Abstract

Geoelectrical survey and Geographic Information Systems (GIS) analysis were conducted in the hard-rock terrain areas of Ondo State, Southwestern Nigeria, with the view to determine impact of rock types on the variability of aquifer characteristics and their influence on the hydro geologic systems. The interpretation of the 414 sounding data indicates occurrence of 17 curve types which were used to infer subsurface lithology and geologic structures. A correlation of sounding curves and rock types revealed that two major aquifers: confined and unconfined aquifers occur mostly on the migmatite, pegmatite, Charnokite and undifferentiated fine-grained quartzite/schists. The third, leaky aquifers, occur predominantly on the biotites gneiss where occurrence of fracture is minimal. Confined aquifers are mostly associated with varying degree of fracturing of crystalline rocks. Estimated average hydraulic conductivity for the aquifer units are 4.43, 0.96 and 4.58 m/day, while their mean transmissivity values are 13.0, 8.71 and 60.18 m²/day respectively. From this work, it is evident that most aquiferous zones have poor to moderate permeability, which revealed why most wells in the area usually have low yield. While moderately permeable unconfined aquifer might be prone to contamination, the leaky or weathered/fractured aquifers can be tapped for groundwater development in the area.

Key words: Geo-electric, Geographic Information Systems, aquifer, transimissivity, hydrogeological.