Arsenic trioxide and underground issues at Giant Mine

Information about arsenic trioxide and other underground issues at the Giant Mine.

The arsenic trioxide challenge

Arsenic comes in many forms and is present in much of the food we consume on a regular basis. Some forms of arsenic can be harmful to people, animals, or plants. The arsenic waste that was created during the mining process is a specific, and potentially toxic form called arsenic trioxide. 

The greatest challenge associated with cleaning up Giant Mine is this arsenic trioxide waste, which is stored underground. Giant Mine holds one of the largest amount of stored arsenic trioxide dust in the world. In addition, it is located close to a populated area and on the shores of a large lake.

A mine worker examines an underground pool of water at the Giant Mine site.

One of Indigenous and Northern Affairs Canada's top priorities for the mine is the effective, long-term management of that waste. There is a need for a long-term strategy to minimize the risk posed by the arsenic trioxide at Giant Mine. Although the 237,000 tonnes of arsenic trioxide dust is currently safely contained and managed, such a large amount of toxic material demands a more comprehensive plan to minimize the risk.

Indigenous and Northern Affairs Canada dedicated more than three years of research and community consultation to help determine which would be the most effective way to protect the health and safety of northerners and the environment. Freezing the arsenic trioxide waste was determined to be the best option.

It is important to note that this solution does not present a "walk-away" option to the Government of Canada. In fact, no total "walk-away" solution currently exists. All the arsenic trioxide management alternatives the Project team explored require ongoing water treatment and monitoring. This includes those options that would take the arsenic trioxide out of the stopes and chambers, which would have the added risk of exposing people and the environment to the substance.

By freezing the arsenic trioxide dust in the stopes and chambers, the majority will be contained. However, a smaller amount remains distributed throughout the other underground mine workings. Ongoing water treatment and monitoring will take place to ensure this arsenic does not leave the site.

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Where does the arsenic on site come from?

The bedrock in the Yellowknife area, including the Giant Mine site, contains arsenopyrite. This naturally-occurring mineral is composed of iron, sulphur and arsenic. The arsenic in the arsenopyrite is in a stable form. It does not present a health hazard.

Arsenic is an element of the periodic table. This element occurs naturally in rock throughout the earth. Arsenic does not decay over time.

When the rock at Giant Mine was crushed and mined out, arsenic was exposed to the environment. Large sections of the underground mine were backfilled with waste rock and tailings. The arsenic concentration in these sources is hundreds of times lower than in the arsenic trioxide waste in the storage chambers. However, the large volumes mean they also could contaminate the surrounding groundwater.

Some of those areas will be frozen as part of the frozen block method. However, mine water that comes into contact with this material will require long-term treatment.

How was the arsenic trioxide waste produced?

Arsenic trioxide waste was created during mining operations. When the arsenopyrite ore was roasted to release the gold, arsenic was also released as a gas, arsenic dioxide. As the gas cooled, it changed to a solid state and became arsenic trioxide dust. Arsenic trioxide presents a health hazard, which is why the Project team carefully manages the waste.

During the life of the mine, the roasting process created about 237,000 tonnes of arsenic trioxide waste. This waste was collected and stored in 14 underground chambers and stopes. No new arsenic trioxide has been produced since Royal Oak Mines went bankrupt in 1999.

The dust stored underground is about 79% arsenic trioxide. It also contains other minerals such as iron, antimony and gold.

Where is the arsenic trioxide now?

There are 11 chambers and five mined out stopes at Giant Mine, built for the purpose of storing waste. Fourteen of these contain a total of approximately 237,000 tonnes of arsenic trioxide waste. The current storage of arsenic trioxide underground at Giant Mine is safe. The chambers and stopes are surrounded by solid rock between 80 and 250 feet below the surface. The chambers are all located near the C-shaft. Cement bulkheads act as plugs and seal the openings to these chambers and stopes.

Arsenic trioxide is soluble in water, though only moderately. However, the pumps at Giant Mine keep the level of groundwater well below the storage chambers. Any water seepage that comes into contact with the storage chambers is collected in the mine water system, pumped to the surface, and treated. The mine and local surface waters are regularly monitored to ensure arsenic trioxide does not escape into the environment.

During remediation, these will be further contained using the frozen block method.

The arsenic trioxide is not in barrels, and the chambers are not under the lake. There are also no chambers or stopes under the communities of Ndilo, Dettah or Yellowknife.

There is very little arsenic trioxide in the tailings. Most of the arsenic in the tailings is in stable forms, including arsenopyrite. The Project completed several studies that look at arsenic contamination on the surface. These are available from the project office.

Why not just remove arsenic out and take it away?

The arsenic doesn't belong anywhere else. It's from here. The arsenic, just like the gold, originates from the local rock. However, Indigenous and Northern Affairs Canada and its technical advisors examined very carefully whether the arsenic trioxide could be removed and disposed of safely elsewhere.

This alternative was rejected for many reasons, including:

Another option examined was whether the arsenic trioxide dust could be removed and placed in a secure landfill on the Giant Mine property. That would eliminate the problem of spills during transport. However, a permanent hazardous waste site would need to be created on the Giant Mine surface. In addition, it would be impossible to remove all the dust because of the irregular nature of the rock around the stopes and chambers. Several thousand tonnes of arsenic trioxide dust would likely remain underground. That would mean there would be two hazardous waste sites: one above ground and one underground.

In the final analysis, all of the alternatives that looked at taking the arsenic out posed more risks to workers, the public and environment than the frozen block method.

What other underground issues require addressing?

Surface openings to the underground are hazards to humans and wildlife. It is important that no person or animal access the openings, especially by accident, once the site is cleaned up. Most openings will be sealed during remediation, once they are no longer used for mine access or ventilation. Gates and door locks will control access to mine openings that require future entrance.

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