O'Toole, D, Harnett, J, Cupitt, P, Georgiou, P & Riascos, A 2022, 'Climate change impact on the closure design for the rehabilitation of the former Rum Jungle uranium mine', in AB Fourie, M Tibbett & G Boggs (eds), Mine Closure 2022: Proceedings of the 15th International Conference on Mine Closure
, Australian Centre for Geomechanics, Perth, pp. 645-660, https://doi.org/10.36487/ACG_repo/2215_46
The former Rum Jungle uranium mine, located approximately 100 km south of Darwin, operated between 1954 and 1971 and underwent rehabilitation from 1983 to 1986. The rehabilitation addressed significant environmental impacts caused by acid metalliferous drainage and achieved objectives related to reducing public health hazards. However, recent studies documented that the original rehabilitation works do not meet modern environmental standards, meaning further rehabilitation work would be required for site closure and relinquishment.
An improved rehabilitation strategy, consistent with the views and interests of stakeholders and that meets contemporary environmental and mined-land rehabilitation standards, was developed in 2020. The strategy involves relocation of potentially acid forming waste rock to new surface waste storage facilities (WSFs) and a water and tailings filled pit, treatment of contaminated pit and groundwater, and the realignment of the East Branch Finniss River (EBFR) to follow its original course back through the pit.
There is now widespread acceptance that human activities are contributing to observed climate change. Continued emissions of greenhouse gases are highly likely to cause changes in all components of the climate system. The Northern Territory Government requested that the effects of long-term climate change on specific aspects of the proposed rehabilitation design be assessed.
Based on the UN’s Intergovernmental Panel on Climate Change Fifth Assessment Report (AR5; IPCC 2014), and Australia’s CSIRO and Bureau of Meteorology (CSIRO & BOM 2015) climate change projections, regional highlights relevant to the project site include warming between 0.5°C and 5.1°C, dry season changes projected to range from 45% drier to 44% wetter, and, for the wet season, 23% drier to 19% wetter. Additionally, fire weather will become more frequent and harsher.
Key climate change indicators likely to influence the rehabilitation design are considered to be climate change–induced rainfall, especially peak intensity rainfall events, and climate change impacts on fire weather, considering both the potential for a catastrophic fire decimating the vegetation throughout the project site catchment, and how the intense heat from an extreme bush fire might change the structure of the soil, resulting in hydrophobic conditions.
The effect of the above potential climate change–related factors has been studied to ascertain the robustness of the specific rehabilitation design elements, including backfilling of the pit, realignment of the EBFR and the erosion of the WSFs.
The key findings of the study have shown that the primary impacts of climate change on the success of the rehabilitation are changes to flow regime and flood levels within the EBFR, with an increase in peak flood levels of between 50 mm and 600 mm. However, the flood level increase would not exceed the defined channel and therefore would not inundate the proposed WSF footprints; hence, no specific mitigation will be required. Increased creek flow velocities would require enhanced erosion and scour protection immediately upstream and downstream of the Main pit. Other drainage structure design upgrades would also be required to mitigate the effects of increased flows.
Modelling of the WSFs to account for climate change effects has shown the maximum gully erosion after 500 years is less than 1.3 m, providing contingency compared to the 2.5 m cover design thickness. Spatially, less than 1% of the erosion will be of a depth greater than 1 m.
Under both base case and climate change scenarios, the type and rate of revegetation is critical to controlling erosion. The final revegetation plan should carefully consider how climate change influences can impact the type and extent of vegetation used.
Keywords: climate change, rehabilitation design, erosion modelling
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