Prepared for INAC by SRK Consulting Inc., lead Technical Advisor to the Giant Mine Remediation Project

Giant Mine is located in Yellowknife, Northwest Territories, and produced gold from 1948 until 1999. After the mine owner went into receivership in 1999, the mine was transferred to Indian and Northern Affairs Canada (INAC). Immediately thereafter, INAC entered into an agreement by which Miramar Giant Mine Ltd. took ownership of the property. Under that arrangement, the mine continued to operate, with the gold ore shipped off-site for processing from 1999 until 2004. Mining ceased in July 2004 and INAC again took on the site one year later, after an orderly transition. The surface land lease was returned to the Government of the Northwest Territories. INAC is looking after the site, and has contracted Deton'Cho Nuna Joint Venture to continue the required maintenance and environmental management activities.

This document presents a Remediation Plan for the Giant Mine site. The plan was prepared for INAC by its Technical Advisor (SRK Consulting Inc.), and reviewed by an Independent Peer Review Panel. Under a Cooperation Agreement between the federal and territorial governments, the Government of the Northwest Territories also contributed to the development and finalization of this plan.

Included in the document is a history of the mine, a description of current site conditions, a review of the state of the surrounding environment, details of the proposed remediation activities, an assessment of the post-remediation conditions, a monitoring plan, and a project schedule. More than 40 supporting documents provide detailed accounts of scientific and engineering studies. The remainder of this Executive Summary will focus on the proposed closure and remediation activities, and their expected effects on the local environment.

Processing of the Giant Mine ore created arsenic trioxide dust as a by-product. Approximately 237,000 tonnes of the dust was produced and stored underground in 15 purpose-built chambers and mined-out stopes, which are step-like parts of the mine where minerals are extracted. The dust is about 60% arsenic. To prevent the release of arsenic into the groundwater around the mine, the Remediation Plan calls for the arsenic trioxide dust and the rock around each chamber and stope to be completely frozen.

The freezing will be accomplished by first installing pipes below and around the chambers and stopes, and then pumping a coolant through the pipes. 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, at a smaller scale, to isolate areas of contaminated soil.

Once the dust and the surrounding rock are completely frozen, the freezing system will be converted to thermosyphons. Thermosyphons are tubes filled with compressed carbon dioxide gas that act as completely passive heat pumps, i.e. they cool the ground without any input of energy. They are a proven technology and have been used to protect frozen ground throughout the North since the 1970's. Thermal analyses and tests carried out at the site show that, even under an assumption of extreme global warming, the thermosyphons will maintain frozen conditions in and around the chambers. The thermosyphons will operate indefinitely, with only periodic maintenance requirements and occasional replacement.

Portions of the underground mine have been backfilled with tailings and waste rock. Although the concentrations of arsenic in these sources are hundreds of times lower than in the arsenic trioxide dust, their large volumes mean that they also have the potential to contaminate the surrounding groundwater. The only practical method to control that potential is to collect and treat groundwater from the mine area. All underground equipment and infrastructure will be removed or de-contaminated prior to allowing the mine to flood, and all surface openings will be sealed. The contaminated mine water will then be extracted through a series of wells, piped to a new water treatment plant, treated to remove contaminants, and then discharged to Yellowknife Bay.

There are eight pits on the site, five of which are substantial in size. The B1 Pit will be backfilled to facilitate installation of the ground freezing system. Contaminated soils from other areas on the site will be in the portion of the pit that will ultimately be within the frozen zone. Waste rock, quarry rock or clean demolition waste will be used to fill the remainder of the pit. The entire backfilled area will then be covered with soil and revegetated. The other pits will be surrounded by berms or fences to prevent inadvertent public access.

There are approximately 16 million tonnes of tailings stored in ponds constructed on the site. The south, central, north and northwest tailings areas cover a total of about 95 hectares. In addition, water treatment sludges are stored in settling and polishing ponds covering an additional nine hectares. Both the tailings and the sludge contain moderate amounts of arsenic. The tailings are subject to wind erosion when dry, and could also be directly taken up by animals looking for salt.

The Remediation Plan calls for the tailings and sludge areas to be covered with one layer of quarried rock and a second layer of fine-grained soil. The lower layer of quarried rock will prevent the upwards migration of contaminants from the tailings, and inhibit the downwards penetration of plant roots. It will also serve as a final protective layer in the event that the soil is removed by erosion.

The upper layer of fine-grained soil will allow for revegetation and future recreational or traditional use of the site. The surface of each tailings area will be graded, and ditches and spillways constructed to limit erosion and to allow water to run off the cover without becoming contaminated.

During and after the remediation, it will be necessary to continue collecting and treating any contaminated water. The plan calls for a new water treatment plant to be constructed. The plant will be used to treat contaminated water extracted from around the arsenic trioxide chambers and stopes during and immediately after the ground freezing. Contaminated surface water will also be collected and treated until monitoring data clearly show that the arsenic levels are low enough to allow direct discharge. Over the longer term, it is expected that water from the underground mine areas outside the frozen zones may continue to need treatment, and the new water treatment plant will remain in operation as required.

