A geoelectrical-hydrogeochemical investigation of shallow groundwater occurrence in Ibadan, southwestern Nigeria |
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Authors: | A I Olayinka A F Abimbola R A Isibor A R Rafiu |
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Institution: | (1) Department of Applied Geophysics, Technical University of Berlin, Ackerstr. 71–76, D-13355 Berlin, Germany, Fax: +49 30 314 72597, e-mail: olayinka@bg.tu-berlin.de, DE;(2) Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-Cho, Okayama 700, Japan, JP;(3) Department of Geology, University of Ibadan, Ibadan, Nigeria, NG |
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Abstract: | An integrated geological, geoelectrical and hydrochemical investigation of shallow groundwater occurrence in the Ibadan area,
southwestern Nigeria, is presented. The primary objective was to characterise the groundwater in a typical low-latitude environment
underlain by Precambrian crystalline basement complex rocks. The dominant rocks comprise suites of gneisses and quartzites.
Chemical analyses of the groundwater show that the mean concentration of the cations is in the order Na>Ca>Mg>K while that
for the anions is Cl>HCO3>NO3>SO4. Statistical analyses, using the product-moment coefficient of correlation, indicate positive correlations between the following
pairs of parameters: TDS and conductivity (r=0.96); Na++Mg2+ and Cl– (r=0.95); Na++K+ and Ca2+ (r=0.43); Na++K+ and HCO3
– (r=0.17); Ca2+ and Mg2+ (r=0.74); Ca2+ and HCO3
– (r=0.33); Ca2++Mg2+ and HCO3
– (r=0.31) and pH and HCO3
– (r=0.54). A very weak negative correlation was recorded between pH and Cl–, with r=–0.003. Five groundwater groups have been identified, namely, (1) the Na-Cl, Na-Ca-Cl, Na-Ca-(Mg)-Cl; (2) the Ca-(Mg)-Na-HCO3-Cl, Na-Ca-HCO3-Cl, and Ca-HCO3-Cl; (3) the Ca-(Mg)-Na-HCO3, Ca-Na-HCO3; (4) Ca-Na-Cl-(SO4)-HCO3 and (5) the Ca-(Mg)-Na-SO4-HCO3. The different groups reflect the diversity of bedrock types and consequently also of the products of weathering. Most of
the water sampled is unfit for drinking on account of the high NO3
– content. It can, however, be used for irrigation purposes as the sodium hazard is low while the salinity hazard ranges from
low to medium. Resistivity soundings indicate the presence of a thick weathering profile, which could be up to 60 m. Such
sites should be the target for any long-term and sustainable groundwater development in the area.
Received: 15 April 1998 · Accepted: 4 July 1998 |
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Keywords: | Crystalline basement complex Resistivity soundings Fracture trends Hydrogeochemistry Ibadan Southwestern Nigeria |
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