The dam, a 40-meter wall of rocks and dirt, gave way without warning, unleashing a torrent of mud. Within a day, some 21 million cubic meters of gray goo and water—the tailings waste left behind by 16 years of copper and gold mining at the Mount Polley mine in western Canada—escaped from a holding pond behind the dam, buried a creek, and poured into Quesnel Lake, home to one-third of British Columbia’s legendary Fraser River sockeye salmon.

The 2014 Mount Polley disaster shocked mining engineers around the world. Many considered Canada a leader in developing rules aimed at preventing the failure of such tailings dams, and respected the mine’s owner, Imperial Metals. “That wasn’t supposed to be able to happen,” Jim Kuipers, an engineer and former tailings dam manager who now consults for environmental groups, recalls a colleague telling him.

Since then, the sense of crisis has deepened. In 2015, a tailings dam in Brazil collapsed, unleashing a mammoth mud spill that killed 19 people, contaminated 668 kilometers of river, and reached the Atlantic Ocean. In 2018, a dam failed at a major mine in Australia; luckily, a second barrier prevented disaster. Last year, a dam disintegrated at a decommissioned Brazilian iron mine, releasing a torrent that killed 270 people.

Engineers fear more catastrophes await, as the world confronts a swelling volume of muddy mine tailings, contained by more and larger dams. Some rise to nearly the height of the Eiffel Tower and hold back enough waste to fill Australia’s Sydney Harbor. “The consequences of a failure are getting much bigger,” says Priscilla Nelson, a geotechnical engineer at the Colorado School of Mines.

In response, scientists, governments, environmentalists, and miners are searching for safer ways to handle the tainted mud. Some are trying to simply inventory the world’s tailings dams—estimates of the number range from 3500 to 21,000—and identify those most at risk of failure. A few have called for a ban on one common but failure-prone design. Others are working on regulatory and management fixes. “The mining industry,” says Joseph Scalia, a geotechnical engineer at Colorado State University, “is realizing they can’t just spend as little as possible and the problem is going to go away.”

TAILINGS ARE THE TRASH of the mining world. To extract most metals, from iron to gold, miners often mix pulverized rock with water, creating a milkshake of silt and gravel. As higher quality mineral deposits run out, miners are turning to lower grade sources that generate more waste. Worldwide, the metal content of copper ore has fallen by nearly half since the mid–20th century. Extracting a single kilogram of copper can now produce 200 kilograms of sludge. The muck is often contaminated with toxic metals or minerals that produce sulfuric acid when exposed to air.

Tailings dams, unlike those built to store water or generate power, don’t earn revenue, creating an incentive for mine owners to minimize costs. Many are built piecemeal throughout the life of a mine. And the barriers are often made from a mixture of rock and the tailings themselves, rather than a more uniform and predictable material such as concrete. Those factors contribute to a failure rate that, over the past century, was more than 100 times higher than that of reservoir and power dams, according to one estimate.

Each disaster has its own constellation of causes, but some arise from seemingly trivial errors. At Mount Polley, investigators led by Norbert Morgenstern, a geotechnical engineer at the University of Alberta concluded that part of the dam was built on a weak patch of silt and clay. Exploratory boreholes drilled prior to construction were too shallow to find the problem. Builders further weakened the dam by making its walls steeper than planned, after the company ran short of rock. One night, the weight of the sludge became more than the dam could bear.

It could have been much worse. No one died. Workers ultimately repaired the dam and shoveled up much of the mud that had buried the creek. (The company says the spill didn’t cause long-term harm to the Quesnel Lake ecosystem, but some ecologists say it’s still too early to tell.)

Morgenstern, who also led the investigations into the 2015 Brazilian incident and the 2018 Australia failure, has found that faulty engineering, including inadequate scrutiny of the underlying geology, was at the heart of all but two of 15 major incidents between 1980 and 2015.

One major problem, he says, is the “normalization of deviance.” The phrase, coined after the 1986 explosion of the space shuttle Challenger, describes how engineers can be lulled into accepting a series of seemingly small risks that snowball into a catastrophe.

There is an unwritten covenant that regulators and mine owners can count on engineers to design a safe tailings system, Morgenstern told a gathering of Brazilian geotechnical engineers in 2018. “That covenant,” he said, “has been broken.”

