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.