Geophysical and Hydrochemical Evaluation of Groundwater Potential and Character of Abeokuta Area, Southwestern Nigeria

Hydrogeological assessment of groundwater resources of part of Abeokuta area was carried out with a view to highlighting the potential of the aquifers to provide portable water supply, chemical character and provenance of the groundwater resources of the area. Seventy-five Vertical Electrical Soundings (VES) were distributed across areas underlain by different rock types. This was complemented with fifty groundwater samples collected from wells and analyzed for the major ions and seventy two minor constituents. Three to five sub-surface geo-electric layers were delineated from the VES. The layers resistivities (Ωm) from top to bottom vary from 24.2- 6428.3, 9.1-2250.0, 13.4-11563.1, 65.1-6654.5 and 400.2-9095.3 while the layers thickness (m) were 0.4-2.5, 0.6-30.0, 1.5-∞,3.4- ∞ and the undeterminable thickness respectively. The bedrock reflection coefficients vary from 0.4-1.0. Areas underlain by porphyritic granite and porphyroblastic gneiss have lower reflection coefficients, (0.82), higher lineament frequency, higher regolith resistivity (av. 119-167 Ωm) and fairly thick regolith (av. 9.2-13.76m). The relative abundance of the cations and anions in groundwater are Na>K>Ca>Mg and HCO3>Cl>SO4 correspondingly. The predominant water types include CaNaHCO3Cl and NaHCO3(Cl). High groundwater potential and yield were associated with areas underlain by porphyritic granite and porphyroblastic gneiss, with CaNaHCO3Cl and NaHCO3(Cl) dominant water-type in areas underlain by gneisses and granite