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

, 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.