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

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

Hydrogeochemical evolution of groundwater in a Quaternary sediment and Cretaceous sandstone unconfined aquifer in Northwestern China

A better understanding of the hydrogeochemical evolution of groundwater in vulnerable aquifers is important for the protection of water resources. To assess groundwater chemistry, groundwater sampling was performed from different representative aquifers in 2012–2013. A Piper trilinear diagram showed that the groundwater types can be classified into Na–SO4 and Na–Cl types. Only one groundwater sample was Na–HCO3 type. The dominant cations for all samples were Na+. However, the dominant anions varied from HCO3− to SO42−, and as well Cl−. The mean total dissolved solid (TDS) content of groundwater in the region was 1889 mg/L. Thus, only 20% of groundwater samples meet Chinese drinking water standards (< 1000 mg/L). Principal component analysis (PCA) combined with hierarchical cluster analysis (HCA) and self-organizing maps (SOM) were applied for the classification of the groundwater geochemistry. The three first principal components explained 58, 20, and 16% of the variance, respectively. The first component reflects sulfate minerals (gypsum, anhydrite) and halite dissolution, and/or evaporation in the shallow aquifer. The second and third components are interpreted as carbonate rock dissolution. The reason for two factors is that the different aquifers give rise to different degree of hydrogeochemical evolution (different travel distances and travel times). Identified clusters for evolution characteristic and influencing factors were confirmed by the PCA–HCA methods. Using information from eight ion components and SOM, formation mechanisms and influencing factors for the present groundwater quality were determined