Groundwater quality and vulnerability assessment in flood-prone areas of the Ikopa watershed, Antananarivo, Madagascar: an integrated hydrogeochemical and statistical approach

This study evaluates groundwater quality and contamination risk in flood-prone areas of the Ikopa watershed in Antananarivo, Madagascar, where rapid urbanization, inadequate sanitation, and recurrent flooding threaten shallow hand-dug wells used for drinking water. The objectives were to characterize groundwater quality, identify major contaminants and their sources, and evaluate the influence of hydrogeological conditions and local well protection measures on contamination risk. A total of 35 water samples, including groundwater from 26 hand-dug wells and 9 river sites, were collected during the rainy season and analyzed for major ions, trace elements, hydrocarbons, and microbiological indicators. Multivariate statistical analyses, including principal component analysis and hierarchical cluster analysis, identified three hydrochemical groups reflecting differences in mineralization, redox conditions, and anthropogenic influences. High concentrations of nitrate, hydrocarbons, and fecal indicators ( Escherichia coli , total coliforms, and intestinal enterococci) were detected, frequently exceeding drinking water guidelines. Nitrate contamination was mainly attributed to manure and septic waste sources based on dual isotope analysis (δ 15 N, δ 18 O), with evidence of denitrification in some samples. Groundwater vulnerability was assessed using the DRASTIC method, while an integrated contamination risk classification considered land use, flood exposure, and well protection conditions. Results showed that well protection measures were associated with reduced bacterial contamination but had limited effects on nitrate and hydrocarbon pollution. These findings highlight that protecting wells alone is insufficient and that integrated interventions targeting sanitation, flood management, land-use planning, and groundwater protection are needed to safeguard drinking water resources in rapidly urbanizing flood-prone regions.