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More than 50 years have passed since the first gold brick was poured at the Giant Mine in Yellowknife, Northwest Territories. The Giant Mine produced 7.6 million ounces of gold between 1948 and 2004, yet the legacy left behind has overshadowed the mine's glorious period as a significant economic driver in the territory's economy.
Much like our territorial neighbour, the Yukon, the search for gold is responsible for drawing people to the territory, and the Giant Mine is one of the main reasons Yellowknife was settled. In fact, the Giant Mine is only a 10-minute drive by car from the city's downtown core – and at 850 hectares by 700 metres deep, it is twice the size of downtown. Its proximity to the capital, however, is only one of many reasons the site remains a concern. The building and surface areas of the mine are contaminated with asbestos as well as arsenic. There are 95 hectares of tailings ponds – enough to cover 300 football fields. In a single season alone, 200 Olympic-sized swimming pools worth (500,000 m3) of arsenic-contaminated water has to be treated. But the issue which generates the most interest is the 237,000 tonnes of arsenic trioxide dust left behind.
The arsenic contamination was a by-product of producing gold at Giant Mine. Gold ore is rich in arsenopyrite, a mineral with a high arsenic content. The roasting process used to extract gold from the ore also produced arsenic trioxide dust (As2O3), a highly toxic substance that was collected and pumped underground into 14 chambers and stopes at the mine site, where it remains sealed today.
In 2004, INAC became responsible for the Giant Mine's site and surface, as well as for the underground clean up it would need. After extensive engagement of Aboriginal groups, the public and other government bodies, INAC moved forward to prepare a comprehensive remediation plan to effectively deal with the mess they had inherited.
INAC and the Government of the Northwest Territories incorporated eight years of research and engineering work to create the Giant Mine Remediation Plan. During its development, the plan evolved and was improved through input sessions from the community and stakeholders. It was also rigorously reviewed and adapted by international experts in the fields of engineering, mine remediation, geology and health sciences, as well as other government departments. The remediation plan outlines all of the activities required to clean up the mine site, including the demolition and removal of all buildings and facilities, and the long-term storage and containment of the arsenic trioxide dust underground.
The latter is perhaps the most interesting and important part of the remediation plan. After all, 237,000 tonnes of arsenic trioxide is equivalent to seven 11-storey office buildings, almost all of which is stored in 14 underground chambers and stopes (irregular, mined-out cavities) cut into solid rock. Cement bulkheads, which act as plugs, seal the openings to these chambers and stopes. The arsenic trioxide dust is totally surrounded by solid rock and concrete. When this underground storage method was originally designed, it relied on the area's natural permafrost, which worked as a frozen barrier. It was believed that, when the time came to close Giant Mine, permafrost would reform around the storage chambers and stopes, and seal in the arsenic trioxide.
To ensure the health and safety of residents and protection of the environment, it was imperative to prevent an emergency situation from arising as the mine infrastructure ages. The Technical Advisor (in fact, a group of companies) and an independent peer review panel of international experts recommended the safest and best option available at this time: the frozen block method. Under the plan, the arsenic trioxide dust will be frozen solid in its sealed 14 underground chambers to create a "frozen block" barrier to prevent any seepage of arsenic from the chambers.
The freezing will be accomplished by first installing pipes below and around the chambers and stopes, then pumping a super-cooled liquid into the series of underground pipes. This will create an impenetrable barrier that will prevent water from entering the chambers and arsenic from leaving the chambers. The technology is similar to that used to create hockey rinks. It has also been used to prevent groundwater inflows to other underground mines and, on a smaller scale, to isolate areas of contaminated soil.
Once the dust and surrounding rock are completely frozen, the freezing system will be converted to thermosyphons – tubes filled with compressed carbon dioxide gas that act as completely passive heat pumps, with no energy input required. Thermosyphons are commonly used in the North to keep ground frozen. A good example is the thermosyphons used in the parking lot of the NWT Legislative Assembly to prevent thawing of the natural permafrost. Thermal analysis and tests carried out at the site show that, even under an assumption of extreme global warming, the thermosyphons will keep the stopes and chambers frozen, and operating indefinitely and requiring only periodic maintenance and occasional replacement.
While the remediation plan undergoes an environmental assessment (EA), INAC has been able to proceed with a Freeze Optimization Study. This study is a test of the freeze process, performed on a smaller scale, to confirm and compare freezing techniques. The study will be used to inform the EA process and provide information about the operation such as power requirements, rate of freezing, as well as more accurate cost estimates.
In order to ensure that people and environment are well protected, care and maintenance activities at Giant Mine have been going on since INAC took it over in 1999. While the remediation plan undergoes the EA process, INAC continues to work to ensure the site is kept in a stable condition by mitigating the risks related to the mine site and its aging infrastructure. A new underground mine pumping system was commissioned in December 2008, doubling pumping capacity, and the effluent treatment plant was also upgraded to improve efficiency. The site has a significant water monitoring program to ensure the site complies with Canada's Metal Mining Effluent Regulations, and groundwater and monitoring of wells that have been established around the periphery of the mine site undergo routine sampling.
These and other care and maintenance activities reduce the potential for emergency measures and mitigate ongoing issues. However, only full remediation of the site can eliminate issues generated by the mine. It will take approximately 10 years to complete remediation of the surface as well as for the complete freezing of the arsenic chambers, after the project has received regulatory approval. Remediation of the Giant Mine site will leave a better legacy that Northerners can be proud of for years to come.