Abstract

Forty (40) Schlumberger depth soundings and inventory of fifty-five (55) hand-dug wells were carried out in Ile-Oluji area, South-western Nigeria, aimed at assessing the groundwater prospect and aquifer vulnerability. The area is characterized by the crystalline basement rocks, mostly of granite gneiss lithology. Major challenges in this area are inadequate municipal water supply coupled with the hydrogeologically difficult terrain, individuals and corporate bodies indiscriminately sinking of tube wells and boreholes within the unconsolidated overburden materials, with obvious lack of concerns for the vulnerability status of aquifers and possible environmental risk. The acquired depth sounding data and wells inventory were critically interpreted and analyzed in order to assess the groundwater prospect in terms of geophysical parameters of the unconsolidated materials overlying the crystalline bedrock. The thickness of the unconsolidated overburden varies from 3 to 54.1 m, where about 35, 47.5 and 17.5% falls within 3 to 4 m, 15 to 30 m and 33 to 54 m brackets respectively, while static water level ranges from 1.7 to 12.9 m. The topmost geoelectric layer has resistivity mostly within 101 to 200 Ωm (50%) and 11 to 100 Ωm (30%) across the area. The resistivity values indicated that the overburden protective cover comprises of clayey sand and clayey materials respectively. This implies that the envisaged protective cover of the aquifers in the study area is generally fairly vulnerable. The interrelationship between the groundwater flow pattern and geoeletrical parameters enabled the rating of the study area into the category of moderate to low groundwater prospect. Finally, results of the geophysical study reasonably provide a basis for which the groundwater potential and vulnerability in the area were appraised.

Key words: Groundwater system, protective capacity, vulnerability, aquifer.