Hydrochemical Characterisation of Groundwater in Parts of the Volta Basin, Northern Ghana

Abstract

Conventional graphical methods were applied to major ion concentrations and stable isotope data to determine the genesis and evolution of the hydrochemistry of groundwater from fractured aquifers in the northern parts of Ghana. The analyses suggest that groundwater hydrochemistry is controlled by the incongruent weathering of silicate minerals in the aquifers. Hierarchical cluster analysis confirms the results and shows that anthropogenic factors also contribute to the groundwater chemistry. Montmorrillonite, apparently resulting from the incongruent dissolution of calcium and sodium rich feldspars in the rock matrix is probably the most stable clay mineral phase in the system. Chloroalkaline indices (CAI) 1 and 2, calculated from the major ion data suggest reverse cation exchange activity between Na⁺ and K⁺ in the water and Ca²⁺ and Mg²⁺ in the rock matrix. This study finds that the groundwater in the area is classified into Ca-Mg-HCO₃, mixed Ca-Mg-Na-HCO₃ and Na-HCO₃ water types, which are typical of groundwaters influenced by silicate mineral weathering and ion exchange. Stable isotopes of oxygen and hydrogen (δ¹⁸O and δ²H) show that groundwater originates from meteoric source, derived from rainfall which rapidly recharges the aquifers through the weathered overburden and ingresses such as joints and fracture systems. Silicate mineral weathering, which appears to be the main controlling process in the hydrochemistry, does not appear to influence the stable isotope data since a plot of δ¹⁸O against EC is a straight line of zero slope.