C. BICKEL/SCIENCE

THE SEARCH IS ON for fixes. Some mining watchdogs are calling for replacing one common type of dam, called an upstream dam, and banning future use of the design. Upstream dams are built in stairlike stages, heading upstream over the accumulating tailings (see graphic, above). Part of the weight of each added step is borne by the tailings below. This approach is often the cheapest, because the tailings serve as construction material.

More than 40% of major tailings dams are the upstream design, according to a global inventory of more than 1700 dams recently launched by pension funds of Sweden and the Church of England, which have pressed the mining industry to strengthen environmental and safety measures. A study of 8000 tailings dams in China found that 95% were upstream dams. And such dams are involved in three-quarters of tailings dam failures, according to one estimate.

The problem is that tailings aren’t a predictable building material, and they are often waterlogged. The water can act like a lubricant, reducing the friction that binds an earthen dam together. Engineering flaws such as poor drainage can exacerbate the problem. In extreme cases—such as the 2019 disaster at the Brazilian iron mine—dam sections simply liquefy.

In Chile, where earthquakes make upstream dams even riskier, the government has forbidden the design since 1970. Brazil banned them in the wake of the 2019 accident, and has ordered the mothballing of all upstream dams by 2027. Worldwide, such a policy could mean the demise of thousands of mines and tailings dams (which could be replaced by dams with different designs). Although such a change might be expensive for companies, right now communities near dams are bearing the costs of cheaper construction, says Payal Sampat of Earthworks, a nonprofit group that promotes mining reforms. “That is unacceptable.”

Some experts caution against a one-size-fits-all approach. Upstream dams can perform safely, particularly in places with dry climates and few earthquakes, says David Williams, a geotechnical engineer at the University of Queensland, St. Lucia. “You can construct [an upstream dam] to be perfectly safe. You can also build it in a not so good way.”

One knowledge gap is an understanding of the forces that can suddenly turn an earthen dam into a liquid river of mud. At the Georgia Institute of Technology, geotechnical engineer Jorge Macedo is stress testing tailings in his lab to document the conditions that trigger liquefaction, particularly in silt, a little-studied material that is common in tailings used to build upstream dams.

Other researchers are looking at better ways to spot dams on the verge of failure. Moe Momayez, an engineer and geophysicist at the University of Arizona, is testing sensors on an Arizona dam that track temperature and moisture levels. Some dams are already equipped with radar or lasers that watch for worrying bulges. Momayez’s goal is to integrate streams of data in a computer system that can spot problems that might escape periodic inspections. “We have a pretty good idea how these tailing dams fail,” Momayez says. “The question is, can we predict that, can we get ahead of the curve?”

Some engineers would like to simply eliminate the need for massive dams. “The best tailings dam is no dam at all,” Nelson says. She is studying whether mine waste can be melted into glasslike fibers that could be used for textiles or reinforcing concrete. In June, mining giant BHP said it would spend $10 million to study such reuse of copper tailings.

A more mature approach is to wring the water from tailings, creating waste the consistency of damp earth, which can be sculpted into mountains. The leftovers can still be toxic, but there’s less danger of a devastating flood, says Jan Morrill of Earthworks. “Filtered tailings should be considered the industry standard,” Morrill says.

Although the approach has been around for decades, it’s rarely used, representing just 4% of tailings systems in the pension funds’ inventory. Filtered tailings systems can cost five to 10 times more than a conventional dam, says Harvey McLeod, a geological engineer who designs tailings dams for Klohn Crippen Berger, a private firm. It’s also an enormous challenge to process tailings at big mines churning out 100,000 tons of waste per day, particularly in wet climates. “It’s easier said than done,” McLeod says.

MANY GROUPS are also pushing for regulatory and management reforms. After the 2019 Brazilian disaster, investment funds worth more than $10 trillion helped bring together officials from industry, government, and the investor group Principles for Responsible Investment to create a set of global guidelines for tailings dam construction. Earlier this month, the coalition issued its plan, calling for stiffer engineering standards for new dams. It also urges top mining executives, rather than lower level staff, to be responsible for tailings safety, and for independent experts to review companies’ waste plans. But it doesn’t push for a ban on upstream dams.

Morgenstern notes that similar reforms he and others suggested in the late 1990s, after an earlier string of dam disasters, were never fully embraced. He expects it won’t become clear until the end of the year whether the new proposals will fare better. Still, he’s heartened that, after the recent tragedies, muddy mine waste is again in the spotlight. “The tree,” he says, “has been shaken.”

Posted in: Earth  doi:10.1126/science.abe3917