WC134 JanFeb 2024 - Magazine - Page 23
Dr. Philip N. Owens
L to R: Researchers from the University of Northern British Columbia collect a sample of plankton from Quesnel Lake. Researchers from
the University of Northern British Columbia, the University of British Columbia, and the federal Department of Fisheries and Oceans
collect a core of the tailings from the bottom of Quesnel Lake.
Bas Vriens, assistant professor of environmental engineering
at Queen’s University in Kingston, Ontario, says submerged
storage is common in mining and the approach seems sensible
for Mount Polley. “If you go down the water column several
tens or even hundreds of metres your oxygen levels are going
to decrease, so you can significantly and sometimes completely
halt the oxidation reactions that would cause metal leaching,”
Vriens says, noting that mining companies sometimes fill old pits
with water and repurpose them for much the same reason. He
adds that removing the slurry from the lakes where it’s become
ensconced presents a much greater environmental hazard due to
risks of triggering oxidation and remobilizing potentially harmful
compounds. “You would be talking about years of dredging
and pumping, and thousands, even tens of thousands, of truck
movements to get all the material from one place to another. You
would have CO2 emissions, noise, dust, vibration from pumping, and traffic. So, it’s a trade-off. What’s worse for the environment—leaving the tailings in place and having an acceptable but
low level of leaching, or performing a very invasive remediation
action where you remove all the material and completely disrupt
the ecology of the area on a broader scale for a long time?” Vriens
says the metal concentrations at Mount Polley were not necessarily problematic given the large volume of water in the tailings
facility, and a more significant challenge was the considerable
WAT E R C A N A D A . N E T
increase in turbidity. “Plants and fish and photosynthesizing
organisms that live in those creeks all of a sudden had this cloudy
water for a long period of time, and that can do more damage
than any contaminants contained within them. Other major solutes like carbonate or sulphate were elevated as well, so the heavy
metal elements were not the only story.”
Mining resumed in 2016, and MPMC rebuilt the pond and
dam to include protective rock berms designed to reduce the elevation of Polley Lake. Water treatment systems, including Veolia
ACTIFLO water treatment plant technology, filter the pond for
contaminants before release. The treated wastewater, now discharged directly into Quesnel Lake through pipes and diffusers at
depth, is sampled regularly, with the environment ministry monitoring factors such as pH, conductivity, turbidity, total suspended solids, total dissolved solids, total organic carbon, hardness,
alkalinity, nutrients, general ions, and total and dissolved metals.
Lake water quality is also routinely monitored and sampled as
part of the mine’s environmental monitoring plan.
Thus far, remediation and other expenses have cost MPMC
and Imperial Metals more than $70 million, a figure that doesn’t
include costs incurred by the province and other public bodies.
However, no charges or fines were levied against either company. Instead, regulators blamed faulty engineering, noting that
the 40-metre-high dam had been built atop a sloped glacial
WATER C AN ADA • JANUARY/ FEBRUARY 2024
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