McArthur River Mine is a zinc-lead mine in the Northern Territory, Australia. When it transitioned from underground operations to open cut mining in 2006, approval was conditional on the appointment of an independent environmental monitor for the life of the mine. The most recent report from the independent monitor, covering the operational period of 2012 to 2013, was based on the monitors’ review of environmental assessments, audits and monitoring data undertaken by us and by the regional government.

The report noted a number of successes in waste management including:

  • Installation of a siltation pond at Barney Creek bridge that reduced contaminants entering the creek, with notable reductions in lead and zinc levels
  • Additional monitoring of ground and surface water and revision of the water balance model
  • Development of an interim cover for the tailings storage facility (Cell 1) and installation of various seepage control measures
  • Construction of an interim clay cover for waste rock material on northern overburden emplacement facility before the wet season
  • Implementation of comprehensive overburden characterisation programme, including the identification of management actions to address
  • the change in geochemical risks
  • Ongoing improvements to minimise fugitive dust emissions

In the period between the 2013 report’s compilation and its publication in late 2014, we addressed a number of concerns it had raised. This included actively meeting the challenge of mitigating emissions from waste rock (the result of a naturally occurring reaction when pyrite‑rich rocks have contact with air and water). This is now under control; emissions have almost completely stopped and our current air quality monitoring data shows no risk to the surrounding community in Borroloola.

Reclassification of overburden

Overburden is one of the chief waste materials in open cut mining. It includes some topsoil, clay and various non-commercial rock types; these are not treated but must be removed to reach the ore.

We worked with specialists performing extensive tests to understand the properties of the overburden at this asset. The most important results include overburden oxidisation, through contact with air and water, which can lead to a leachate liquid forming that contains acid. Previously we used three classifications of overburden: alluvium (or clays), non-acid forming (NAF) rock and potentially acid forming (PAF) rock. Now six classifications are used, two of which have benign properties and four have non-benign properties.

This re-classification led us to redesign the overburden emplacement facility; this process will include extensive consultation with traditional owners, other community members and other stakeholders.