Groundwater resources assessment of part of Ilesa, Southwestern Nigeria.
An integrated geophysical survey involving the electromagnetic (EM) and direct current resistivity methods was carried out in part of Ilesha, Osun State, underlain by schists, quartzite and quartz schist complex, with a view to assessing the groundwater potential. The EM survey was conducted along eighteen (18) EM profiles with the Geonics EM34-3 using both 20 m and 40 m coil separations at 20 m station interval. The EM measurements were presented as conductivity profiles and were qualitatively interpreted for high conductivity zones. The resistivity survey involved Schlumberger Vertical Electrical Soundings (VES) with maximum AB/2 of 600 m at fourteen (14) locations that were constrained by the EM anomalies. The VES data were quantitatively interpreted by partial curve matching and computer assisted 1-D forward modelling. Some anomalous conductivity zones that were typical of fractured basement were identified from the EM profiles. The VES delineated fractures at deep depth within areas underlain by quartzite or quartzitic schists. Two test boreholes (BH-1 and BH-2) were drilled with mud rotary/down-the-hole hammering methods and pump tested using standard procedure. BH-2 drilled at location with very high EM conductivity (375 mhos/m) and thick fractured basement gave very high groundwater yield of 65 m3 /h (18 l/s), while BH-1 drilled at location with relatively low conductivity (40 mhos/m) and thin fractured basement gave low yield of 1.6 m3 /h (0.46 l/s). The study concluded that contrary to previous view of generally low prospect for groundwater development in the basement complex of Ilesa, some part of the study area was characterized by high groundwater potential.