Geoelectrical characterisation of aquifers in Bauchi-Alkaleri-Kirfi geological transition zones, Northeast Nigeria

Geological transition zones (GTZ) have proved to be hydrogeologically problematic, owing to poorly defined basal and lateral contacts between the rocks. Several occurrences of abortive wells in GTZ overtime has necessitated the need to characterise groundwater potential zones (GPZ) in these zones. Attempts have been made to delineate the complex lithological contacts in GTZ using geophysical and geological methods. However, the characterisation of groundwater prospective zones in these complex lithological contacts, which are very significant to the sustainable management of groundwater in GTZ, have not received much attention. Therefore, this study focused on the characterisation of the groundwater prospects of aquifers in a typical GTZ in Bauchi-Alkaleri-Kirfi areas, Northeastern Nigeria. Geological mapping was employed for the lithological delineation of rocks while geoelectrical mapping using 94 vertical electrical sounding (VES) was adopted for characterisation of subsurface media (aquifers). Three thematic maps comprising of aquifer resistivity, thickness and lithology were integrated using overlay analysis in the GIS environment to generate a GPZ map. Geologically, the basement part of the GTZ is underlain by migmatite, gneiss, granite and charnockite while the sedimentary areas were underlain by the sandstone units of the Tertiary Kerri Kerri Formation. The VES data indicated a generally 3–5 geoelectric layer system within the sedimentary zones with aquifer thickness of 11–158 m in the sandstones. However, the Basement Complex areas revealed saprolite thicknesses of 2.4–83.8 m (migmatite/gneiss), 1.3–67.3 m (granite) and 1.2–44.6 m (charnockite), suggesting a better aquiferous medium in the migmatite/gneiss units. Consequently, integration of thematic maps revealed three GPZ namely: poor zones (charnockite and granite), moderate zones (migmatite/gneiss and granite), and good zones (dominantly sandstone). This study shows the efficacy of applying GIS to VES data in groundwater exploration for better management of groundwater resources.