Seasonal groundwater contamination and well vulnerability in flood-prone areas of the Ikopa watershed, Antananarivo, Madagascar: a multi-tracer hydrogeochemical and statistical study

This study examines groundwater residence time and seasonal contamination in shallow wells across the floodplain and adjacent uplands of Antananarivo, Madagascar, to clarify how the river–aquifer interactions and rapid recharge affect drinking water quality. The area combines a flood-prone alluvial plain with informal settlements and on-site sanitation, conditions that place pressure on groundwater quality. Groundwater and river water from 35 sites were sampled in three campaigns covering different seasonal conditions, namely the end of the rainy (April 2022) season, the beginning of the rainy season (January 2023), and the dry season (September 2023). Samples were analyzed for various geochemical parameters, hydrocarbons and microbiological indicators, together with residence time tracers (δ2H, δ18O, 3H, 14C, CFCs and SF6). Multi-tracer groundwater residence time interpretation shows that most wells contain young water with mean residence times (MRTs) below 30 years, mixed with an older component in some cases, whereas a few wells have MRTs of several decades. Seasonal and spatial differences in contaminants were evaluated with multivariate statistics comparing floodplain wells, upland wells and rivers. Seasonal contrasts indicate that nitrate, hydrocarbons and fecal bacteria peak at the onset of the rainy season, when intense recharge and flooding mobilize contaminants from the land surface and favor direct intrusion into poorly protected wells, while concentrations decline during the dry season. Floodplain wells do not consistently display more isotopic or chemical similarity to rivers than upland wells, suggesting that large-scale riverbank filtration or floodwater intrusion is not the main control of groundwater composition in these riverine wells. Instead, the combined isotopic, chemical and statistical evidence points to rapid recharge and wellhead intrusion of stormwater (rainfall) as the dominant pathways for contaminant transport to wells.