Comparative study of aquifer systems in parts of the basement complex and coastal plain sands of southeastern Nigeria using geophysical techniques and geochemical parameters

Abstract

This study presents the robust, multi-parameter comparative evaluation of the geophysical and geochemical characteristics of two distinct geological formations in Southeastern Nigeria. The fractured crystalline basement of the Southern Oban Massif (Awi Community) and the porous sedimentary media of the Nigeria Delta Basin (Calabar Municipal). Twenty (20) vertical electrical sounding (VES) points under Schlumberger array, and sixteen (16) groundwater samples were analyzed for the modeling of the groundwater potential, water quality and access irrigation suitability. The geoelectrical modeling delineates high yield, porous aquifers within the sedimentary gravel/sand units in Calabar at depths between 19 and 42.9 m, in contrasts with structurally controlled, weathered to fractured regolith units in Awi. The geochemical analysis of the two formation depicts a host freshwater (TDS ˂ 500 mg/l). Aquifers of both formations suffer from severe geogenic iron contamination up to 5.18mg/l and widespread anthropogenic nitrate loading exceeding the 50mg/l standard. The evaluated water quality through the weight arithmetic water quality index (WQI) yielded values consistently exceeding 300, classifying the untreated groundwater across both formations as unsuitable for direct consumption. Irrigation indices calculated for water samples from both formations presented a critical divergence; low sodium adsorption ratio (SAR) values suggested chemical safety, whereas highly elevated magnesium adsorption ratio (MAR) scores of up to 76.4 % posed a severe long-term risk of soil alkalization and structural degradation. Gibbs and Piper diagram visualization confirmed a transitional lifecycle in Calabar from precipitation-dominated to rock-water interaction-dominated facies, while Awi basement remains tightly cluster within the precipitation- dominated facies regime. Both aquifer systems are characterized by a highly mineralized Ca-Mg-Cl-SO water type, highlighting permanent hardness.