Geology Reference
In-Depth Information
14.8. MINING ACTIVITIES
The environmental and geotechnical problems associated with other mineral exploitation
activities center around (i) water supply, (ii) waste disposal, and (iii) contaminants migra-
tion through or over permafrost terrain. These are briefl y discussed.
Many mining operations involve the disposal of waste material either in rock piles or
in settling ponds (“tailings” ponds). Containment is required because of possible toxic
components to the waste, therefore any interconnected surface water bodies such as lakes
and streams are clearly inappropriate. In many cases, impoundments or dams are created
for the specifi c purpose. In permafrost terrain this creates a problem because the water
in the tailing pond is a heat sink and leads to the thaw of underlying permafrost, raising
the possibility of eventual groundwater contamination. For example, this is a potential
problem at the Red Dog lead-zinc mine in the Brooks Range, central Alaska (Weaver,
2003; Weaver and Kulas, 2003).
There are several possible procedures that minimize these sorts of problems. One solu-
tion is to divide the containment area into a number of sectors, which are used at different
times of the year, allowing sediment to freeze under natural conditions before use once
again the following year (Perlshtein and Pavlenkov, 2003). In this way, no water body, or
heat sink, is created. In the case of the Ekati diamond mine, NWT, Canada, an earthen
dam was constructed across a small drainage channel to create the containment pond.
Here, the problem was not one of thaw of underlying bedrock, because this was imperme-
able Shield rock, but of possible leakage around and through the dam. To solve this
problem, a series of thermosyphons were installed along the dam to transmit heat into the
atmosphere and to keep the dam frozen (see Figure 14.6B).
Opencast mining presents stability problems for the large rock piles that are produced,
together with issues as to the eventual closure of the pit following exhaustion of the
resource. As regards rock-pile stability, passive methods (see Figure 14.5) are being used
at Ekati to stabilize (i.e. freeze) the edge of the rock waste. The latter is placed in succes-
sive layers or terraces, each 15-20 m high. Convective air circulation through the rock piles
is enhanced by the emplacement of less porous and less permeable layers at the base of
each terrace level. The closure of an open pit associated with the Raglan Mine, Ungava
Peninsula, northern Québec, was safely accomplished by a permanent freezing of the pit,
which was backfi lled with frozen rock waste from the adjacent rock-waste piles. The
potential problem that was avoided was the creation of “acid-rock drainage” induced by
the relatively high sulfi de content of the waste rock (Szymanski et al., 2003).
A different set of problems related to opencast mining in permafrost occurs when salt-
induced cryopegs, present in the bedrock strata that are being removed, are exposed in
the pit wall. Seepage into the pit has to be pumped out but the problem remains to dispose
of the water. Surface impoundment is not an option because of contamination of surface
water systems, and the possible creation of a heat sink if impounded. One solution used
at the Udachnaya kimberlite mine in Yakutia, Russia, has been to pump more than 10
million cubic meters of brine solution into a frozen aquifer at depth in the surrounding
terrain (Alexeev and Alexeeva, 2003).
Containment problems are especially acute in the near-surface. Although perennially-
frozen ground is an effective barrier against downward movement of solutes, thawing of
the active layer and the ice contained at its base in the so-called “transient layer” greatly
increases the lateral hydraulic conductivity in this zone during the thaw season. Leaching
is also enhanced by snowmelt runoff. Therefore, a general problem is the leaching and
movement of ions and contaminants away from containment areas. For example, the
movement of potassium chloride from drilling mud contained in an unfrozen state in
