HYDROGEOLOGIC CHARACTERISATION OF CRYSTALLINE BASEMENT AQUIFERS OF PART OF IBARAPA AREA, SOUTHWESTERN NIGERIA

Abstract

Globally, nearly half of human population relies on groundwater for various uses. However, the occurrences of this vital resource are usually constrained in areas underlain by crystalline bedrocks. This is more critical in the southwestern Nigeria (SN), where about half of the landmass is underlain by crystalline bedrocks; hence the need for proper hydrogeological characterisation of crystalline basement aquifers. Therefore, this study was designed to focus on geological, geomorphological, geophysical and hydraulic characterisations of the crystalline basement aquifers of part of Ibarapa area located in the SN in order to determine the groundwater potential and ensure sustainable groundwater development. The study utilised geological, remote sensing, Vertical Electrical Sounding (VES) and pumping test methods. The geological approach involved geological field mapping, while the remote sensing method included the use of Radar and Landsat imageries from which thematic maps (drainage, elevation, lineament/fracture, and vegetation index) processed with ArcGIS and Erdas imagine software were generated. Eighty-five VES were carried out using Schlumberger array. The field data were processed by partial curve matching and computer iteration using Winresist 1.0 software. Longitudinal conductance derivatives were estimated for assessment of recharge potential from the VES data. Single-well pumping tests with constant discharge and corresponding recovery tests were carried out on twenty-three boreholes for the determination of hydraulic (transmissivity and hydraulic conductivity) parameters. Amphibolite, gneisses, migmatite and porphyritic granite were the underlying rock units delineated. Weathering potential was higher in amphibolite and gneisses due to foliation and abundance of ferromagnesian minerals. Geomorphological characterisation revealed two hydro-geomorphic units: the high-lying areas underlain by migmatite (characterised by ridges and inselbergs of porphyritic granite) and the low-lying areas underlain by gneisses and amphibolite representing groundwater discharge zones. The dominant VES curve types were three-layer H-type (91.5%), characterised by moderate to thick weathered regolith with thickness >18m in amphibolite and gneisses terrains, compared to <13m in migmatite and porphyritic granite areas. The corresponding resistivity (Ωm) for the weathered layers were 9.1 – 225.2, 18.8 – 810.6, 28.1 – 565.5 and 26.5 – 294.0 for amphibolite, gneisses, migmatite and porphyritic granite, respectively. The estimated total longitudinal conductance (Ω-1) was 0.02 – 0.60 in migmatite and porphyritic granite, and 0.01 – 1.42 in amphibolite and gneisses which is an indication of thicker clayey weathered units. The estimated transmissivities (m2/day) were 0.25 – 11.65 in amphibolite and gneisses area; and 0.56 – 2.95 in migmatite and granite terrains. The corresponding hydraulic conductivities (m/day) were 0.54 – 66.80 × 10-2 and 2.31 – 19.69 ×10-2. These hydraulic results indicated that boreholes in amphibolite and gneisses terrains have good water transmission capacity. Thirty-five percent of all the tested boreholes attained dynamic equilibrium water level at low drawdown during pumping; an indication of higher yield of the penetrated fractured bedrock and rock contacts especially in gneisses and amphibolite terrains. Areas underlain by amphibolite and gneisses in Ibarapa region were characterised by good hydrogeological attributes that may guarantee sustainable productive boreholes, while areas underlain by migmatite and porphyritic granite exhibit low groundwater potential that can only sustain hand-dug wells. Keywords: Bedrock delineation, Hydro-geomorphic, Longitudinal conductance, Hydraulic parameters, Fractured bedrock Word count: 491