How water quality under extreme floods compromises ecosystem services, sanitation and One Health

In May 2024, Porto Alegre, Brazil, experienced the most intense large-scale precipitation event recorded in the country, exposing major vulnerabilities in flood-protection and sanitation infrastructure. Using high-resolution spatial sampling across 92 sites during the flood peak, we investigated physicochemical parameters, microbiological indicators, ecotoxicity, antibiotic resistance genes, and pathogenic viruses in floodwaters. An integrated analytical framework combined standardized physicochemical analyses, culture-based microbial quantification with antibiotic susceptibility testing, molecular detection of resistance genes and pathogenic viruses by qPCR/RT-PCR, ecotoxicological assays, and principal component analysis to identify contamination pathways and associated environmental and public health risks. Results revealed severe water quality degradation, with turbidity exceeding 470 NTU, mean total phosphorus reaching 1.07 mg/L (21× the legal limit), and mean ammoniacal nitrogen reaching 12.74 mg/L (3.4× the legal limit). E. coli concentrations exceeded U.S. EPA Recreational Water Quality Criteria by more than one order of magnitude, indicating severe fecal contamination. Ecotoxicity assays using Daphnia magna indicated acute toxicity in stagnant floodwaters. Microbiological analyses detected multidrug-resistant bacteria, with carbapenemase genes ( bla KPC and bla NDM ) widely detected across flooded urban zones, including hospital-adjacent areas. Pathogenic viruses, including Norovirus GI and GII, and Hepatitis A and E, were also detected. Water quality deterioration contributed to the shutdown of five of six drinking water treatment plants, depriving 1.23 million residents of potable water, while wastewater system failures discharged approximately 50,000 m³/day of raw sewage into flooded neighborhoods and the Jacuí River. Unlike most flood studies, this study provides quantitative and in situ evidence collected during the flood peak. By integrating real-time environmental, microbiological, molecular, and ecotoxicological indicators, the proposed framework offers a replicable approach for water quality assessment, risk evaluation, and urban water management under extreme flood conditions.