Mine tailings are fine-grained and usually have high concentrations of metals and metalloids, making tailings storage facilities reservoirs of contaminant-bearing dust-sized material. Although there is a general understanding of the risks posed by tailings dust, there are still major gaps in our knowledge concerning the chemical form or speciation of dust. This is an especially important consideration at mine sites located near populations where contaminant-bearing dust can have lasting impacts on the surrounding environment and can pose serious health risks on local residents.
One such site is the Giant mine, an abandoned mine in the Northwest Territories of Canada. A gold mine that was in operation from 1948 to 1999, where roasting was used to liberate submicroscopic gold from arsenopyrite ore. Oxidation of the ore generated arsenic-bearing iron oxides and arsenic trioxide as byproducts. In the early days of operations, arsenic trioxide was emitted uncontrolled into the atmosphere and was co-deposited with the tailings on site. Arsenic trioxide is the most toxic and bioaccessible solid form of arsenic.
Presently, the tailings at Giant are left vulnerable to high velocity winds after the snow melts in late spring, when it is too cold to apply a dust suppressant at the surface. This poses a significant risk as two populations (Yellowknife and Ndilo) are located less than 5 km downwind of the mine site. Community concern regarding arsenic-bearing dust from Giant prompted a study to investigate the tailings dust, focused on characterising the arsenic speciation or dust content.
The findings from this study show that despite the high concentration of arsenic in the tailings dust, arsenic trioxide is a very rare constituent in the dust. However, the soils near the Giant mine tailings contain abundant arsenic trioxide from historic roaster emissions and exhibit very high concentrations of arsenic that are often similar in scale to the tailings. The fact that arsenic trioxide can persist in these surface soils suggests that they might actually pose a greater risk to the community than the tailings themselves, and yet the soils are not actively monitored by the remediation project.
Legacy mine sites such as Giant pose unique challenges when it comes to characterising and containing toxic material from the site; the extent of contamination may not be well understood.
Environmental contamination is not always obvious, particularly where it concerns fine-grained particulate; thus, it is important to fully understand metal(loid) speciation and mobility before considering and employing long-term remediation strategies.
