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

Aas soon as the barrier broke, a flood of poison brought death into the river. Crossing the fragile wall built to contain mining waste in Zambia’s copper belt in February 2025, more than 50 million cubic liters of acid and heavy metals spilled into Chambishi Creek, a tributary of the Kafue River, the country’s longest river.

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

For the millions of Zambians who rely on Kafue, the collapse of the tailings dam at China’s Sino-Metals Leach copper mine triggered a national environmental emergency that has yet to end. The spill disrupted the drinking water supply to Kitwe, Zambia’s third-largest city, home to half a million people.

Signs of pollution were detected 60 miles downstream from the collapse. Helicopters continued the spill downstream, dropping lime into the water to try to neutralize its corrosive power.

The affected region is home to rare wildlife, including the Kafue lechwe antelope, Zambian barbet and wattled crane.

“It looked like diesel mixed with oil. We had already planted our crops, but they died. When you now turn the soil to plow it for sowing, it has turned yellowish and has a pungent smell,” says Mary Milimo, a 65-year-old small-scale farmer near where the Mwambashi River joins the Kafue.

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

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

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

Nearly a year later, the Kafue disaster has become another black mark for the mining industry and its long history of environmental disasters caused by improperly stored waste. Tailings dams – deposits of often toxic mining waste stored underwater – litter landscapes around the world. Often, they contain enormous amounts of toxic and harmful materials.

While tailings dams are theoretically built to last forever, more extreme weather brought on by the climate crisis has changed the risk profile of many structures. Floods, intense rains and other extreme weather events make many of them more unstable, experts say, increasing the risk of future disasters.

Analysis carried out for the Guardian by researchers Tim Werner and Victor Wegner Maus, who have played a leading role in establishing the true scale of the world’s mining industry, found that at least 108 tailings dams are located in key biodiversity areas across the world, although this figure is likely to be a significant underestimate due to lack of data. This represents approximately 5% of known tailings facilities in the Global Tailings Portal database.

In 2019, 272 people died near Brumadinho, Brazil, when a tailings dam burst, releasing a torrent of mud onto a miner’s canteen and communities below. Four years earlier, another burst dam in Mariana, Brazil, killed 19 people, spreading pollution along more than 600 kilometers of rivers and streams. The breach caused widespread ecological devastation, increasing the risk of extinction for 13 aquatic species and negatively impacting 346, according to subsequent 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 toxic forms, building up food chains, inhibiting plant growth, and altering populations of soil microbes.

Professor Elaine Baker, a marine scientist at the University of Sydney who helped develop the world’s first public database of mine tailings dams, says: “The way we mine is still very similar to how the Romans did it. We get a lot of waste and we dump it somewhere and we hope it doesn’t hurt anyone.”

“They don’t just disappear,” she adds. “They have to be maintained in perpetuity. So we leave our descendants with huge piles of waste.”

“They’re inherently less stable than water dams. We often build them in valleys where you build a dam wall and dump the tailings behind it. They’re some of the largest man-made structures on the planet. When they burst, you get this muck of mud that will just go down the hill,” she says.

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

Bora Aska, a doctoral student at the University of Queensland, studied the extent of tailings dams in protected areas. Its work found that many of them are in important areas of high biodiversity and are at disproportionate risk of collapse.

“Shockingly, 9% of all tailings dams worldwide were in protected areas. The majority were created after the protected area was created. These were also very high-risk tailings facilities, by industry standards,” she says.

Institutional investors, such as the Anglican Church Pension Fund, have sought to push for greater transparency on tailings dams in the mining sector, launching a safety initiative after the Brumadinho disaster. Together with the Swedish National Pension Fund Ethics Council, they brought together investors who oversee a total of $25 trillion (£18.5 trillion), urging the companies they invest in to adopt the highest standards in residue management.

Illegal and artisanal mines, which have few protocols for dealing with mining waste and even fewer incentives, also contribute to the problem.

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

“Any loss of life at a tailings management facility is unacceptable. We have seen many improvements in the standards for conventionally managed tailings,” she says, detailing a 73-point standard the mining industry has developed for best practices in waste management.

“Even though it may seem disastrous,” she said. “I think we’ve made really significant progress since the standard was put in place.”

Despite the industry’s efforts, Gagen acknowledged that most mining companies were not ICMM members and that a minority of the 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 conditions due to climate change, which are expected to place additional pressure on tailings management facilities.

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

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

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