Impact of extreme flood on fecal and metal contamination in Lake Guaíba, southern Brazil

In April and May 2024, unprecedented rainfall pounded the southern Brazilian state of Rio Grande do Sul, leaving 95% of its municipalities affected by the largest and most destructive floods in its history. Porto Alegre, the state capital and largest city, is bordered by Lake Guaíba and was severely affected by the flood. Floodwaters resulting from extreme weather events may contain high concentrations of fecal bacteria and other contaminants, including metals. This study aimed to assess contamination levels in Lake Guaíba following the flood event. Surface water samples were collected at two locations: Site 1, situated in the urban area of Porto Alegre, and Site 2, located in a more preserved area. The samplings were performed during pre-flood periods (December 2023 and March 2024) and post-flood period (June 2024). The following water quality parameters were analyzed: total coliforms, Escherichia coli , Al, Cd, Cu, Fe, Pb, Mn, Hg, Ni, Cr, and Zn. In the post-flood period, total coliforms and E. coli concentrations increased respectively by factors of 20.2 and 2.3 at Site 1 and 6.2 and 7.5 at Site 2, compared to the average for the pre-flood periods. Among the metals analyzed, Cd, Pb, Cu, Cr, Hg, and Ni were not detected during any sampling period. Among the quantified metals, Zn was only detected in the post-flood period at both sampling sites. These findings indicate that the flood event was associated with substantial increases in fecal contamination, whereas no clear and consistent pattern was observed for metal contamination.