Effluent from the treatment plant will be discharged via a diffuser system into Yellowknife Bay. Discharge to the bay, rather than to Baker Creek, will allow year-round treatment of the extracted mine water. The year-round treatment will remove the current requirement to store large amounts of contaminated water on the surface. It will also allow operation and maintenance of the water treatment and ground freezing systems to be carried out by a permanent year-round staff.

Baker Creek has areas of significant sediment contamination. Also, the current alignment poses a risk of flooding into the C1 Pit and the connected underground workings. The Remediation Plan calls for diversion of the creek into a new channel around the areas where it poses a risk to the underground workings. Options for dealing with the contaminated sediments are under further investigation. Options include removal of heavily contaminated sediments, and diversion of the creek to uncontaminated areas. However, continuing arsenic inputs, primarily from areas upstream of the site, will limit the level to which the sediments can be cleaned.

A number of surficial materials, from natural soils to tailings to mine rock, are present in various areas across the site. An estimated 328,000 cubic metres of material is contaminated with arsenic at levels that exceed the GNWT criterion for industrial land use. Areas of hydrocarbon contamination are also present, but largely overlap the arsenic contaminated areas. The Remediation Plan calls for contaminated soils and mine rock to be excavated and disposed of within the frozen portion of B1 Pit, which will subsequently be covered with non-contaminated material. Additional contaminated soils and spilled tailings will be excavated and moved into the most appropriate tailings or sludge impoundment.

More than 100 buildings, supported by associated infrastructure and utilities, remain on the site. Many of these buildings pose a hazard to the public. The Remediation Plan calls for all buildings and infrastructure to be removed. Any arsenic-contaminated materials will be removed and placed in the empty chamber 15. The public highway through the site will be relocated to keep traffic away from the demolition, soil cleanup and ground freezing activities. Options for the new highway alignment are being discussed with the GNWT Department of Transportation.

After the remediation activities are completed, the site will consist of a small area that will need to remain under active management, and a broader area that is available for other uses. The actively managed area will be centered around the current C-Shaft, and will allow for both maintenance of the ground freezing system and long-term treatment of contaminated mine water. The remainder of the site will include areas along the current highway corridor that will be available for industrial use, open pits surrounded by rock berms or fences, and broad areas of covered tailings. The tailings areas, in particular, are expected to be open to broader uses after the Remediation Plan has been fully implemented.

The remediation activities will decrease but not completely eliminate arsenic releases from the site. In quantitative terms, the arsenic releases from the site will decrease from the current level of approximately 500 kilograms per year to less than 200 kilograms per year. In the absence of the proposed remediation measures, arsenic releases from the Giant Mine site could increase to many thousands of kilograms per year.

The post-remediation arsenic release from the site and, equally importantly, areas upstream, will mean that Baker Creek will remain contaminated with arsenic. Ecological risk assessment calculations show that there will continue to be a potential for adverse effects on bottom-feeding fish and terrestrial animals living in the Baker Creek area. Human health risk assessment calculations indicate that arsenic intakes by humans will remain within the range estimated for Canadians, and that there will be little risk of adverse health effects. There may, however, need to be some restrictions on future activities at the site until monitoring programs can demonstrate that arsenic levels are within safe levels.

A detailed plan for monitoring the site during and after implementation of the Remediation Plan has been developed. It includes sampling and analysis of groundwater and surface water, air quality monitoring, environmental effects monitoring, and monitoring of ground temperatures within and around the frozen arsenic trioxide chambers and stopes. It also includes regular inspections of remaining pit walls, as well as the covers, ditches and spillways associated with the remediated tailings impoundments. The monitoring will be sufficient to allow post-remediation performance to be compared to both predictions and license requirements. Monitoring reports will be prepared and submitted to the Mackenzie Valley Land and Water Board.

Remediation of the Giant Mine site, as outlined in this plan, will require approval pursuant to the Mackenzie Valley Resource Management Act. It is anticipated that this Remediation Plan will be submitted to the Mackenzie Valley Land and Water Board in 2007, as part of a water license application.

Throughout the licensing process, INAC intends to continue public consultation on the Remediation Plan. Reviews by the regulatory agencies (or Boards) will also have formal requirements for public input. The schedule of the water licensing process will ultimately be determined by the regulatory agencies.

Once the licensing process is complete, the project will be presented to the Treasury Board for final funding approval. Final engineering design of the approved remediation measures will commence at the same time.

The major surface remediation activities should be mostly completed within five years; and the ground freezing should be substantially completed within 10 years. Maintenance of the ground freezing system would continue indefinitely, as would mine water treatment and long-term monitoring.

Giant Mine Remediation Joint Project Office

2^nd Floor, Waldron Building, 5103 - 48th St.,
Yellowknife, NT X1A 1N5
Tel.: (867) 669-2426
Fax: (867) 669-2439
Email: giantmine@ainc-inac.gc.ca
www.giant.gc.ca
QS-Y289-002-EE-A1 Cette publication est aussi disponible en français sous le titre: Résumé – Plan de restauration du site de la mine Giant