A perspective and roadmap for emerging toxic flood hazards affecting ecosystems and society

Abstract Floods increasingly interact with anthropogenic pollution sources, remobilize legacy pollution, release emerging pollutants and harm critical infrastructure, transforming hydrologic extremes into compounding pollution disasters. Here we introduce the term ‘toxic floods’ to provide a perspective on emerging floods which mobilize, transport, and deposit hazardous contaminants, creating exposure pathways for ecosystems and humans. We propose that toxic floods constitute a multi-hazard risk across riverine, urban, and coastal settings, because contaminant (re-)distribution can generate persistent hotspots and secondary remobilization sources that outlast the primary flood event, producing acute and chronic exposure with long-term adverse environmental impacts, socioeconomic disruption, and public-health implications. Drawing on interdisciplinary evidence, we synthesize the key controls on toxic-flood dynamics, such as erosion and remobilization, infrastructure failure, phase partitioning, and deposition in low-energy zones, to provide a mechanistic understanding of when and where flood-related toxicity emerges. However, critical research gaps remain, including sparse observations and datasets, limited coupling of hydro-sediment dynamics with contaminant transport and fate, and inadequate integration of exposure and health risks into flood risk assessment. Building on these mechanisms and the prevailing research gaps, we outline a roadmap for actionable research and decision-making spanning pollution hotspot and pathway characterization, adaptive monitoring and modeling, and coordinated mitigation and governance strategies. This conceptual framework establishes a scientific foundation for developing evidence-based mitigation strategies and engineering solutions to address the projected increase and intensification of future floods, one of the most pressing environmental challenges of the twenty-first century.