Long Term Water Quality Dynamics in Flooded Mine Pit at Gunnar Uranium Site (N. Saskatchewan)

Uranium U2020, 2020

Alexey Klyashtorin and Anton Sizo

The Gunnar uranium mine/mill site was operated on the north shore of Lake Athabasca from 1953 to 1964. The ore was mined from the open pit >100 m deep interconnected with mine workings >500 m deep. Following the mine closure in 1964, both pit and mine working were intentionally flooded through a channel blasted between the pit and Lake Athabasca. At present, the pit contains about 3.2 million m3 of water contaminated by U and Ra-226 that periodically spills over the pit rim into Lake Athabasca.The water column was sampled by different research groups in 1981, 2002, 2011, and 2017. By 1981, the pit has already been found stratified, with a chemocline at ~75 m identified from a sharp change in dissolved oxygen and conductivity. The uranium concentration in 1981 was ~700 µg/l on the surface and 2,900 µg/l at 110 m. The corresponding concentrations of Ra-226 were 0.1 and 0.4 Bq/l. By 2002, the chemocline stabilized at ~60-65 m marked by a sharp change in uranium, Ra-226, oxygen, and conductivity. By 2017, the highest and lowest concentrations of uranium were attributed to the epilimnion and chemocline (1,200 µg/l and 200 µg/l, respectively). The uranium content in the column was aligned mostly with the oxidation-reduction potential. Ra-226 content in the epilimnion was ~0.4 Bq/l; the maximum concentration was attributed to the chemocline (5.4 Bq/l). The total amount of uranium and radium accumulated in the pit water by 2017 was estimated as 3,420 kg and 2,163 KBq, respectively. The uranium and radium longterm dynamics are best approximated by ascending linear trends (R2 =  0.906 and 0.878, respectively). It is anticipated  that uranium and radium in the pit water are likely to continue increasing for the coming decades if no remediation occurs.
Keywords: Gunnar, Mine, Flooded pit, Uranium, Ra-226, Chemocline, Accumulation, Dissolved oxygen
cim.paperattributes.header.paperkeywordsfrench: Gunnar, Mine, Flooded pit, Uranium, Ra-226, Chemocline, Accumulation, Dissolved oxygen