Regulator tells Diavik to consider longer-term climate change in closure plan

An N.W.T. regulatory board has asked Rio Tinto to revisit its proposed closure plan for its Diavik diamond mine and to extend its climate change projections further into the future. 

The Wek'èezhìi Land and Water Board (WLWB) is organizing a workshop in Yellowknife next week where parties are expected to make some progress on that plan — and to talk about elements of it that have not yet been decided. 

Though the focus of the upcoming meeting is about water quality, documents uploaded to the registry earlier this summer show that the mine's plans to account for a changing climate have also triggered some debate.

Back in July the WLWB returned the first version of Diavik's final closure and reclamation plan to the company, outlining 24 decisions it had made and 53 revisions that were needed. 

One of those changes, it said, was that the mine needed to update its climate change modelling to show the stability of its plans to promote freezing at three facilities: the north country rock pile, the processed kimberlite containment facility and the landfill. 

It also asked the mine to provide model reports for all three facilities that go beyond 100 years. 

"The board recognizes that uncertainty exists with climate change modelling and updated models will continually be improved upon," the WLWB wrote in its letter. "However, the board emphasizes that the onus is on DDMI (Diavik Diamond Mine Incorporated) to ensure that closure designs will be appropriate in the long-term."

The board said the Tłı̨chǫ Government, the territorial government and the Environmental Monitoring Advisory Board all raised concerns about the duration of the climate projections that Diavik had been using. 

The board notes that Diavik has argued climate projections can only be reasonably modelled for 100 years. 

A climate projection report, prepared for the company by a third party, says that the 2120s are the "furthest period in the future with available projections."

What Diavik is doing already

During a media tour earlier this summer, Diavik officials talked about how the closure plan includes reinforcing two spots so they aren't at risk of thawing permafrost. 

The mine is located on East Island in Lac de Gras, about 300 kilometres northeast of Yellowknife, near the  Nunavut border. It's in an area of continuous permafrost, meaning the ground is frozen year-round. 

Gord Stephenson, Diavik's manager of surface operations and closure, said the processed kimberlite containment facility and the north country rock pile are both making use of the natural cold temperature of the ground.

The north country rock pile is a place where waste rock is stored and capped. When open-pit mining ended in 2012, the pile was 500 meters tall at its highest point and contained 140 metric tonnes of waste rock and 4.5 metric tonnes of till, according to that first closure plan the company filed in 2022. 

Stephenson said the pile contained "potentially acid-generating rock" and that it needed to be covered in order to stay frozen. That cover, he said, is 4.5 metres in depth. At the time of the tour, officials said the closure of the pile was 97 per cent complete. 

Stephenson said one of the challenges of the closure has been making the middle area of the processed kimberlite containment facility stable in the long term. 

He said some of its walls were built on "ice-rich till," and that there's a "minute" chance that if that till were to thaw, those walls could shift. That's why it's reinforcing that area too. 

"What we're doing is a defence for that scenario of thaw. We're buttressing and stabilizing the dam embankments to prevent cracking so if it does thaw, everything still stays stable there." 

If there were to be thawing, Stephenson said there's a risk the company would have to come back and do some remediation work. "If we saw some settlements that needed to be addressed, we would have to come in an put some more rock in place."

The WLWB is giving Diavik until the end of next April to address the decisions and revisions it's made. Its letter says "climate change was identified repeatedly as an area of uncertainty" that could have "significant implications and contingency considerations." 

The WLWB said updated climate modelling was needed to "understand the long-term stability" of the three facilities and they would be needed before Diavik's security was returned.