Aquifer characterization and DRASTIC vulnerability assessment within the low-permeable Enugu shale formation, Anambra Basin, Nigeria using electrical resistivity method

Abstract

This study focuses on aquifer characterization and vulnerability assessment modeling of the low-permeable Enugu shale, Anambra Basin, Nigeria, utilizing the Electrical Resistivity Method (ERM). The Enugu shale formation presents a unique challenge due to its low permeability, which impacts groundwater movement and storage. We employed ERM, specifically Vertical Electrical Sounding (VES), to determine the subsurface resistivity variations, which aided in identifying aquiferous zones and their depths. The findings reveal significant variability in aquifer depths ranging from 4.0 to 44.5 m. This suggests that most of the underground water in the region is from shallow sources. The apparent resistivity model reveals variation in the resistivity of the aquifer ranging from low resistivity (10 to 60 m) to high resistivity (100 to 190 m). The depth to the water table map reveals higher depths (40 m to 75 m) at Abapka, Emene, and part of New Heaven. The relatively low values of transmissivity (200 m2/day–900 m2/day) and hydraulic conductivity (0.08 m/day–1.0 m/day) recorded are clues of prolific regions for groundwater development with low water yield capacity. Areas with higher transmissivity and thicker aquifers may provide better water supply potential, but overall, the low permeability suggests limited groundwater movement, requiring careful management to avoid over-extraction and contamination. The DRASTIC index risk (DIR) model indicates low to high susceptibility to contamination (with DIR ranging from 28 to 201), necessitating strategic groundwater protection measures. The low-permeable Enugu shale exhibits significant variability in its hydraulic properties, impacting its vulnerability and usability.