Assessment of aquifer vulnerability in fractured rocks in the Abakaliki area, southeastern Nigeria, using geophysical and geological data

A combination of the integrated electrical conductivity (IEC), aquifer vulnerability index (AVI), and the groundwater hydraulic confinement-overlying strata-depth to water table (GOD) pollution indices were used to assess the vulnerability of the fractured rock aquifer in the Abakaliki area to pollution from anthropogenic sources. The interpretation of the vertical electrical sounding (VES) data indicates that the topsoil ranges from laterite to silt to silty-clay. The geoelectric sections have varying numbers of layers (1–6 layers) with their resistivity ranges as 29–3949 Ω, 5–1631 Ωm, 0.35–3,767 Ωm, 2–7926 Ωm, and 1–3060 Ωm, respectively. These have been grouped into four main lithologic units, namely the topsoil, indurated/baked shale, fractured shale/sandstone (the aquiferous unit), and the consolidated sandstone units. The resistivity of the vadose zone (RVD) ranges from 34 to 3383 Ωm with a mean value of ± 347 Ωm. The thickness of the vadose zone (hVD) is generally thin (< 18 m), while their hydraulic conductivity (k) is relatively high (0.20–16.11 m/day) for argillaceous rocks. The IEC values (≤ 0.85 S) suggest a weak to fair protective capacity for the vadose zone. The result of the hydraulic resistance (HR ≤ 56) indicates that the underlying aquifer is high to extremely vulnerable, and the GOD values (> 0.7) agree that they are extremely vulnerable.