGICAL MAPPING OF GROUNDWATER RESOURCES OF ERO, ONDO STATE, SOUTHWESTERN NIGERIA

Abstract

A combination of hydrogeologic and geophysical investigation was employed in Ero, Southwestern, Nigeria in an attempt to evaluate the area’s groundwater potential alongside the interference of aquifer susceptibility to contamination. The deployed technique entailed Electrical Resistivity method with hydrogeological measurement and hydrochemical analysis of hand dug wells in the area. The longitudinal unit conductance values which were corroborated with the top soil resistivity and vadose zone thickness was employed in the aquifer vulnerability assessment of the area. Qualitative and quantitative interpretation of schlumberger depth soundings delineated the subsurface into: topsoil, weathered layer, weathered/fractured basement and fresh bedrock. The interpretations reveal resistivity values in the range of 24 to 1112 Ohm-m for top soil, 12 to 1402 Ohm-m for weathered layer, 533 to 913 ohm – m to for weathered/fractured basement and 1020 Ohm-m to ∞ for fresh bedrock while thicknesses in the range of 0.4 to 1.3m for top soil, 0.7 to 10.2m for weathered layer and 0.4 to 16.2m weathered/ fractured basement. Hydrogeological measurements yielded thickness are in the range of 0.1-0.7m in terms of water column. The developed flow direction map depicts flow pattern from the northern and southwestern to southeastern and western part of the area. Hydrochemical analysis reveals a higher physico-chemical parameter concentration in the southern part than the northern part of the area. The resistivity distribution values of the aquifer units, its thickness together with the overburden thickness show that most of the study areas have low groundwater potential. In terms of aquifer vulnerability, it was revealed that the overburden protective capacity of the area can be classified into poor, weak and moderate protective capacity zones.