Northern News Services
In a plan submitted for restoration of the gold mine's property, the company set 370 parts per million of soluble arsenic as an acceptable arsenic concentration to leave in soils on the mine property once the mining closes.
Based on a 1998 study by a private consultant, the estimate assumes "that the site would be used in the future for a similar development," Miramar stated in its interim abandonment and restoration plan.
Canadian soils typically have arsenic levels of 4-14 ppm. A Royal Military College report released earlier this year indicated arsenic occurs naturally in the Yellowknife area in concentrations of 3-150 ppm.
The job of determining safe levels of arsenic is complicated further by the fact that some kinds of arsenic are more harmful than others.
One of the most toxic forms, arsenic trioxide, was emitted from the gold roaster stacks at Giant and Con for decades.
The poisonous dust particles settled on the land around the mines in varying concentrations.
Con mine manager John Stard emphasized Miramar's interim plan for reclamation of Con is a "living document."
The standard it recommended was based on the best data available, he said.
Another set of soil remediation standards is offered in a draft report now before a committee of industry and government officials. Miramar is a member of the Yellowknife Soil Arsenic Remediation Committee.
The report, prepared by private consultant Risk Logic, recommends standards for arsenic in soil based on land use.
Different levels are proposed for industrial, residential and recreational uses.
Committee chair Steve Harbicht said the committee is aiming to complete a technical review by the end of the month and release the final report early in the new year.
"Once we have completed the task of developing these guidelines, then we turn them over to the various regulatory authorities, who may or may not make use of them," the Environment Canada official said.
Stard suggested standards based on three land use categories may not be adequate for reclamation of Con.
"I think it's going to have to be almost site specific," said Stard.
In his response to Miramar's interim abandonment and restoration plan, Harbicht stated that Miramar assumes too much when it assumes the entire area need only be restored for industrial uses.
Land use for the area will have to be discussed with the city, government of the Northwest Territories and other parties who will become responsible for the land, Harbicht noted.
Most of the soil contamination from Con, which has operated for half a century, occurred before Miramar took over the property in 1993.
If the standard recommended by Con is accepted, the company estimates it will have to excavate and remove 98,000 cubic metres of soil from the site.
All of the soil would come from within a metre of the surface.
In its interim plan, Miramar calculated total cost of the cleanup would be $7.7 million.
Surface cleanup alone at Giant is anticipated to cost $16.3 million.
The Mackenzie Valley Land and Water Board in October rejected the plan.
The board felt more detail was needed and that the plan failed to address a number of key issues, such as how the company would deal with the excavated soil.