Entropy-weighted water quality index assessment of groundwater in Ibadan metropolis, Southwestern Nigeria

An entropy-weighted water quality index (EWQI) was used in this study to evaluate the quality of groundwater in parts of the Ibadan metropolis, Nigeria. Seventy-five groundwater samples were collected and analysed for pH, electrical conductivity, total hardness, alkalinity, turbidity, total dissolved solids, cations (Na+, K+, Ca2+, Mg2+), anions (HCO3−, Cl−, SO42−, NO3−, F−), Fe and Total Heterotrophic Bacteria Content (THBC). The pH of the groundwater in the study area ranges from neutral to slightly alkaline. Based on the average concentrations, the abundance of anions is in the order as follows: Cl− > HCO3− > SO42− > NO3− > F−, while that of cations is Na+ > K+ > Ca2+ > Mg2+. The groundwater type was predominantly of mixed and Na-Cl types, likely controlled by multiple processes, such as water–rock interaction and mineral weathering. Anthropogenic activities, including improper waste disposal and sewage contamination, were also identified as significant contributors to groundwater quality degradation. The prevalent bacteria in this study are Klebsiella pneumonia, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus. The average EWQI value was 32.8, with values ranging from 2.2 to 143.6. Most of the groundwater in the research area, according to the EWQI, has good to excellent quality for drinking, while only 20% of the samples were medium to low quality, necessitating minimum treatment.