Geoelectrical and hydrogeochemical characterisation of the basement complex aquifers in the area around the Abuja city centre, Nigeria.

Abstract

Geophysical and hydrochemical data had been acquired and analysed in the area around Abuja Centre, Nigeria. This was with a view to characterizing the Basement Complex aquifers and assessing its groundwater quality. Secondary parametric Vertical Electrical Soundings (VES) were acquired at fifteen previously drilled borehole locations within the study area. Groundwater samples were also collected from same boreholes and analysed for physio-chemical and microbiological parameters. The VES data were quantitatively interpreted for subsurface sequence delineation and aquifer identification while the chemical elements were analysed using histograms. The VES interpretation results revealed three (H and K-type), four (KH, HA and AA-type) and five (HKH-type) layers geoelectric models. The VES interpretation results delineated five subsurface layers composed of the topsoil, laterite, weathered layer, partially weathered/fractured basement and fresh basement. The resistivity of the topsoil ranged between 36 Ωm and 924 Ωm with thicknesses of between 0.3 m to 3.5 m. The laterite layer resistivity ranged between 408 Ωm and 1878 Ωm, with thicknesses of 2.2 m to 8.5 m. The resistivity of the weathered layer ranged from 15 Ωm to 509 Ωm while the thickness ranged between 3.7 m and 64.7 m. The partially weathered/fractured basement had resistivity values that ranged from 14 Ωm to 734 Ωm and thicknesses of 7.9 m to 22.1 m. The resistivity of the fresh basement ranged between 414 Ωm and 99061 Ωm with depth to bedrock of between 7.9 m and 73.5 m. All the analysed chemical elements had concentrations that were within the WHO permissible levels with the exception of very few boreholes showing relatively high nitrate and total iron concentrations. The study concluded that the study area was characterized by both the weathered layer and partly weathered/fractured basement aquifers. The groundwater was also found to be generally potable.