AN INTEGRATED HYDROGEOPHYSICAL, HYDRAULIC, AND HYDROCHEMICAL FRAMEWORK USING GSI TOOL FOR ASSESSING AQUIFER SUSTAINABILITY IN THE COASTAL SEDIMENTARY BASIN OF AKWA IBOM STATE, NIGERIA

Abstract

Groundwater resources in coastal sedimentary basins are increasingly threatened by rapid urbanization, excessive groundwater abstraction, saline intrusion, and hydrogeological heterogeneity, posing significant challenges to long-term aquifer sustainability. This study develops an integrated hydrogeophysical-hydraulic-hydrochemical framework for groundwater sustainability assessment and introduces a novel Groundwater Sustainability Index (GSI) for delineating groundwater management zones and supporting evidence-based groundwater resource assessment in the coastal aquifer systems of Akwa Ibom State, southeastern Nigeria. Thirty Vertical Electrical Sounding (VES) datasets were integrated with borehole hydraulic records, groundwater hydrochemical analyses, and hydrogeological information to characterize aquifer geometry, hydraulic properties, groundwater quality, aquifer protective capacity, and susceptibility to saline intrusion. Three hydrogeological management zones were identified. The northern inland zone is characterized by confined sandy aquifers with depths of 18.6-102.7m, hydraulic conductivity of 12-19mday⁻¹, transmissivity of 6.5×10⁻²m²s⁻¹, and longitudinal conductance exceeding 1.0mhos, indicating excellent aquifer protection and high groundwater sustainability. The central transitional zone comprises semi-confined interbedded sand-clay aquifers with moderate transmissivity (4.6×10⁻³-2.3×10⁻²m²s⁻¹) and aquifer protective capacity (0.19-0.86 mhos). In contrast, the southern coastal–estuarine zone contains shallow unconfined aquifers with water-table depths of 0.7-3.5m, low aquifer resistivity (25.3-271.4Ωm), and chloride concentrations exceeding 350mg L⁻¹, indicating high vulnerability to saline intrusion. The GSI classified groundwater sustainability as very high (0.87) in the northern inland zone, high (0.62) in the central transitional zone, and low (0.36) in the southern coastal zone. The results demonstrate that groundwater sustainability is governed by the combined influence of aquifer productivity, aquifer protective capacity, groundwater quality, overburden characteristics, and resistance to saline intrusion rather than by aquifer productivity alone. The developed framework and GSI provide a practical, transferable decision-support approach for groundwater sustainability assessment, aquifer vulnerability evaluation, and prioritization of groundwater monitoring and management in hydrogeologically complex coastal sedimentary aquifer systems.