Abstract
The present study involved the use of 82 geo-electric soundings, and the measurement of well inventory and conduct of yield tests in 19 wells across the various bedrock terrains of Ibarapa region of southwestern Nigeria. The aim is to proffer solution to the unsustainable yield of the available boreholes in order to effectively exploit the existing groundwater resource in the area. From the geological reports, the area is underlain by four principal crystalline rocks that include porphyritic granite, gneisses, amphibolite and migmatite. The geo-electric studies revealed that the degree and extent of development of the weathered–fractured component varied, leading to diversity in groundwater yield and in aquifer vulnerability to contamination. The thickness of the weathered layer is greater than 18 m in areas underlain by amphibolite and gneisses and less than 13 m within migmatite and porphyritic granite terrains. High groundwater yield greater than 70 m3/day was recorded in wells within the zones of rock contacts and in areas with large concentration of bedrock fractures and elevated locations across the various bedrock terrains. Aquifer vulnerability is low in amphibolite, high in granitic terrains, low to moderate in gneisses and high to moderate in migmatite. Also, wells’ depths and terrain elevation have a moderate to strong indirect relationship with groundwater yield in most bedrock terrains, except in high topographic areas underlain by porphyritic granite. Therefore, there is need for modification of well depth in accordance with the terrain elevation and hydrogeological complexity of the weathered–fractured components of the variuos bedrock terrains, so as to ensure a sustainable groundwater yield.
Highlights
Large parts of southwestern (SW) Nigeria are completely underlain by intrusive crystalline rocks that can neither store nor transmit water, except when fractured, or where there is significant regolith thickness over the basement
Hydrogeological characterisation of the basement aquifers and their prospects have been investigated from geo-electric studies, groundwater yield tests, and from well inventory data of existing boreholes across the various bedrock terrains
The strong indirect relationship existing between groundwater discharge and well depth is an indication of shallow occurrence of the water-bearing zones in the basement terrain of Ibarapa region
Summary
Large parts of southwestern (SW) Nigeria are completely underlain by intrusive crystalline rocks that can neither store nor transmit water, except when fractured, or where there is significant regolith thickness over the basement. For hydrogeological characterisation of the basement aquifers in Nigeria, the applications of electrical resistivity methods have been used for decades (Olorunfemi and Olorunniwo 1985, 1990; Ako et al 1990; Malomo et al 1991; Olayinka and Mbachi 1992; Olorunfemi and Okhue 1992; Olayinka and Olayiwola 2001; Oseji et al 2005; Alile et al 2008) Most of these investigations used the primary geo-electric parameters, which are the layer resistivity and the corresponding thickness for interpretations of the hydrogeological settings of the crystalline basement aquifers. These variables were derived from the integration of primary geo-electric parameters (i.e. the layer thicknesses and their corresponding true resistivities) and comparison of the passage of electric current across and parallel to the geo-electric boundaries to water passage through subsurface layers was made
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