Geophysical investigation of aquifer flow unit characteristics in northern parts of Akwa Ibom State, Southern Nigeria

Abstract

The heterogeneous nature of the aquifer system poses significant challenges to both aquifer delineation into hydraulic flow units (HFUs) and the assessment of groundwater flow efficiency. This heterogeneity leads to changes in aquifers’ hydraulic parameters, which include porosity, flow and storage capacities, permeability, flow Zone Indicator (FZI) and aquifer quality index (AQI). Thus, the key aim of this study is to examine aquifer flow unit characteristics in Ikot Ekpene and Obot Akara municipalities by categorizing the units into distinctive HFUs via the combined application of the electrical resistivity technology (ERT), Stratigraphic Modified Lorenz Plot (SMLP) and Flow Zone Indicator (FZI) techniques. The area is shown to comprise successions of three to four sandy layers with minor clay interbeddings at few locations from the ERT results. The aquifer units are found in layers two and three respectively at depths of between 1.6 and 30.2 m with 30.2–89.0 m thickness. The results of the FZI technique identify two distinct HFUs in the area with respective discrete rock typing (DRT) values of 14 and 15 whereas that of the SMLP method reveal 5 unique HFUs, each characterised by similar flow characteristics. The delineated flow units from the SMLP results are classified as conductors with a fair efficiency ranking. This ranking seems to correlate with the DRT results for the delineated unconsolidated sandy and gravelly sand aquifers, which are shown to be perfectly heterogeneous. These findings from this maiden study in the area are very promising and provide useful insights on groundwater flow characteristics in the area to assist groundwater resource regulating authorities and policy makers in making decisions on appropriate location of new water boreholes to satisfy the mushrooming water needs of the local population.