Integrated Hydrogeophysical and Hydrogeochemical Assessment of Aquifer Characteristics and Groundwater Vulnerability in Parts of Asaba, Niger Delta, Nigeria

Abstract

 In highly developed areas, groundwater resources are becoming more vulnerable to both natural and anthropogenic sources andaquifer architecture, intrinsic vulnerability and hydrochemical controls are important for sustainable groundwater use. The studycombined the Vertical Electrical Sounding (VES), hydrochemical analysis, ionic ratios, and the GOD vulnerability index to evaluateaquifer characteristics, groundwater vulnerability and the hydrochemical evolution in the part of Asaba, Niger Delta, Nigeria.Fifteen VES locations were investigated and compared to 15 groundwater samples taken from surrounding wells. The VES datadelineated a multi-layered aquifer system that is heterogeneous, with principal groundwater-bearing horizons occurring atapproximately 32.7–42.7 m in VES 1–8 and deeper saturated sandstone aquifers at 70.89–84.28 m in VES 9–15, exhibitingunconfined, semi-confined and confined conditions. High lithological variability was observed with apparent resistivity valuesbetween 5.31 and 2738.42Ωm. The GOD index ranged from 0.048 to 0.49, indicating moderate vulnerability at VES 1 and 5, lowvulnerability at VES 2, 3, 6, 8, 9, 11 and 12, and negligible vulnerability at VES 4, 7, 10, 13, 14 and 15. The hydrochemicalcharacterisation revealed that the groundwater was mainly Ca–Mg–HCO₃ type, and a few samples were of mixed Ca–Mg–Na–HCO₃facies. The ionic-ratio analysis suggested that the dominant hydrochemical processes were hydrochemical evolution due to silicateweathering, ion exchange, reverse ion exchange and mineral dissolution, while a few samples (L5–L7) were found to be enriched inchloride, suggesting a localized influence within the groundwater system. The integrated VES–GOD–hydrochemical assessmentshows that the deeper, better-protected aquifers are more intrinsically protected, while the shallower, less protected aquifers needmore attention for groundwater protection and land use management. The results offer a holistic perspective on enhancinggroundwater-resource assessment and management in the complex sedimentary aquifer system of the Niger Delta.