YELLOWKNIFE - Approximately 37,000 square metres of land and lake were contaminated when processed kimberlite tailings overflowed a contaminant wall at the Ekati diamond mine two weeks ago.

"An investigation is still ongoing and we don't know the cause of it," said Deana Twissell, superintendent of external affairs with Ekati.

Approximately 4.7 million litres of kimberlite tailings from Ekati Mine spilled into a nearby lake about two weeks ago. - photo courtesy of BHP

She said the spill could have been discharged from a spigot (valve) or by shifting tailings pond water levels from the spring melt.

Processed kimberlite tailings - which is a by-product of the diamond extraction process - is processed ore that has been crushed to a half a millimetre in size, according to Twissell. It consists of water, sand and silt containing ferisilicon, which Twissell described as "an inert alloy of iron and silicon, which is not harmful to the environment."

Around 4.7 million litres of the tailings spilled onto tundra and adjacent frozen Fay Lake.

"The flow was limited to the edge of the frozen lake," said Twissell.

The spill was reported by a worker who saw it from the air when he was flying in on May 16.

As of May 27 - when an NWT spill report update was issued - 2,300 cubic metres of tailings remained to be cleaned up.

Ekati is responsible for all costs associated with the clean-up. It is still ongoing.

According to Annette Hopkins, director of operations with Indian and Northern Affairs Canada, Ekati is permitted to discharge tailings into four different ponds, or cells.

"They have been discharging in cell B (where the incident occurred) for more than a month," she said.

The tundra from the containment area to Fay Lake is very rocky, said Hopkins.

"I don't believe there was a lot of vegetation between the containment area and the lake," she said.

Hopkins said water samples have been taken from the site and have been sent to laboratories.

Although processed kimberlite is not toxic, the spill is still an issue, said Marc Lange, area director of the Department of Fisheries and Oceans (DFO).

"We call any material that gets deposited in water, even clean sand, a harmful alteration of habitat, because it can smother eggs and displace fish," he said. "Even if it was inert, it would be a concern of ours to get that off the lake. (Fay Lake) is definitely fish bearing, from what folks have told me."

Lakes in that area are typically home to lake trout, whitefish and Arctic grayling, said Lange.

Workers built a temporary road down to Fay Lake and a fence on the lake to contain vehicles and clean-up operations.

Heavy equipment scraped lake ice and dumped excess processed kimberlite into Ekati's Long Lake Containment Facility.

Twissell said they cleaned up as much on Fay Lake as they could while it stayed frozen. A vacuum sucked melt water off the surface of the lake and the water was taken to the contamination facility.

Workers also installed silt nets on the tundra slope and shore of Fay Lake to act as a barrier for silt movement. A dyke was put up to contain the cell and stop additional flow.

DFO has the authority to fine perpetrators of spills.

"At this point, no charges have be laid," said Lange, adding that he will know in a couple weeks if there will be a fine levied. Fines under the Fisheries Act can range from demands to help recover and regenerate the contaminated habitat to cash penalties of up to $100,000 for a first offence summary conviction or $500,000 if charged as an indictable offence.