Increased river and coastal flooding could expose people to legacy toxic pollutants that were long buried in water and sediments, the United Nations Environment Programme warned in its report Frontiers 2025: The Weight of Time.
These legacy chemicals include heavy metals (like lead, cadmium, which are toxic at even low concentrations), certain persistent organic chemicals (like pesticides , synthetic chemicals, by-products of industrial processes and waste incineration). Both these categories do not breakdown easily and are prone to accumulate in sediments of polluted rivers, lakes and estuaries.
International and national regulations have banned some of them, but the older chemicals continue to stay even decades after the ban. They can cause neurotoxicity, immunotoxicity, hepatotoxicity, reproductive toxicity, and various cancers.
Floods, the UNEP explained, can re-suspend, disperse and re-deposit the pollutants from the contaminated sediment, which can spread across the landscape. “Many sediment-bound pollutants can also enter the food chain and accumulate in plants and animals, including humans,” read the report.
The UNEP has cited some examples of this. After striking the Texas coast in August 2017, the Hurricane Harvey triggered floodwaters, which released an enormous load of sediment which carried significantly high concentrations of carcinogenic chemicals and mercury into the Galveston Bay area of Texas.
Similarly, a catastrophic flood event in the Niger Delta in 2012 mobilised sediments contaminated with carcinogenic polycyclic aromatic hydrocarbons and deposited them over extensive portions of the floodplains.
A slightly different problem occurred in Pakistan. In 2010, the Pakistan flood flooded around a fifth of the country’s total land area, sweeping away a significant but unknown portion of 2,835 metric tonnes of obsolete pesticides and other persistent organic pollutants that were kept in storage facilities for proper disposal. This could further contaminate soils, water and sediments. The UNEP stressed that damage caused by the event, and the 2022 floods that affected a full third of Pakistan’s area, needs to be monitored and assessed.
The report explained that many regions are increasingly exposed to river and coastal flooding due to the upward trend in rainfall intensity and magnitude associated with tropical storms.
India, too, is at risk. The report, which analysed various scientific studies, showed that Ganga, Hindon and Vaigai had concentrations of cadmium in sediments above which harmful effects sediment-dwelling organisms are likely to occur frequently. Cadmium is a carcinogenic with potential endocrine disrupting effects. It can cause kidney and bone damage as well as adverse pregnancy outcomes.
UNEP calls for more studies evaluating the polluted sediments and other pollutant deposits in inland and coastal waters for their ability to release contaminants. This, they stated, is crucial to understanding the potential hazards to human and environmental health from flood events. “Future studies should prioritise investigating these aspects,” it highlighted.
Current sources of chemicals are also a concern. For example, millions of tons of persistent organic pollutant waste from organochlorine and organofluorine production are deposited in landfills globally, with 4.8 to 7 million tonnes.
The experts called for flood-control infrastructure and protection measures guided by nature-based solution to reduce the number and intensity of flood events and associated damages. They also suggested traditional flood control measures such as polders, dikes, and retention basins to restore riparian ecosystems by allowing floodplains, wetlands and forests to re-emerge or establish water-conscious urban development.
But they acknowledged that this will not be enough. These measures, they added, simply relocate contaminated sediments and confine the problem to areas that pose less threat to human and environmental health. So, they recommended developing river basin management plans that balance flood retention, river conservation and the multiple pressures on water resources.
The UNEP also stressed the importance of regular monitoring of river basin-specific pollutants. “As climate-related storms intensify, an adaptive management approach is essential for addressing the compounded and cascading effects of flood events, especially when addressing contaminant remobilisation risk,” read the report.
This involves continuously monitoring and reassessing the effectiveness of implemented measures and adjusting as science evolves as well as integrating local knowledge and engaging local communities, including citizen science efforts, in environmental monitoring and decision-making processes.