A new report by the United Nations Environment Programme (UNEP) has sounded the alarm on a hidden threat exacerbated by climate change legacy toxic chemicals buried in river and coastal sediments that could resurface during floods, endangering public health and the environment. Titled Frontiers 2025: The Weight of Time, the report highlights how intensifying floods driven by tropical storms and heavier monsoons can mobilize long-banned pollutants from sediments, releasing them into soil, water, and food chains.
These legacy pollutants include heavy metals like lead and cadmium, and persistent organic pollutants (POPs) such as obsolete pesticides and synthetic industrial chemicals. These substances, banned decades ago, still persist in riverbeds and estuaries due to their non-biodegradable nature. Once stirred by floodwaters, they can spread across landscapes, re-enter ecosystems, and accumulate in plants, animals, and humans, causing a range of health problems including cancers, neurological damage, organ toxicity, and birth defects.
UNEP highlighted past disasters that show the scale of this risk:
•Hurricane Harvey (2017) released mercury and carcinogenic chemicals into Galveston Bay, Texas.
•Niger Delta floods (2012) mobilized hydrocarbons across wide areas.
•Pakistan floods (2010 and 2022) potentially swept away over 2,800 tonnes of banned pesticides and POPs from storage sites, further polluting ecosystems.
The report emphasizes that India is also at risk, pointing to studies that show dangerous levels of cadmium in sediment samples from the Ganga, Hindon, and Vaigai rivers. Cadmium is especially dangerous as it can damage kidneys, bones, reproductive health, and disrupt hormones.
UNEP urges urgent attention and calls for:
•Scientific studies to evaluate the risk of toxic sediment release from inland and coastal waters.
•Nature-based solutions like restoring wetlands, floodplains, and riparian forests to absorb floodwaters.
•Traditional flood infrastructure like polders and dikes, integrated into river basin management plans.
•Regular monitoring of river-specific pollutants and adaptive flood management to adjust strategies as climate risks evolve.
•Community involvement and citizen science in data collection and environmental decisions.
The report also warns of current sources of pollution millions of tonnes of POP waste still sit in global landfills, with 4.8 to 7 million tonnes linked to organochlorine and organofluorine production.
While climate adaptation measures can reduce damage, the UNEP cautions that merely shifting polluted sediments away from populated zones is not a permanent fix. Instead, it calls for a comprehensive approach that balances flood prevention, ecosystem restoration, and public health protection.
In the face of worsening climate-linked floods, the message is clear: the past’s pollution is not behind us—and unless addressed, it could re-emerge with devastating consequences.