Characterization of crystalline basement and carbonate aquifers, Southeastern Nigeria, using hydrogeoelectrical and hydrogeochemical methods

This study aims to delineate aquifers, determine their properties, and evaluate surface water and groundwater quality across two geological areas underlain by granitic basement (BA) and carbonate (CA) rocks in southeastern Nigeria, where groundwater and surface water serve as vital resources for drinking, industrial, and agricultural needs. It also assesses spatial and seasonal variations, identifies geogenic and anthropogenic influences, and evaluates potential human health risks. A total of 6 vertical electrical sounding (VES) surveys and 32 surface and groundwater samples were collected during the wet and dry seasons. Water samples were analyzed for physicochemical parameters and heavy metals using standard protocols. The assessment combined the Water Quality Index (WQI), irrigation indices, hydrochemical facies, and multivariate statistical modeling. The geoelectrical measurements delineated three aquiferous layers in BA and four in CA. Field data indicate that deeper-water levels occur in the dry season (DS) compared to shallower levels in the wet season (WS). Aquifer properties, including transmissivity, hydraulic conductivity, and volume of water, are higher in BA relative to CA. Estimates of groundwater reserves indicate higher water volumes during the wet season and greater volumes in BA compared to CA. The dominant cations follow the order Ca 2+ Na + Mg 2+ K + , whereas anions rank HCO₃ − Cl − SO₄ 2− NO₃ − , with the prevailing water types being Ca-HCO 3 and Na-HCO 3, revealing carbonate weathering and silicate weathering as the main processes controlling water chemistry. Concentrations of some heavy metals (Fe, Cr, Ni) are above the maximum admissible limits. Hydrochemical analysis using Schoeller diagrams revealed both forward and reverse ion exchange processes. The WQI classification showed the majority of samples as excellent for drinking. Multivariate analysis identified strong correlations among TDS, EC, hardness, Ca 2+ , Mg 2+ , Cl − , and SO₄ 2− , indicating combined natural and anthropogenic impacts. Groundwater is excellent for drinking, whereas irrigation indices confirmed the suitability of the majority of samples for irrigation and industrial use. Potential health risk assessment suggests both non-carcinogenic and carcinogenic health risks. These findings align with the UN SDG 6 (Clean Water and Sanitation) and SDG 13 (Climate Action) and are essential for ensuring sustainable groundwater protection and enhancing climate resilience in vulnerable coastal aquifer systems.