The impact of rainfall on drinking water quality in Antananarivo, Madagascar

Abstract

Low-income cities that are subject to high population pressure and vulnerable to climate events often have a low capacity to continuously deliver safe drinking water. Here we report the findings of a 32-year investigation of the temporal dynamics of indicators of drinking water quality in the city of Antananarivo, where we assess the long-term evolution of supplied water quality and characterize the interactions between climate conditions and the full-scale supply system. A total of 25,467 water samples were collected every week at different points in the supplied drinking water system. Samples were analyzed for total coliforms (TC), Escherichia coli (EC), intestinal Enterococci (IE), and spores of Clostridia (SSRC). Nine-hundred-eighty-one samples that were identified as positive for one or more indicators were unevenly distributed across the series. The breakpoint method identified four periods when the time series displayed changes in the level and profile of contamination (i) and the monthly pattern of contamination (ii), with more direct effects of rainfall on the quality of supplied drinking water. The modeling showed significantly different lags among indicators of bacteria occurrence after cumulative rainfall, which range from 4 to 8 weeks. Among the effects of low-income urbanization, a rapid demographic transition and urban watershed degradation are progressively affecting the quality of supplied water and resulting in the more direct effects of rainfall events.