WC137 JulyAug 2024 - Magazine - Page 30
MINING
Measuring a mine’s impacts
Through the Quesnel River Research Centre at the University
of Northern British Columbia, Ellen Petticrew and Philip
Owens have measured the mine’s impacts on area waterways over the last decade. Owens recalls early discussions,
soon after the dam failure, where proponents and regulators
discussed various options, including capping or even dredging
the spilled tailings, which had mostly settled on the basin
bottom roughly 100 metres beneath the surface. “If it was
dredged it could have actually made the situation much worse
for a short term because all this contaminated fine material
would have inevitably been resuspended into the water column,” said Owens. “So, the decision (to leave the waste at the
bottom) wasn’t necessarily wrong. But I think the idea of potentially capping and amending it with carbon or something
ecologically suitable might have been a better option.”
In a posting on its website, however, MPMC says analyses
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WATER C AN ADA • JULY/AUGUS T 2024
Leave it or move it?
Dumaresq offers three-fold reasoning for mines storing waste
in situ rather than removing it for disposal off-site as urban
industrial brownfields often do. First, modern mines produce
tens of thousands of tonnes of tailings daily, depending on
their size. “Considering that a typical dump truck suitable
for use on public roads holds about 20 tonnes, the logistics
and environmental impact (e.g. greenhouse gas emissions) of
transporting tailings to off-site locations would be immense,”
WAT E R C A N A D A . N E T
James Rankin, Ellen Petticrew
Ellen Petticrew and Philip Owens of UNBC perform fieldwork in the
Quesnel River. UNBC researcher Jordan Lindgren studies sediment
samples from Quesnel Lake. If lakes have high sediment levels, it
is sometimes preferable from an ecological point of view to leave
toxins alone.
UNBC researcher Kyle da Silva recovers a measuring device used
to track pollutants from the Mount Polley mine tailings spill in 2014.
Depending on the depth of the lake, removing pollutants is sometimes a less preferable option than dredging and trucking them.
of water, sediment, benthic organisms, and fish determined
that the tailings at the bottom of the lake are chemically and
physically stable, so the best environmental option was, in
fact, to leave the slurry undisturbed, let natural sediments
slowly cover it, and allow the lake and adjacent waterways to
continue to recover naturally. Mount Polley’s tailings don’t
contain the ingredients needed to produce acid rock drainage,
so Quesnel Lake has never faced that particular nightmare
scenario. However, the UNBC researchers have documented
copper-rich sediments in upper levels of the water strata, as
well as elevated nutrients such as phosphorus in the water
column and higher metal concentrations in phytoplankton
and zooplankton, which fish consume. Petticrew and Owens
attribute this largely to reversals of the lake’s water columns as
temperatures shift each spring and fall, driving the contaminants upwards, allowing transport uplake, downstream or,
alternatively, resettling.
“We’re picking up elevated metal concentrations in the
lower levels of the food web,” said Petticrew, noting that fish
such as trout should be monitored for any metal content.
Although he has concerns specific to Mount Polley and
Quesnel Lake, Owens hesitates to take the notion of deep-water tailings storage completely off the table. “It could be the
lesser of two evils,” he said. “Terrestrial tailings ponds have a
high risk of failure. So, in some situations it may make some
sense, depending on the lake, where it’s located, and any
cultural or ecological significance. If there’s a high natural
sedimentation rate, it would probably bury it quite quickly,
particularly if it’s given artificial help through capping or
something like that.” Petticrew points to modern innovations
such as dry stacking, where tailings are dewatered, compacted,
and stored benignly above ground.
While Mount Polley’s reliance on Quesnel Lake seems set
in stone, at least for now, mine owners almost uniformly use
the facilities that handled tailings during the mine’s productive years to accommodate waste post-closure. “One thing
that’s increasingly done with tailings at closure is recontouring
the facilities and starting to move towards facilities that mimic
more natural landforms,” added Dumaresq. “That’s one way
to help reduce physical risk, and also a way to potentially
reduce chemical risk, depending on what’s in the tailings.”
