Detrimental effects of acidic drainage on the environment are well recognized in areas not encompassed in permafrost in Canada. There are numerous abandoned, operating and proposed mines in Canadian permafrost regions. These northern mines have an additional tool to combat acid generation by freezing and keeping the wastes in a frozen state. This tool requires careful evaluation to determine where and how it may be used economically by the mining industry for the closure of tailings impoundments and waste rock piles.
Covers placed over reactive tailings and waste rock in non-permafrost regions have been studied and field-tested for many years. Many operating mines in the Canadian permafrost region have adopted, or are considering, encapsulating reactive tailings in permafrost. This can be accomplished in permafrost areas with suitable cold air temperatures and incorporating a suitable design for the active layer. This layer located at the ground surface, thaws every summer, and thereby develops a potential condition for tailings or rock waste to oxidize during the summer. To achieve encapsulation of tailings or rock waste in permafrost it is necessary to place material (e.g. a cover) over the tailings or waste rock that will contain the seasonally annual thawed zone, or the active layer, within it. This will prevent thawing of the tailings or rock waste near the surface.
Encapsulating in permafrost appears to be a good option for reactive tailings in continuous permafrost because it does not require either, 1) a dry cover such as a clayey till material, a material that is generally lacking in continuous permafrost, or 2) a water cover that is difficult to maintain because of high evaporation rates observed in most of the Canadian permafrost region. In addition, water covers are costly for remote Arctic sites because of higher construction, inspection and maintenance costs.
Four recent case histories from cover test pads constructed over reactive tailings in continuous permafrost provide information on parameters that govern the design of a cover to maintain the tailings in a frozen state. Case histories from Nanisivik, Raglan, Lupin and Rankin Inlet represent different tailings operations; cover design approaches, and physical and climate conditions.
Naturally, the first condition for encapsulating tailings in permafrost is to have sufficiently low air temperatures that create continuous permafrost. While permafrost is defined as a ground that remains at or below 0ºC for two consecutive years, it is suggested that for permafrost encapsulation the ground temperature at a depth of zero annual amplitude be at or at least -2ºC. This ground temperature translates to a mean annual air temperature (MAAT) of about 8ºC when short-term annual air temperature fluctuations of about 1.5ºC and the difference between MAAT and the mean annual ground surface temperature (MAGST) of about 4.5ºC are applied.
The above criterion does not consider global warming which may have important implications for permafrost. Early documentation of global warming was done by Nichols (1975) who showed through radioactive carbon dating of peat that the earth experienced a little Ice Age about 300 years ago and has been warming ever since. Three progressively increasing warming trends have been recorded since.
Observations indicate that about 50% of the glaciers in the Swiss Alps have melted between 1850 and 1985 and an additional 25% have melted since 1985 (2003 Permafrost Conference address). Anisimov and Fitzharris (2001) have reported a warming trend in air temperatures of 2 to 4ºC worldwide and up to 5ºC/100years since the beginning of the 20th century at selected locations in northern Siberia and Alaska. An increase in the warming trend has been observed in both Alaska (Esch 1990) and Canada since 1980 (Dyke & Brooks 2000). MAAT records for selected Canadian sites are given in this report.
Under present conditions, encapsulation of tailings is generally feasible northwest of the Mackenzie River in the Northwest Territories, all of Nunavut and the most northern region of Quebec. However, with the present global warming trend, permafrost encapsulation in some of the northern regions of the Northwest Territories and Quebec may not be sustainable in 100 to 200 years. Mines considering encapsulating tailings in permafrost will need to determine what impact global warming will have on their design.
The design concept for encapsulating reactive tailings in permafrost is to provide a sufficiently thick cover material that will maintain the active layer (annual thaw) within the cover. The thickness of the active layer is determined by site air temperature, moisture content in the soil/rock stratigraphy, surface vegetation, slope orientation, colour of the surface material (albedo), snow depth that varies with local topography, and other factors. Because these factors vary greatly from site to site, and even sometimes across a given site, the thickness of the active layer may vary from 0.5m to 5m. Site-specific factors are important considerations for the design of covers.
The dominant factors that govern the thickness of the active layer in the four case histories presented are air temperature and water content. The other factors are of less importance for the following reasons:
• Surface vegetation – Establishing a vegetative cover in permafrost regions is a slow process. It can be assumed that there will be no vegetation on top of the tailings or waste rock surface for some time.
• Slope orientation – Most of the tailings are deposited at near horizontal slope.
• Colour – Most of the soil cover materials have similar colours. However, colour could be a factor as discussed in the Nanisivik case history.
After establishing the air temperature distribution for the site, the moisture content of the cover material is the most important parameter governing the maximum thaw depth or the required cover thickness.
The effect of the moisture content within the cover material on the thickness is illustrated by the four case histories discussed in this report and the Diavik proposed cover design:
Raglan – A cover design of 2.4m was selected. It consists of a 1.2m layer of mine rock underlain by 1.2m of crushed sand and gravel esker. The measured active layer base is at a 1.9m depth (based on 0ºC freezing temperature). The thaw depth is 1.9m.
Nanisivik – Constructed five test pads with a 2m thick cover with varied stratigraphies. The active layer in the five test pads varied depending on the moisture content. It ranged from initial moisture content of about 34% at 1m to about 7% at 1.5m. The average thaw depth at the five test pads varied from 1.0 to 1.4m. The smallest thaw depth corresponding to the cover with the highest moisture content.
Lupin – The cover ranges from 0.6 to 1.6m of sand and gravel esker material. The active layer was observed to be a function of the location of the groundwater surface. It varied from 1.3m for no cover and fully saturated tailings to a depth of 1.8m when groundwater surface was likely at the base of the 1.6m thick cover.
Rankin Inlet – A 1m sand and gravel esker material was used for a cover. The freezing point was depressed to about 4ºC due to seawater infiltration and oxidized sulphides being dissolved. The active zone is estimated to be at about 5m based on the depressed freezing point and visible ice. At 0ºC the thaw depth would be estimated at 2.7m.
The four case histories demonstrate that to minimize the active layer thickness within a cover, the moisture content within the cover has to be maximized. Naturally, the highest moisture content that can be reached within the cover is complete saturation. However, partial or complete saturation of the cover could result in the establishment of a stagnant water layer within the cover that prevents oxidation without the need for freezing (Li et al 1997). It has been demonstrated in previous MEND studies (MEND 2.22.2) that an effective barrier to oxygen diffusion will result if the degree of saturation of a soil layer can be maintained greater than 85 to 90%.
Based on a design concept given in MEND 6.1, work done on a stagnant water cover by Li et al (1997) and column testing by CANMET (MEND 2.12.1e), Lupin selected for its closure of their filled tailings cells, a saturated zone cover design. (Holubec 2002). The tailings will be covered with 1m of esker sand and gravel that will maintain a saturation zone at the base of the cover. Monitoring of the thermal conditions and groundwater level within the cells covered in 1995 and 1998 has supported this concept.
