Spatio-temporal dynamics of water quality during and after an extreme flood event in southern Brazil

In April-May 2024, a compound flood event in southern Brazil produced the highest water levels ever recorded in Porto Alegre, overwhelming urban drainage, sewage, and water treatment infrastructure. This study evaluates the spatiotemporal response of raw water quality in the Guaíba system-the primary drinking-water source for 1.32 million people-during and after this unprecedented event. We conducted seven monitoring campaigns (S1-S7) over one year, from the flood peak (May 2024) to post-event conditions (April 2025), across 22 sampling sites covering six water supply systems. Physicochemical parameters, nutrients, metals, and microbiological indicators were assessed using repeated-measures ANOVA, principal component analysis, and two-way cluster analysis. Results revealed a strong temporal gradient separating the flood phase (S1-S4) from post-event conditions (S5-S7). During peak flooding (S1), turbidity reached 475 NTU, aluminium 9.6 mg·L^-1, total phosphorus 11.9 mg·L^-1, and total coliforms 616,459 MPN/100 mL-exceeding Brazilian regulatory limits and World Health Organization risk thresholds by one to two orders of magnitude. Spatially, extreme hydrological forcing homogenized water quality degradation across most supply systems, except Ilha da Pintada (4.4% impervious surface), which maintained consistently lower contamination. Most critically, water quality did not recover to baseline one year after the event: during S7 (April 2025), E. coli reached 14,878 MPN/100 mL-the highest campaign mean-while mercury tripled and ammoniacal nitrogen returned to flood-peak levels, demonstrating a concentration effect driven by reduced dilution under low-water conditions. These findings demonstrate that post-flood low-flow periods represent an underrecognized vulnerability window for urban water supply, and that monitoring must extend well beyond the recession phase. The rapid response framework applied here provides a replicable approach for assessing compound flood impacts on water quality in other metropolitan regions exposed to climate-driven hydrological extremes.