Mining’s toxic timebomb: dams full of poisonous waste are dotted around the world. What happens when they burst?

While tailings dams are meant to last for ever, extreme weather events are making many unstable – with devastating consequences for nature and humans

As soon as the barrier broke, a flood of poison brought death to the river. Gushing through the fragile wall built to hold back mining waste in Zambia’s copper belt in February 2025, more than 50m cubic litres of acid and heavy metals poured into the Chambishi stream – a tributary of the Kafue River, the country’s longest waterway.

Thousands of lifeless fish rose to the surface as a plume of acid floated downriver, leaving dead crocodiles and other wildlife in its wake.

For the millions of Zambians that depend on the Kafue, the tailings dam collapse at the Chinese state-owned Sino-Metals Leach copper mine triggered a national environmental emergency that is yet to end. The spill shut down drinking water supplies for Kitwe, Zambia’s third-largest city, home to half a million people.

Signs of pollution were detected 60 miles downstream from the collapse. Helicopters chased the spill downriver, dropping lime into the water in an attempt to neutralise its corrosive potency.

The affected region is home to rare wildlife, including the Kafue lechwe zntelope, the Zambian barbet bird, and the wattled Crane.

“It looked like diesel mixed with oil. We had already planted our crops, but they died. When you now turn up the soil to till it for planting, it has become yellowish and has a pungent smell,” says Mary Milimo, a 65-year-old smallholder close to where the Mwambashi River joins the Kafue.

“There are no more fish here,” says Patrick Chindemwa, 66, who farms nearby. “I planted maize in October using irrigation. All the maize dried up.

“The ground is yellow and soil here is like grease; it is slippery and when it rains, it melts. We need help,” he says.

Sino-Metals did not respond to a request for comment.

Almost a year later, the Kafue disaster has become yet another black mark against the mining industry and its long history of environmental disasters caused by poorly stored waste. Tailings dams – repositories of mining waste that is often toxic and stored under water – litter landscapes around the world. Often, they hold huge quantities of poisonous, harmful material .

What are tailings dams? 

Tailings dams are structures designed to store mining waste. They are meant to last forever. Some are built like traditional dams that hold back water, while others are constructed with rock and other waste materials. Some are enormous, numbering among the largest engineered structures on the planet. 

What do they hold? 

While the contents depend on the type of mine, most store mud, rock and waste water. However, high concentrations of heavy metals and other substances harmful to humans and nature are often unearthed during the mining process, and these frequently form part of a tailings dam's contents. 

What happens if they fail? 

When tailings dams fail, the consequences can be disastrous. Huge amounts of pollution can rapidly enter the surrounding environment, poisoning water, soil and wildlife. In the worst cases, hundreds of people have been killed. In 2019, 272 people died near Brumadinho, Brazil, when a tailings dam burst, releasing a torrent of mud on to a mineworkers’ canteen and communities below.

While tailings dams are theoretically built to last for ever, more extreme weather driven by the climate crisis has changed the risk profile of many structures. Flooding, intense rainfall and other weather extremes mean that many of them are more unstable, according to experts, increasing the risk of future disasters.

Analysis produced for the Guardian by researchers Tim Werner and Victor Wegner Maus, who have played a leading role in establishing the true scale of the mining industry around the world, found that at least 108 tailings dams are situated in key biodiversity areas around the world, although this is probably a significant underestimate due to data limitations. This is about 5% of known tailing facilities on the Global Tailings Portal database.

In 2019, 272 people died near Brumadinho, Brazil, when a tailings dam burst, releasing a torrent of mud on to a mineworkers’ canteen and communities below. Four years earlier, another burst dam in Mariana, Brazil, killed 19 people, spreading pollution along more than 400 miles of river and watercourses. The breach brought widespread ecological devastation, increasing the extinction risk of 13 aquatic species and negatively impacting 346, according to later studies.

The impact of tailings dams on the environment can last for decades, often with disastrous consequences for nature. Heavy metals do not degrade over time and can evolve into many poisonous forms, accumulating up food chains, inhibiting plant growth and altering populations of soil microbes.

Prof Elaine Baker, a marine scientist at the University of Sydney who helped develop the first public database of mine-tailings dams around the world, says: “The way we do mining is still very similar to the Romans. We get a whole lot of waste and we dump it somewhere and we hope that it’s not going to hurt anybody.

“They do not just go away,” she adds. “They have to be maintained in perpetuity. So, we’re leaving our descendants huge piles of waste.

“They are inherently less stable than water dams. We often build them in valleys where you make a dam wall and dump the tailings behind it. They are some of the largest engineered structures on the planet. When they burst, you get this slurry of mud which will just go down the hill,” she says.

Due to the mining industry’s secretive nature, the true global scale of tailings dams is still poorly understood. But with growing demand for construction materials and those needed for the renewable energy transition, huge quantities will be extracted in the coming decades.

Bora Aska, a PhD student at the University of Queensland, has been researching the scale of tailings dams in protected areas. Her work has found that many are in important areas of high biodiversity, and at disproportionately greater risk of breaking down.

“Shockingly, we found that 9% of all tailings dams worldwide were in protected areas. The majority had been created after the protected area was formed. They were also very high-risk tailings facilities, according to industry standards,” she says.

Institutional investors, such as the Church of England’s pension fund, have sought to push for greater transparency about tailings dams in the mining sector, launching a safety initiative after the Brumadinho disaster. Along with the Swedish National Pension Funds’ Council on Ethics, they have brought together investors who oversee a combined $25tn (£18.5tn) – urging firms they invest in to adopt the highest standards of tailings management.

Also contributing to the problem are illegal and artisanal mines, which have few protocols to deal with mining waste and even less incentive.

Emma Gagen, research director at the International Council on Mining and Metals (ICMM), an industry body that aims to improve sustainable development in the sector, says Brumadinho has been a turning point for mining companies in their approach to tailings dams.

“Any loss of life from a tailings facility is unacceptable. We’ve seen a lot of improvements in standards for conventionally managed tailings,” she says, detailing a 73-point standard that the mining industry has developed for best practice in managing waste.

“As much as it might look dire ,” she says. “I do think we have made really significant progress since the standard has come into place,.”

Despite the industry efforts, Gagen acknowledged that most mining companies were not ICMM members, and a minority of tailings dams they oversee were likely to meet the council’s standards. The benchmarks are designed to adapt to the challenges of more extreme weather from climate breakdown, which is expected to put extra pressure on tailings facilities.

Last year, an investigation into the Kafue disaster by Zambian authorities found no evidence that the tailings dam was managed by qualified engineers, with cracks and uncompacted walls found in the structures. Experts warn that without radical action, similar disasters could occur.

Baker says: “There’s no reason why we’re seeing tailings dams in so many wilderness areas and protected areas. The industry goes where it wants to go, wherever they find deposits. They don’t really care.”

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