It was once dubbed New Zealand’s most contaminated site. But after more than two years of work, the major risks to community health and safety and environmental damage posed by the Tui Mine site on Mt Te Aroha have been removed.
The successful conclusion of the complex project to remediate the abandoned Tui Mine on Mt Te Aroha was celebrated in May 2013. About 100 people gathered at Tui Pa before travelling up the mountain to view the remediated site. Among the guests were Environment Minister Hon Amy Adams, local Members of Parliament, community leaders and contractors.
The Tui Mine site consists of underground mine workings (adits), waste rock dumps and ore stockpiles, and deposited tailings from the ore processing. There are a number of water discharges from the site including drainage from adits, contaminated under drainage from waste rock and tailings, and natural sources.
There were three main issues arising as a result of the mining activity at this site:
The Tunakohoia stream, located on Crown land managed by the Department of Conservation (DOC), was affected by heavy metals leaching from the adits and from the tailings dam. These heavy metals, including lead and cadmium, were contaminating the Tunakohoia stream which flows through the centre of the Te Aroha township.
The Tui catchment is adjacent to but separate from the Tunakohoia catchment. The Tui catchment was also affected by heavy metals arising from the tailings dam.
There is an abandoned mine tailing impoundment in the Tui catchment, on land owned by the Matamata-Piako District Council (MPDC). Technical reports found that this structure was at risk of collapse in a moderate seismic event or an extreme weather event. Such events could have resulted in over 90,000 m3 of mine waste liquefying and flowing down the Tui stream past the edge of Te Aroha.
contain the tailings within a stable and secure location
reduce the release of contaminants into the Tui and Tunakohoia streams, thereby improving the water quality in those streams
improve the geotechnical stability of the tailings impoundment
improve the safety and security of the site
improve the visual appearance and aesthetics of the site
address as far as practicable, within the limitations of the project, the impacts of the Tui mine on the taonga of the Te Aroha maunga (Mt Te Aroha) for iwi.
The Waikato Regional Council has been involved in the management of the Tui Mine remediation works since June 2007, when the Ministry for the Environment (MfE) announced funding for the project.
The project is overseen by a governance group comprising the senior management of Waikato Regional Council, MfE, the landowners, Matamata-Piako District Council (MPDC) and the Department of Conservation (DOC). Local iwi Ngāti Rāhiri Tumutumu are also to be part of project governance. A steering group including senior staff from the main parties will oversee the project’s implementation. A representative from the Iwi Advisory Group, comprising members of interested iwi including Ngati Rahiri Tumutumu, Te Kupenga O Ngati Hako and Ngati Haua, is also part of the project governance and steering group.
The project has been completed in two phases:
Phase 1 involved preparatory work (detailed design, site establishment, access road) and the treatment of the old underground mine workings. The old mine workings discharge contaminated water into the Tunakohoia Stream. The workings were partially plugged with engineered concrete bulkheads, and an alkaline solution (lime or cement) was injected into the ground to counter the strongly acidic conditions.
Phase 2 involved remediating the tailings dam area. The initial plan was to contain the tailings in a new dam built to modern standards. Instead, a one-metre cap of clean fill was placed 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 was carried out in May 2013, while the community will be invited to join in planting at the rest of the site in the spring 2013.
The project numbers
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 trucked 8000 tonnes of lime, 14,000 tonnes of rock and gravel, 10,000 tonnes of clay and 10,000 cubic metres of topsoil to the 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 project took about 160,000 man hours of planning, management, engineering and construction time.
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.
Phase 1 of the project: Completed within a budget of $6.5 million.
Phase 1 - Clearing Raise 3 ore chute
Phase 2 of the project: Started in October 2011 and is due to be fully completed in December 2013, within an estimated budget of $15.2 million.