GEOPHYSICAL AND HYDROCHEMICAL INVESTIGATIONS OF ABEOKUTA AND IKORODU AREAS OF SOUTHWESTERN NIGERIA

Abstract

Most studies on groundwater in Nigeria have always been limited to either their occurrence or chemistry, with insufficient attention on potential, chemical character and evolution. This study therefore was designed to evaluate groundwater in Abeokuta (Basement Complex) and Ikorodu (Sedimentary) areas with the aim of highlighting groundwater potential, chemical characteristics and provenance in the two geologically contrasting terrains. A total of 125 Vertical Electrical Soundings (VES) in Abeokuta (75) and Ikorodu (50) areas were carried out using Schlumberger array. The field data were curve-matched and computer iterated. Fifty groundwater samples were collected from shallow hand-dug wells in Abeokuta area while 50 groundwater samples were collected from shallow wells (16) and borehole (34) from Ikorodu area and analysed using the Inductively Coupled Plasma Optical Emission Spectrometry and Ion Chromatography. Stable isotopes (18O and 2H) of 10 water samples from each geological terrain were determined using the Optima Dual-Inlet Mass spectrometer. Data were analysed using linear regression. Three to five sub-surface geoelectric layers were identified in Abeokuta area. The layer resistivities (Ωm) from top to bottom ranged from 24 - 6428, 9 - 2250, 13 - 11563, 65 - 6655 and 400 - 9095 while the thicknesses (m) were 0.4 - 2.5, 0.6 - 30.0, 1.5 - ∞, 3.4 - ∞ and the undeterminable thicknesses respectively. The bedrock reflection coefficients vary from 0.4 - 1.0 delineating confined, unconfined and weathered/fractured basement aquifers. Areas with confined and weathered/fractured basement aquifers have high groundwater potential. In Ikorodu, three to six sub-surface layers of alternating sequences of clay, sandy-clay, ferruginised sands and sands were identified. The resistivities (Ωm) of these layers are 11.2 - 588.6, 3.3 - 3787.7, 7.4 - 7789.4, 5.7 - 3592, 27.8 - 5785.1 and 9.8 - 822.9 while the thicknesses in metres were 0.5 - 4.4, 0.8 - 60.1, 1.5 - 101.9, 6.6 – 154.1, 35.5 – 112.4 and undetermined respectively. Areas delineated as confined and unconfined aquifers in Ikorodu have high groundwater potential. In Abeokuta, Ca(Na)2(HCO3)2(Cl)2 and NaHCO3(Cl) water types predominate in locations underlain by gneisses and granites respectively, whereas in Ikorodu, the water types were predominantly NaHCO3(Cl) and NaCl (lagoon area) and Ca(Na)2(HCO3)2 (inland) area. These reflected influence of silicate weathering and ion exchange in Abeokuta while dissolution of calcite and dolomite, silicate weathering and ion exchange controlled the chemistry in Ikorodu area. The ∂2H, ∂18O and the deuterium excess respectively vary from -7.6 to -11.1 ‰, -2.3 to -2.7 ‰, 9.6 to 11.4 in Abeokuta and -14.2 to -18.5 ‰, -2.5 to -3.3 ‰ and 5.6 to 7.8 in Ikorodu. In Abeokuta, relationship between 18O and 2H (∂2H = 7.6 ∂18O + 9.3) indicates recharge dominated by precipitation formed by Rayleigh process with little or no influence of evaporation, while in Ikorodu area, the relationship (∂2H = 5.8 ∂18O + 0.8) shows isotopic exchange between groundwater and aquifers. High groundwater potentials were associated with confined and weathered/fractured basement aquifers, with Ca(Na)2(HCO3)2(Cl)2, NaHCO3(Cl), NaCl and Ca(Na)2(HCO3)2 water types. Provenance indicated recharge by precipitation and isotopic exchange between groundwater and aquifer materials. Word count =500 Keywords: Geophysical, Aquifers, Geochemical, Groundwater, Hydrochemical facies