Speedy acid rock drainage is top priority at the Faro mine site

When Yukon prospector Al Kulan stumbled across a lead-zinc deposit downstream of Ross River in 1953, it's unlikely he imagined his discovery leading to the most environmentally tainted mine site in the territory.

When Yukon prospector Al Kulan stumbled across a lead-zinc deposit downstream of Ross River in 1953, it’s unlikely he imagined his discovery leading to the most environmentally tainted mine site in the territory.

With an estimated cost of at least $700 million, the remediation of the Faro mine is expected to continue for hundreds of years. In operation from 1969 to 1998, the Faro mine produced between 5,000 and 9,300 tonnes of ore per day when it was at its peak, churning out approximately 15 per cent of the world’s zinc and lead output.

These days, the Faro site is buzzing with a different kind of activity. A water treatment plant has replaced the sound of rocks rumbling through ore mills and miners with hard hats have been traded in for water quality testers and environmental consultants.

The Yukon government is currently in the design and planning stage for remediating the site. Temporary structures will be put in place in 2015 and more permanent ones will likely appear in 2020, says the Yukon’s director of assessment and abandoned mines, Stephen Mead. Remediation of the site is a challenging prospect simply because of the amount of mine tailings that need to be dealt with, he says.

There are 320 million tonnes of waste rock (rocks that were removed to access the ore) and 70 million tonnes of tailings (the byproduct of milling ore) on the Faro site. This material is rapidly deteriorating in a process know as acid rock drainage, which is threatening surrounding plants and animals.

Acid rock drainage is a natural process that takes place when sulphur-containing rock is exposed to oxygen and water. It’s a chemical reaction that’s similar to rusting and weathering. However, when sulphurous rock is degraded it’s released as sulphuric acid into the surrounding environment.

The acid dissolves metals in nearby rocks and the resulting mixture is particularly harmful to aquatic life. The process is common to many mine sites in the Yukon, but not to the same extent as at the Faro mine.

In a natural setting, acid rock drainage happens to such a slow degree that the environment can neutralize or adapt to the acidity. But when large amounts of ore are brought to the surface, the chemical reaction is significantly sped up, says Mead. During the process of mining, the rock is hauled to the surface and crushed, creating more surface area for oxygen and water to react with.

The release of these metals as a result of acid rock drainage will continue at the Faro site for the next 400 to 800 years, scientists have estimated.

Acid rock drainage severely affects the pH of surrounding water. “We’ve got all sorts of different levels of pH on the site depending on what material water is stored in or comes in contact with,” says Mead. In some of the tailing ponds – there are three on site – the pH hovers around 4 or 5 (the pH of freshwater lakes and ponds is generally 6 to 8). At a pH of about 5 fish populations begin to disappear in a lake and at around 4.5, the water becomes devoid of fish.

When the water reaches a pH of 3.5, an iron bacterium, Thiobacillus ferroxidans, can further dissolve metal sulfides, such as pyrite, producing ferric hydroxide which smothers vegetation. “In some of the more contaminated sources of water, these are small sources, we occasionally see pH levels as low as 2 which is about the pH of a can of Coke,” says Mead.

But it’s not only water in direct contact with sulphur-containing rock that is affected. Fine-grained metals are also dispersed off-site to surrounding areas, says Mead. Plants uptake those metals which are then eaten by land mammals. “Moose, for instance, eat those plants and then accumulate metals in their body,” he says.

The government does an extensive survey of large mammals in the area, taking tissue samples to look for any signs of contamination that would be harmful to humans who may hunt the animal. Although scientists have seen metals accumulate in moose, it hasn’t been at levels that pose any risk to people, says Mead.

Contamination to surrounding water is also something that needs to be taken into consideration. A water treatment facility on the Faro site is being used to treat surface water that moves through the complex. Nearby creeks and streams are diverted away from the mine while any snowmelt, rainwater or runoff that trickles into the area is treated. “All the contaminated water is constrained to the mine’s footprint where it is intercepted and sent to the treatment plant and when it’s clean, we discharge it beyond the footprint,” says Mead. Once the water is discharged it meets national environmental standards, he says.

But groundwater which seeps through layers of soil is trickier to decontaminate. “There’s a large layer of organic material in the ground that slows the contaminants and then over tens of years those contaminants start moving into underground aquifers,” says Mead. Several years down the road that water will return to the surface again with contaminants in tow.

Scientists regularly test more than a hundred groundwater wells on the Faro site to check for contamination. In the next five years, as groundwater contamination is expected to increase, the government will install pumps to intercept tainted water and circulate it through the treatment facility.

One way of slowing the process of acid rock drainage is adding a thick soil cover to the sulphidic waste. In 2010 Energy, Mines and Resources installed one of these covers on a section of the waste rock at Faro. Engineered soil covers are made of gravel and soil and slow the rate at which contaminants are created by minimizing the contact between the waste rock and water and oxygen.

A plan is in place to put a larger soil cover over the remaining waste rock by 2020. That’s when other remediation features such as a new water treatment plant, upgraded dams to keep tailing ponds from leaking and a re-sloping of waste rock will occur.

Stop-gap actions may need to happen between now and then, however, says Mead. “The waste rock piles are slowly oxidizing, slowly reacting. The water drains through them and occasionally we see contaminated water appear in places that it’s never appeared before. It seeps through rock – if you can imagine pouring water on the top of a sponge – and at some point water drips out of the corner of that sponge when it gets full up.”

Scientists are constantly monitoring the site to look for these kinds of changes, he says. “We have to position ourselves to be flexible to react to whatever the environment delivers us.”

This column is co-ordinated by the Yukon Research Centre at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at

http://www.yukoncollege.yk.ca/research/publications/newsletters_articles

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