T
he dam, a 40-meter wall of rocks
and dirt, gave way without warn-
ing, unleashing a torrent of mud.
Within a day, some 21 million cu-
bic meters of gray goo and water—
the tailings waste left behind by
16 years of copper and gold min-
ing 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 sup-
posed to be able to happen,” Jim Kuipers,
an engineer and former tailings dam man-
ager 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 Aus-
tralia; luckily, a second barrier prevented
disaster. Last year, a dam disintegrated at a
decommissioned Brazilian iron mine, releas-
ing 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 conse-quences of a failure are getting much bigger,”
says Priscilla Nelson, a geotechnical engineer
at the Colorado School of Mines.
In response, scientists, governments, en-
vironmentalists, 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 num-
ber range from 3500 to 21,000—and iden-
tify those most at risk of failure. A few have
called for a ban on one common but failure-
prone design. Others are working on regu-
latory and management fixes. “The mining
industry,” says Joseph Scalia, a geotechnical
engineer at Colorado State University, “is re-
alizing they can’t just spend as little as pos-
sible 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, min-
ers 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 con-
taminated 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 reve-
nue, creating an incentive for mine owners
to minimize costs. Many are built piece-
meal 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 ma-
terial such as concrete. Those factors con-
tribute to a failure rate that, over the past
century, was more than 100 times higherA string of catastrophic failures
has raised alarm about dams meant
to contain muddy mine wastes
By Warren Cornwall
A DAM BIG
PROBLEM
Mud released by a burst tailings dam at an iron mine
near Brumadinho, Brazil, killed 270 people in 2019.NEWS | FEATURES | MUDPHOTO: WASHINGTON ALVES/REUTERS
2 1 AUGUST 2020 • VOL 369 ISSUE 6506 907
Published by AAAS