Groundwater Exploration in Afi Mountain (Obudu Plateau), South Eastern Nigeria: Integrating Vertical Electrical Sounding (VES), Bore Hole drilling and Pump Testing

Abstract

Groundwater development in crystalline basement terrains is often constrained by complex subsurface geology, where groundwater occurrence is controlled by weathered and fractured basement units. This study evaluated the effectiveness of Vertical Electrical Sounding (VES) for groundwater exploration in the Afi Mountain area, Boki Local Government Area, Cross River State, Nigeria, through integration with borehole drilling and pump testing. A total of 41 VES stations were conducted and interpreted using Zoody and WinResist software to delineate aquifer horizons and recommend drilling depths. Based on the VES results, 41 boreholes were drilled using the rotary air-compression method, followed by pumping tests conducted between 2 and 16 July 2017. Pumping test data were analysed using Aquifer Test Pro Version 2011.1 to determine static water level (SWL), drawdown, borehole yield, and pump installation depths. The interpreted geoelectric sections identified weathered regolith with resistivity values of approximately 200–300 Ωm and fractured basement aquifers with resistivity values of 300–600 Ωm, while fresh basement generally exceeded 600 Ωm. Recommended VES drilling depths ranged from 30 to 65 m, whereas actual drilled depths ranged from 35 to 70 m. Comparison of VES predictions with drilling records showed that 30 of the 41 boreholes (73.2%) exhibited excellent agreement, with depth deviations of 1–5 m, while 11 boreholes (26.8%) showed poorer agreement, with deviations of 5–10 m. Static water levels ranged from 3.62 to 20.78 m, borehole installation depths from 30.6 to 68 m, and recommended pump capacities from 0.5 to 1.5 hp. Pumping tests revealed that 30 boreholes produced sustainable yields ranging from 0.71 to 1.50 L s⁻¹, whereas 11 boreholes recorded low yields of 0.0074–0.35 L s⁻¹ and were classified as unsuccessful despite intercepting groundwater. The results demonstrate that VES is a reliable technique for locating groundwater-bearing zones within crystalline basement terrains and significantly improves drilling success. However, the occurrence of low-yield boreholes despite successful aquifer interception confirms that geophysical interpretation alone cannot adequately predict aquifer productivity. The study therefore emphasizes that pump testing remains indispensable for verifying hydraulic performance and ensuring sustainable groundwater development in basement complex environments.