The United Nations Environment Programme (UNEP) has raised serious concerns about the hidden threat of toxic chemicals resurfacing during floods, in its latest Frontiers 2025: The Weight of Time report. The report highlights how rising river and coastal flooding can disturb long-buried pollutants in sediments posing renewed health and ecological risks worldwide.
These “legacy chemicals” include heavy metals such as lead and cadmium along with persistent organic pollutants (POPs) like banned pesticides and industrial by-products. Despite regulations and bans in place for decades, these substances remain lodged in riverbeds, lakes, estuaries, and coastal zones refusing to degrade and continuing to accumulate. During flood events, these pollutants can be resuspended and transported across the landscape, potentially contaminating food chains, water sources, and ecosystems.
The report cites past flood disasters to illustrate the scale of the threat. In 2017 Hurricane Harvey in the United States stirred up toxic sediments containing mercury and cancer-causing compounds, which then entered the Galveston Bay. Similarly Nigeria Niger Delta floods in 2012 mobilised carcinogenic pollutants across vast floodplains. In Pakistan, the 2010 floods swept away an unknown quantity of over 2,800 tonnes of obsolete pesticide waste raising long-term contamination risks.
UNEP analysis warns that climate change is making these events more frequent and intense. As rainfall patterns grow more extreme due to warming temperatures, many regions including India face heightened exposure to this hidden hazard. Rivers such as the Ganga, Hindon, and Vaigai already show cadmium levels in sediments high enough to threaten aquatic life and potentially human health. Cadmium exposure can damage kidneys and bones, disrupt hormones, and harm pregnancies.
Current chemical storage practices are also a concern. The report notes that 4.8 to 7 million tonnes of persistent organic pollutant waste particularly from organochlorine and organofluorine industries remain in landfills worldwide. These can easily be mobilised by flooding, increasing the risks of widespread contamination.
To address this growing problem, UNEP recommends a multi-layered approach. This includes mapping and monitoring contaminated sediments, creating adaptive river basin management plans and integrating traditional flood control methods like polders, dikes, and wetlands with modern nature-based solutions. However, experts caution that merely relocating contaminated sediments is not enough it only shifts the problem to less vulnerable areas.
The report also emphasizes the role of local communities and citizen science in monitoring efforts. As climate-related storms intensify, an adaptive, informed, and inclusive strategy is essential to manage both the visible and hidden risks posed by floods in an era of escalating environmental challenges.