Groundwater Quality Assessment in a Semi-Arid Area Western Central Senegal Using Water Quality Index (WQI) and Multivariate Statistical Analysis

Water quality is a very significant parameter that determines its suitability for various uses. In this study, 42 groundwater samples were collected, 39 of which were ultimately used after calculating the ion balance error, from wells and boreholes during a sampling campaign conducted from May 30 to June 1, 2023, to assess quality and identify the main factors controlling groundwater chemistry. The results show that the pH ranges from 6.89 to 8.2, indicating that the groundwater is neutral to alkaline in nature. The order of abundance of ions in groundwater is defined as follows: HCO 3 − > Cl − > SO 4 2− > NO 3 − for anions and Na+ > Ca2+ > Mg2+ > K+ for cations, indicating that ( HCO 3 − ) and ( Na + ) are respectively the most abundant anion and cation in groundwater. The piper diagram shows four water types: Ca-HCO3 water type, Na-Cl water type, mixte-Ca-Mg-Cl water type, and Ca-Cl water type, of which Ca-HCO3 is the dominant water type. Gibbs diagrams, binary diagrams, and chloro-alkaline index values indicate that water-rock interaction, evaporation, and ion exchange are the main processes controlling groundwater mineralization. Principal Component Analysis (PCA) identified two principal components accounting for 80.54% of the total variance. The first component indicates that water quality is controlled by natural processes such as water/rock interaction and evaporation, but also by anthropogenic pollution. The second component is associated with fluoride-rich alkaline waters, suggesting the influence of fluoride-bearing minerals dissolution. Furthermore, Hierarchical Cluster Analysis (HCA) classified the waters into two groups: highly mineralized waters with a mean TDS value of 2642.5 mg/L and moderately mineralized alkaline waters with a mean TDS value of 666.7 mg/L. The water quality index (WQI) of the study area ranged from 4 to 414 and shows that the majority of water (64%) is unfit for human consumption.