Integrated Physicochemical and Microbiological Assessment of Shallow Water in Agbangnizoun, Southern Benin

Abstract

Background and Aim: Safe drinking water remains a major challenge in rural sub-Saharan Africa, where people rely on shallow groundwater sources. Groundwater quality is influenced by geology, recharge, land use, and human activities, affecting its vulnerability to contamination. This study quantitatively and qualitatively assessed the physicochemical and bacteriological quality of shallow well water (17 to 45 m total depth) in the commune of Agbangnizoun and evaluated its spatial variability using an integrated Pollution Index. Study Design: This was a dry-season field sampling survey of shallow wells used for domestic water supply in Agbangnizoun." Place and Duration of Study: The study was carried out in the commune of Agbangnizoun, southern Benin, and sampling was performed during the dry season (December 2023 to March 2024). Methodology: Twenty-seven shallow wells were sampled and analyzed for pH, electrical conductivity, major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃⁻, Cl⁻, SO₄²⁻, NO₃⁻), total hardness, and fecal indicator bacteria using standard physicochemical and microbiological methods. A Pollution Index based on standardized parameter scores was calculated, and heatmap analysis of the index scores was used to visualize spatial variability and identify dominant contamination drivers. Results: The groundwater showed predominantly acidic conditions, with a mean pH of 5.86, and low to moderate mineralization dominated by calcium (Ca²⁺) and magnesium (Mg²⁺). Nitrate (NO₃⁻) concentrations exceeded 25 mg/L in most wells, indicating significant anthropogenic influence. All samples were contaminated with fecal indicator bacteria, with Escherichia coli concentrations ranging from 20 to 290 CFU/100 mL. The Pollution Index revealed strong spatial heterogeneity, with wells W6–W9 mainly affected by elevated sulfate (SO₄²⁻) and nitrate (NO₃⁻) levels, while well W26 showed dominant bacteriological contamination. Conclusion: Shallow well water in Agbangnizoun is highly vulnerable to both chemical and bacteriological pollution and is unsuitable for direct consumption without treatment. The Pollution Index, derived from standardized parameters and visualized by heatmap analysis, effectively identifies pollution hotspots and supports groundwater management in rural settings.