The population of Yellowknife is approximately 17, 300. Yellowknife is
located on the north shore of Great Slave Lake, 5 miles south of the mouth
of the Yellowknife River. The city is still young; a permanent settlement
was not established until the discovery of gold in 1934. There are currently
two gold mines located in the city, Royal Oak Mines' Giant Mine and Miramar
Con Mine. Con Mine is currently in a care and maintenance phase and is
not mining/milling at this time.
The average precipitation in the Yellowknife area is 267 mm/yr. August
receives the most precipitation. The annual potential evaporation rate
is approximately 483 mm/year (Reid, pers. comm.). Evaporation exceeds precipitation
in the region and the climate is therefore classified semi-arid. The July
high is 21ºC and the January high is -25ºC. The City is located
on Back Bay of Great Slave Lake resulting in temperature moderation. The
lakes around Yellowknife freeze between October 15 and November 15 and
break up at the end of May.
The city is located on the Canadian Shield and is underlain by volcanic
and plutonic igneous rocks. Groundwater movement occurs along the joints
and fractures of the low porosity rocks. The volcanics are approximately
2, 700 million years old (Archean) from the Precambrian. The plutonic rocks
are also from the Precambrian; however they are 100 million years younger
than the volcanics. The geology of the region consists chiefly of basalt
with smaller amounts of granite, gabbro, cherty tuff and dacite. Proterozoic
diabase and gabbro dykes are also found in the area. Quartz veins are observed
throughout the region. The region is underlain by permafrost, which restricts
the infiltration of precipitation and groundwater recharge. The semi-arid
climate, presence of permafrost and igneous geology result is a slow groundwater
recharge rate in the natural environment.
Water is drawn from the Yellowknife River, 5 km away from the city. Great
Slave Lake was polluted by the mining industry and is not suitable for
drinking water. The water consumption totals were 2,560,443 m3,
2,586,161.5 m3 and 2,981,841.8 m3 in 1995, 1996 and
1997 respectively (English, 1998). This includes the water use by the two
mines. Municipal wastewater is treated in a wetlands system and discharged
into Great Slave Lake.
There are two gold mines in Yellowknife. The mining industry is a potential
source of groundwater pollution via mine drainage, leachate from tailings
disposal areas and the use of explosives underground. Contamination of
surface waters has been documented from Miramar Con Mine and Royal Oak
Mines Giant Mine (Bastedo, 1994). It has recently been discovered that
substantial seepage from Giant Mine's Tailings Ponds is occurring. Minimizing
the contamination of inland waters by the NWT mining industry is part of
the mandate of the Regulatory Approvals Section of Water Resources within
the Department of Indian Affairs and Northern Development. Giant Mine has
over 260, 000 tonnes of arsenic trioxide stored underground in large vaults.
The arsenic trioxide is a byproduct of the milling/roasting process. The
deposit mined by Royal Oak is rich in arsenopyrite whereas the deposit
mined by Miramar is not and does not yield arsenic trioxide as a byproduct.
A substantial portion of mine inflow comes from near-surface sources. According
to the current water balance, 1537 m3/d of inflow into the mine
workings is from groundwater seepage, 698 m3/d from seepage
of the largest tailings pond and 786 m3/d from service water
from Great Slave Lake (Gibson et al., 1998). It is estimated therefore
that when the sumps are turned off upon abandonment, that groundwater seepage
will be the major source of filling with the majority coming from the upper
levels of the mine. It is predicted that a decreasing hydraulic gradient
associated with a rising water level will not significantly change the
rate of water groundwater seepage. The seepage from the tailings pond however
should decrease substantially upon abandonment. It is expected that the
open volume space of the mine estimated at 135,989,980 ft3 (3,853,766
m3) will fill in five to seven years to a static water level
close to the surface. At this time the arsenic trioxide storage chambers
will be submerged (Gibson et al., 1998). There is still little information
on what the environmental effect will be. The Department of Indian Affairs
and Northern Development is currently investigating the effects in a joint
effort with Royal Oak Mines as well as searching for a solution to the
problem.
Permafrost makes groundwater development difficult in the Yellowknife region.
The igneous rocks of the area outcrop frequently and the choices for building
sites are already limited. Heat from structures can melt the top layer
causing foundations to crack and buildings to sink. Damage can be seen
on many of the older buildings. Roads constantly need reconstruction. Permafrost
is especially dangerous to the mining industry. Proper precautions must
be taken to ensure that dams supporting tailings containment areas are
constructed properly, since the melting of permafrost can cause the dam
to fail.
To maintain the frozen state of the ground, the top layer is insulated
thereby making it strong enough to support construction. Gravel pads 3
to 4 feet thick are frequently placed below smaller structures; pilings
are driven into the permafrost to elevate larger buildings, thereby preventing
heat from penetrating to the ground below. For more expensive buildings
themosyphons are used. This is the case for the Legislative Building and
Museum.
Bastedo, J., Shield Country; Life and Times of the Oldest Piece of
the Planet. The Arctic Institute of North America. 1994.
English, C., City of Yellowknife Water Licence Compliance Report.
Water Resources, Department of Indian Affairs and Northern Development.
1998.
Gibson, J. & Frackflow Consultants Inc., Preliminary Hydrogeological,
Geochemical and Isotopic Investigations at the Giant Mine, Yellowknife,
NWT. Prepared for Indian and Northern Affairs Canada. June
30, 1998.
Reid, B., Regional Hydrologist, Water Resources, Department of Indian
Affairs and Northern Development. Personal Communication, 1998.
