Extreme weather and drinking water safety: impacts of the 2023 flood Hans in Norway

Climate change is expected to increase the frequency of extreme weather events, challenging the provision of safe drinking water. In August 2023, the extreme flood event Hans severely affected infrastructure in south-eastern Norway. We assessed the impact on drinking water quality in 63 municipalities receiving red flood warnings, covering 138 water supply systems serving ∼845,000 people. Data on E. coli, Clostridium perfringens and intestinal enterococci were retrieved from 93 utilities that reported results before, during, and after the flood. In total, >7000 water samples were analysed. Positive samples were more frequent in raw water (14.6 % E. coli, 32.2 % Clostridiumperfringens, 9.6 % enterococci) than in treated water (≤1.2 %). Significant correlations were observed between river flow and faecal indicator exceedances in both raw and treated water, particularly in the Drammen river basin. These findings indicate increased public health risk during extreme flooding and underscore the vulnerability of drinking water systems to climate-sensitive hazards. The results provide baseline data directly relevant for hazard identification and risk assessment in climate-resilient water safety planning and for implementing the European Drinking Water Directive (EU, 2020/2184).