| Summary: | The Soil and Water Assessment Tool (SWAT) was used to assess total phosphorus concentrations (TOT-P) and fluxes (TOT_P) in the Paraopeba River basin, located in the state of Minas Gerais, Brazil, following the collapse of tailings dam B1 in Brumadinho. The model was calibrated and validated for periods before (2000–2018) and after (2019–2021) the collapse of Brumadinho dam in 25 January 2019, with great accuracy measured by various performance indicators (e.g., R2 ≈ 0.8). The flow of phosphorus-containing sludge from the iron-ore tailings explored in the Córrego do Feijão Mine of Vale, SA, and released after the B1 dam break has impacted the Paraopeba River water through large increments in the TOT-P near the dam site (60–100%, with pre-rupture values varying between 0.06 and 0.1 mg/L). But other major sources were flagged, namely urban sources from the Betim region that raised TOT-P to ≈ 0.9 mg/L periodically during the entire simulation period. The study also revealed controls of TOT-P concentration in the basin, namely the coverage by forests that lowered down the TOT-P at 0.5–0.8 μg/L.km2. The lowering rate was, however, dependent on the occupation by argisols. As per the simulation results, the larger the percentage of argisols in a region the larger the TOT-P will be in the surrounding water courses, meaning that the argisols are prone to erosion and phosphorus leaching. A cluster analysis of input (e.g., terrain slope, soil type) and output (e.g., runoff) variables from the SWAT allowed relating TOT-P with surface- and TOT_P with underground-dominant hydrological processes, respectively runoff and groundwater flow, linking them to specific environmental variables such as argisols and steep slopes in the first case and latosols and smooth landscapes in the second case. The management implications retrieved from this holistic assessment were discussed. Finally, the TOT-P were checked against Brazilian environmental standards. In that regard, the concentration of total phosphorus was compared to limits established in Resolution 454/2012 of the National Environmental Council – CONAMA. Some sub-basins exhibited levels above the legal threshold, and the contamination was viewed as systemic requiring immediate action (e.g., implementation of sewage treatment and best management practices in agriculture), as well as monitoring in the spatial and temporal frames. The existence of extreme rainfall events in the basin was the main cause of SWAT model inaccuracies, namely of overestimated TOT-P.
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