The Hidden Threat of Remobilized Legacy Pollutants During Flood Events
SETAC Members Contribute Chapter to UNEP Report “Frontiers 2025: The Weight of Time”
Markus Brinkmann, University of Saskatchewan
As climate change intensifies extreme weather events, a long-overlooked environmental hazard is reemerging with potentially alarming consequences: the remobilization of legacy pollutants from contaminated sediments during floods. The 2025 edition of the United Nations Environment Programme’s (UNEP) Frontiers report brings this issue to the surface, highlighting the urgent need for integrated flood and pollution management strategies. Led by Markus Brinkmann, Associate Professor and Centennial Enhancement Chair in Mechanistic Environmental Toxicology at the University of Saskatchewan and Director of the School’s Toxicology Centre, a team of environmental toxicologists and chemists, as well as environmental management and hydraulic engineering specialists, contributed Chapter 4 of the report titled “Forgotten but Not Gone: Remobilization of Legacy Pollutants by Flood Events.”
Floodwaters are not just a physical threat – they are chemical vectors. Sediments in riverbeds, floodplains, estuaries and behind dams often act as long-term sinks for environmental pollutants, such as heavy metals, persistent organic pollutants (POPs) and polycyclic aromatic hydrocarbons (PAHs). These contaminants, some deposited many decades ago from industrial, agricultural and mining activities, can be suddenly remobilized during high-flow events, reentering ecosystems and food chains.
The report cites several case studies that underscore the scale and severity of this issue. For example, Hurricane Harvey in 2017 delivered massive sediment loads into Galveston Bay, Texas, along with elevated concentrations of mercury and PAHs. Similarly, catastrophic flooding in Nigeria’s Niger Delta in 2012 redistributed PAH-contaminated sediments across vast floodplains, exacerbating ecological and human health risks in an already vulnerable region.
Legacy pollutants are not confined to sediments only. In Europe, the 2002 Elbe river floods remobilized hexachlorocyclohexane (HCH) residues from former pesticide production sites, increasing contaminant levels in fish more than twentyfold. These events illustrate how historical pollution with chemicals that have since been banned, and which were once thought to be contained, can reemerge by climate-driven hydrological extremes.
The toxicological implications are profound. Due to their physicochemical properties, many sediment-bound contaminants also tend to bioaccumulate in aquatic organisms, disrupt ecosystems and pose long-term health risks to humans. Yet, these indirect effects of flooding are often overlooked in disaster response and environmental planning.
The authors call for a paradigm shift in flood risk management – one that integrates sediment quality assessments, contaminant mapping and nature-based solutions. Evaluating the stability and pollutant load of sediments in flood-prone areas is essential for anticipating and mitigating future hazards. Moreover, the report emphasizes the need for international cooperation, particularly in transboundary river basins, where upstream pollution can have downstream consequences. There is also potential for the impacts of flood-driven pollutant mobilization to intensify in the future as an increasing proportion of the world’s population settles in low-lying and flood-prone areas, potentially amplifying the issue.
For the SETAC community, this chapter serves as a call to action to expand the scope of environmental risk assessments and restoration efforts. It challenges scientists, regulators and practitioners to consider not only the pollutants of today but also those buried in the sediment archives of the past, waiting for the next flood to resurface.
The full report can be accessed on the UNEP website.
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