We all love gold mines. Nothing sets off a good staking rush like a hint of the mother lode. But along with the yellow metal sometimes come unwanted byproducts, notably arsenic. Such is the unfortunate case at the Giant mine in Yellowknife. Besides yielding more than 7.1 million oz of gold during 50-plus years of mining, the operation has left behind some 237,000 tonnes of arsenic trioxide to be dealt with.
The Giant mine developed in the mid-20th century when roasting was the accepted method of removing arsenopyrite from gold ore. The resulting arsenic trioxide dust was collected in a baghouse and returned underground to mined-out stopes. It was sealed in with concrete bulkheads, and that was supposed to be the end of it. However, under certain conditions arsenic trioxide is soluble in water, and it hasn’t stayed put. It now seeps as a slimy, toxic, ankle-deep ooze through the drifts. The gruesome mess needs cleaning up.
The first line of defense has been to keep the arsenic in the stopes. This involves monitoring the water level in the mine and continuous pumping to keep it below the containment areas. Frequent testing indicates that so far there has been no contamination of Great Slave Lake, other surface water or groundwater. That’s a good start, but only a start.
The search for a long-term solution is being spearheaded by the federal DEPARTMENT OF INDIAN AND NORTHERN AFFAIRS (DIAND). The problem has been under continuous review by DIAND, other government branches, universities and consultants since 1997. In 2000, DIAND retained the services of SRK CONSULTING to act as lead advisor in the search for a solution to the arsenic problem. Also on the team were SENES CONSULTANTS, H.G. ENGINEERING and LAKEFIELD RESEARCH.
The report delivered to DIAND late last year examined seven possible means of reducing the hazard. Alternative A involves continued collection and various treatment methods for contaminated water. Alternative B involves freezing the surrounding ground, then allowing water to seep in and encase the arsenic dust in ice. Alternative C would remove the dust and redeposit it at the bottom of the mine. Alternative D would remove the dust and ship it to a hazardous waste disposal site. Alternative E involves mining the dust and reprocessing it to recover gold and a high-purity arsenic trioxide. Similarly, Alternative F involves mining the dust and treating it to recover gold and create a more stable chemical called scorodite, which would still have to be stored in a hazardous waste landfill. And the final possibility, Alternative G, involves removing the dust and encasing it in a more stable material that would be stored on-site in a hazardous waste landfill.
The alternatives were rated by the probability of significant arsenic release over the short- or long-term; worker heath and safety risks; and the costs. The long term outcomes with the least probability of arsenic release are those that involve moving the material to the bottom of the mine, a hazardous waste dump or reprocessing. The risk of exposing workers to arsenic is higher if the dust is moved than if it is left in place. Needless to say the costs are pretty much all over the monetary map: from perhaps $30 million for basic water treatment (Alternative A) to $1 billion for removal and surface disposal (Alternative D).
Whether the arsenic trioxide remains underground or is removed will be the topic of public debate; SKR recommended one solution for each scenario. If the dust is left where it is, the best solution would be to freeze it and the surrounding ground in a block. This plan comes with an estimated cost of $90-120 million. If the dust is removed, they recommend encapsulating it in a stable material, perhaps bitumen. This option has a price tag of between $230 million and $280 million.
The SRK report leaves governments, local communities, and us with plenty to think about. Everyone agrees that the present situation needs remediation, but the decisions on how and how much it will cost cannot be made overnight. Anyone desiring a more thorough knowledge of the situation should visit the DIAND web site, www.nwt-tno.inac-ainc.gc.ca/giant. It’s well worth an hour’s reading.