Northwest Territories




62º31'N 114º21'W

Information supplied by

Jennifer Gray

1470 Midland Ave.#906 Scarborough ON, Can. M1P 4Z4


Dated Sat Dec 12 10:04:32 1998 

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City Description:

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.

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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.

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Basic Hydrogeology:

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.

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Water Use:

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.

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Groundwater Issues:

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.

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Groundwater Problems:

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.

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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.

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References and Other Author(s):

  1. Bastedo, J.,  Shield Country; Life and Times of the Oldest Piece of the Planet.  The Arctic Institute of North America.  1994.
  2. English, C.,  City of Yellowknife Water Licence Compliance Report.  Water Resources, Department of Indian Affairs and Northern Development.  1998.
  3. 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.
  4. Reid, B.,  Regional Hydrologist, Water Resources, Department of Indian Affairs and Northern Development.  Personal Communication, 1998.
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