SOMBA K'E/YELLOWKNIFE - Beneath Highway 4 and the old Giant Mine site lies 14 huge chambers filled with deadly arsenic trioxide dust.

Martin Gavin, project manager for the Giant Mine Remediation Project, stands at Giant Mine on July 31. - Tim Edwards/NNSL photo

The chambers are sealed, but groundwater may get in and become contaminated, so the federal government plans to encase the 237,000 tonnes of dust in huge blocks of ice.

"There was arsenic trioxide produced as a byproduct of producing gold and that was stored in underground chambers which are essentially rock chambers, which are several hundred feet underground," said Martin Gavin, who took over as project manager for the Giant Mine Remediation Project in November 2008. "You have 14 chambers roughly the size of the Precambrian building under the ground out there."

The doors to the chambers are blocked off with large concrete bulkheads that range from four to 10 feet in thickness, and are bolted into the rock.

"They did a great job engineering the (chambers), these are all really solid structures, so this stuff is not free-flowing underground. It's very well contained," said Gavin.

Despite those safeguards, there's still potential for groundwater to flow through the chambers.

Arsenic trioxide is can be deadly if ingested in doses as small as one milligram. However, it is rarely fatal when it contacts the skin or eyes, or if it's inhaled.

Indian and Northern Affairs Canada (INAC), which is overseeing the clean-up project, is the federal department that chose the "frozen block method" from among more than 50 approaches that were considered, according to Gavin.

"Essentially what we're going to do is we're going to make an underground freezer," said Gavin. "Basically what we do is drill underground and put in brine pipes ­ the same type of brine pipes you'd find at the Multiplex or at the arena, or in a deep-freeze in your house.

"We add a brine system to that and then we turn on the fridge, add water, and make a big ice cube. The groundwater will come, hit the ice cube, and will find its way around."

After the initial freezing, thermosyphons will be used to keep the temperature below zero. Thermosyphons are long, vertical cylinders that passively control temperature by circulating liquid inside, a cycle using heat to evaporate it and condensation to bring it back down to the bottom. No pump or power is required to keep the system going.

Gavin estimated the project will cost about $400 million, but there are still lots of details and design specifics to be figured out before they can nail down the exact cost.

Mayor Gord Van Tighem told Yellowknifer in January that city council was worried INAC might "freeze it and forget it." The city requested an environmental assessment, which was granted.

At that same time, former city councillor Kevin O'Reilly said INAC was just getting into a "holding pattern" and that he thought it wasn't a very responsible way to deal with the arsenic situation.

O'Reilly also expressed concern about the ice melting.

If for any reason the blocks were to thaw out, Gavin said the situation would be just as it is today, once again creating the potential to contaminate groundwater.

Gavin also said that the project will be kept up indefinitely, and the threat of thaw is low with thermosyphons in place.

"Upon completion of environmental assessment it should take eight to 10 years to essentially reach substantial completion," said Gavin.

He said INAC hopes to start the project around 2012-13 but is not sure how long the environmental assessment will take.

Other aspects of the project will include removal of buildings on site; the tailings pond will be capped and covered and vegetation will be placed atop it; the water treatment facility will be moved to a new location and modernized; and the highway will be re-routed, according to Gavin.

Right now, the project's crew are testing different drilling technology and types of drill heads, among other things, to make sure that they are efficient when the time comes for the project to start, he added.