AN APPRAISAL OF GROUNDWATER RESOURCES OF THE BASEMENT COMPLEX OF ABEOKUTA, SOUTHWESTERN NIGERIA: USING ELECTRICAL RESISTIVITY AND BOREHOLE DATA EVALUATION

Access and viability of aquifer in Abeokuta, Southwestern Nigeria varies tremendously. Available data suggests that wrong point selection, poor data quality from the subsurface, lack of technical knowhow and poor development of drilled holes among others are responsible for low yielding aquifers. This problem is common in basement complex terrains where most of the aquifers are localized. This work describes the reason why some aquifers are productive and others are not. We estimated the depths of various boreholes, the resistivity values at which water can be obtained, thickness of each layer found and the efficiency of electrical resistivity method in the location of boreholes within the study area.

Using the Hummel method (modified Schlumberger method) to address accessibility constraint, the potential electrodes were temporarily fixed while the current electrode was kept fixed throughout the sounding operation at a distance equal to or greater than the mobile current electrode spacing (preferably at infinity) and perpendicular to the line of electrode spread. A few soundings were established and lithologs were correlated, using already existing boreholes. It was observed that the thickness of the top soil within this area was between 0- 2.5m, which was followed by the lateritic sandy clay having a general thickness ranging from 2.5m- 5.5m (depth of 2.5m- 8.0m). The weathered basement occurred at a depth ranging between 8.0m- 34.0m, and the fresh basement was encountered at a depth below 34.5m. The resistivity values at which water can be obtained ranged from 35.0 - 150.4 Ωm. Some of the wells investigated have quartz vein and mottled clay.

In conclusion, drilling in the area should be below 40m. Correlation between the lithologs and the sounding locations established have proved that Vertical Electrical Sounding (VES) is a very efficient method for the location of boreholes because it provides detailed geo-information on the type of layers to be encountered before drilling.