Hydrogeochemistry, ionic speciation, controlling processes and agricultural suitability of groundwaters in sections of the semi-arid basement complex of north–central Nigeria

Abstract

In order to ascertain the governing mechanisms, sources, and speciation of ionic species, identify hydrochemical facies, and assess their appropriateness for agricultural use, groundwaters from several portions of the semi-arid basement complex of north–central Nigeria were analyzed based on chemistry. Basement aquifers made of gneiss-migmatites, metasediments, and granitoids store groundwater. Standard analytical techniques were used to analyze the fluids for main cations, anions, and physical characteristics. The results showed that the waters were slightly acidic (with a mean pH of 5.38, below the permissible range of 6.5–8.5), and that the predominant cations and anions were Na⁺ >Ca²⁺ >K⁺ >Mg²⁺ and Cl⁻ > NO₃^- > HCO₃^- > SO₄²⁻, respectively. Analysis of the ions' stoichiometric ratios reveals that alkali elements predominate, making up about 55.3% of the ions and being connected to silicate weathering. Based on ionic ratio calculations, it was determined that ion exchange was a key factor controlling water chemistry. Ionic species cross plots show that silicate weathering (sodic and calcic plagioclase) predominates. Hydrochemical facies, Gibbs plots, and principal component (correlation, cluster and factor) studies all show that ionic elements are geogenic, essentially coming from the weathering of silicates with ion exchanges. Based on predicted saturation indices, hydrochemical modeling by the computer program VISUAL-MINTEQ reveals that the majority of main ions occur in free mobile states with associated mineral species, all at undersaturated levels. Based on measurements of the sodium adsorption ratio (SAR), sodium percentage (% Na), and chloro-alkaline indices (CAI), the waters have been evaluated for their suitability for agricultural use.