Tree water use and sensitivity to contaminated mine water

Riverbank, or riparian, vegetation provides many ecosystem benefits that keep creeks healthy, including habitat and food for fish and wildlife, shade to cool water temperatures and roots to stabilise banks. Because groundwater likely meets more than half of the dry season needs of riparian vegetation in the Top End, groundwater contamination here could significantly impact riparian vegetation and associated river health.

Weathering of waste rock from the Ranger Uranium Mine releases contaminants, including magnesium sulfate. These contaminants are washed out by rain and are predicted to move through the local groundwater towards Magela Creek. Depending on the concentration, magnesium sulfate (a salt) has the potential to affect the trees, other plants and ecosystems along riverbanks downstream from the Ranger mine site.

The Ranger Uranium Mine is due to cease operations in 2021 and be revegetated by 2026. This study will lead to better knowledge of common riparian woody species’ groundwater uptake, and therefore their relative dependence on it, to help predict contaminant impacts and inform mine closure and monitoring. The project will also increase our understanding of the sensitivity of riparian and forest ecosystems to changes in groundwater levels, which can then be applied to changes caused by other factors such as irrigated agriculture developments or climate change. The work will help improve our understanding of surface water–groundwater interactions in similar environments.

This project will:

  • identify where the trees along the creeks in the Magela catchment get their water from – from the soil, from the creek itself or from the shallow groundwater – and the relative quantities of each
  • assess how sensitive common tree species are to magnesium sulfate levels
  • assess risks to riparian vegetation from the contaminated water, predict its impact and identify where ongoing monitoring should be focused
  • identify which tree species grow best in salty areas if rehabilitation is needed

Project activities

  • Compare the isotope signature of trees in the catchment to the isotope signatures of soil, creek, and shallow and deep groundwater to determine the age and source of water that the trees use and their relative dependence on groundwater over a wet–dry cycle
  • Estimate the water use of riparian vegetation through calibrating remotely sensed images
  • Assess the sensitivity of common riparian woody species to magnesium sulfate using greenhouse-based trials
  • Quantify risks to riparian vegetation associated with the discharge of mine-related contaminants into surface and groundwater through analysis and surface and groundwater modelling.

Anticipated outputs

  • Technical reports and associated scientific papers
  • Recommendations for monitoring riparian health
  • Maps of areas that mine-related contaminants are likely to most affect.
  • Communication products for policy-makers, ecological restoration consultants and for the mining and agricultural industries.

Fieldwork will take place at the Ranger Uranium Mine as well as upstream and downstream of the mine area.Map depicting the location of Ranger Uranium mine

This project is being led by Professor Lindsay Hutley from Charles Darwin University (CDU). Professor Hutley will be assisted by researchers from CDU, the University of Western Australia and the Supervising Scientist Branch of the Department of the Environment and Energy and by the Gundjeihmi Aboriginal Corporation.

Contact

Lindsay Hutley, Charles Darwin University

[email protected]

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  • An upstream monitoring site in Magela Creek, Kakadu. Photo: Supervising Scientist Branch (Commonwealth of Australia).
  • A monitoring site in Magela Creek, Kakadu. Photo: Supervising Scientist Branch (Commonwealth of Australia).
  • Gulungul creek floodplain between Baralil and Magela. Photo: Supervising Scientist Branch (Commonwealth of Australia).
  • Magela Creek in Kakadu. Photo: Supervising Scientist Branch (Commonwealth of Australia).
  • Riparian vegetation serves a critical function as habitat and protection for Top End waterways. Photo: Michael Douglas
  • For extended periods of the year, riparian vegetation relies on groundwater. Photo: Michael Douglas
  • Kakadu National Park surrounds the Ranger Uranium Mine, so effective rehabilitation is essential. Photo: Michael Douglas
  • Assessing riparian vegetation in the field and testing tolerances in the lab will guide rehabilitation criteria. Photo: Michael Douglas