The successful conclusion of the complex project to remediate the abandoned Tui Mine on Mt Te Aroha is being celebrated today.
The remediation has removed the major risks to community health and safety and environmental damage posed by the mine site.
Tunakohoia Stream was contaminated with heavy metals leaching from the mine and was unsuitable for swimming, fishing, drinking or irrigation. The Tui Stream was dead and unable to support any aquatic life. These streams flow into the Waihou River and eventually into the Firth of Thames, so the mine’s toxic legacy extended well beyond Te Aroha.
There was also a risk that the tailings dam would collapse in an extreme weather event or earthquake, sending up to 90,000 cubic metres of mine waste toward Te Aroha.
“For a long time this was dubbed the most contaminated site in New Zealand. As a result of the work carried out over the past two and a half years that is no longer the case,” said Waikato Regional Council chairman, Peter Buckley.
About 100 people are to gather at Tui Pa before travelling up the mountain to view the remediated site. Among the guests are Environment Minister Hon Amy Adams, local Members of Parliament, community leaders and contractors.
“Today’s function has been an opportunity to acknowledge the contribution so many organisations have made to this important project,” Cr Buckley said.
“Cleaning up and remediating the Tui Mine site has taken a collaborative effort as well as some innovative engineering solutions and very clever thinking to a technically complex issue.
“It is the first time in New Zealand such work has been undertaken and as a result we’ve not only been able to protect a community but also provide people with a place to visit in the future.
“A healthy environment is critical to wellbeing and our economic future. In the Waikato we are fortunate to be rich in natural resources but we need to find ways to grow our wealth while at the same time protecting our environment. A Tui Mine will not be allowed to happen ever again,” Cr Buckley said.
The project, the first of its kind in New Zealand, took about 160,000 hours of planning, management, engineering and construction time to successfully remediate the contaminated site on the mountain’s western slope.
Since work began, heavy trucks have carried more than 10,000 tonnes of cement to the site – that’s enough to build a 40-storey building.
Contractors have also trucked 8000 tonnes of lime, 14,000 tonnes of rock and gravel, 10,000 tonnes of clay and 10,000 cubic metres of topsoil up Tui Road to the contaminated mine site.
Construction crews have used these materials to stabilise 115,000 cubic metres of toxic mine tailings and 8000 cubic metres of stockpiled waste rock.
Their work has included injecting a mixture of cement and lime (limestone slurry) into the old underground mine workings to reduce the amount of contaminants leaching into Tunakohoia Stream.
“The mass stabilisation of material and chemical treatment and neutralisation of acidic environments at this scale was unique,” said project manager, Waikato Regional Council’s Ghassan Basheer.
“Chemical treatment is normally carried out in a laboratory environment, but in this case we had to treat massive quantities of toxic material on site, which was technically very challenging.
“To make sure we got it right we carried out extensive work to identify the best treatment method for this site. We then trialled our method on 4000 cubic metres of tailings over a three month period, and only once the trial was successful did remediation work really get underway.”
Mr Basheer said while there were plenty of international case studies for the treatment of underground mine workings, there was very little international experience to draw on in terms of in situ stabilisation of toxic material.
The final stage of the project, carried out over the past two months, has involved placing a one-metre cap of clean fill over the newly shaped land. This has been grassed and is designed to stop oxygen and water entering the stabilised tailings in the short term until vegetation cover establishes.
Initial planting of natives at the site’s old processing plant will be carried out this month, while the community will be invited to join in planting at the rest of the site in the spring.
The $21.7 million project has involved the Ministry for the Environment, Waikato Regional Council, Matamata-Piako District Council, the Department of Conservation and local iwi.