Tree water use and sensitivity to contaminated mine water

Ranger uranium mine is surrounded by Kakadu National Park. The mine has ceased operations and rehabilitation works are due to be completed by 2026. Spring-fed monsoon vine forests and riparian (riverbank) vegetation depend on soil and groundwater and provide essential habitat for the highly diverse aquatic ecosystems of the freshwater Magela Creek, which flows through the mine lease. Contamination of shallow groundwater with mine wastewater after rehabilitation of the site could have significant impact on this riparian vegetation and stream health.

This study has improved the knowledge of how common woody riparian plants use and depend on groundwater, which will help predict impacts from contaminants and inform mine closure and monitoring.

The project:

• used different isotope tracers to quantify the groundwater dependence of riparian vegetation in the Magela Creek catchment, which flows through Kakadu National Park and the Ranger uranium mine lease
• tested the sensitivity of common woody riparian species to the contaminant magnesium sulfate (MgSO₄; a salt) to provide a risk assessment of impact from potentially contaminated surface and/or groundwater
• quantified risks to riparian vegetation associated with the discharge of mine-related contaminants into surface and groundwater through analysis and modelling of and surface and groundwater.

Location of Ranger uranium mine adjacent to Kakadu National Park.

• Riparian trees in the creek channel, upper banks and floodplains of Magela Creek predominantly access shallow (~0.7–1.5 m deep) soil water sources.
• Trees in the creek channel (on the ridges, islands and lower banks) also use shallow groundwater contained in the sand-bed aquifer (1.5–2 m deep), while some of the trees located on the floodplain (about 40 m away from the channel) use groundwater contained in the deeper weathered bedrock aquifer (3–5 m deep).
• Riparian trees are most likely to draw on potentially contaminated shallow groundwater during the dry season and trees on the floodplain may draw on potentially contaminated deeper bedrock groundwater in very dry conditions.
• Riparian species are unlikely to be severely impacted by MgSO₄ at (and at even greater than) the concentrations they are predicted to be exposed to after rehabilitation. No significant growth impacts were observed with MgSO₄ concentrations of up to 7,095 mg/L MgSO₄ (equivalent to 1,433 mg/L Mg) across the eight species tested in the greenhouse sapling experiments. Current modelling suggests peak magnesium loads from the rehabilitated landform could reach 500 mg/L Mg in shallow groundwaters although further hydrological work is required to increase the confidence in these modelled loads.
• Additional work is required to determine in situ responses of more mature vegetation in contrast to the sapling-focussed greenhouse trials from this project.

Analysis of the different compositions of isotopes suggests that riparian trees along Magela Creek predominantly access shallow (~0.7–1.5 m deep) soil water sources. However, the origin of this shallow soil water could be either wet-season rainfall or capillary rise from the groundwater below. Aside from this shallow soil-water dominance, we found that trees located on the ridges, islands and lower banks of the creek channel also use shallow groundwater contained in the sand-bed aquifer, while some of the trees located on the floodplain (about 40 m away from the channel), use groundwater contained in the weathered bedrock aquifer.

The isotopes also point to the potential flow pathways within and between geological layers. There is no direct evidence for connectivity between the weathered bedrock and sand-bed aquifers, but the slope of the water table suggests that sub-surface flow from shallow parts of the weathered bedrock aquifer to the sand-bed area occurs during the wet season and as the flood flows recede. During the dry season, sideways connectivity is low as the creek ceases to flow and vertical groundwater flow becomes dominant.

Given the potential for the flow of contaminated groundwater from the weathered bedrock to the sand-bed aquifer during the wetter months, risk may be high for trees located in and closer to the channel (i.e. on ridges, islands and lower banks) due to their potential use of contaminated water present in the sand-bed aquifer and the soils that sit above the sand. Risk may also be high in floodplain areas, where trees are likely to use some bedrock groundwater under very dry conditions.

In summary, riparian trees predominantly use soil water during the dry season and varying amounts of groundwater. If these sources are contaminated (as they are presently at one location in Magela Creek receiving contaminated mine-derived groundwater), then mature trees will be exposed to that contamination. During the wet season, exposure may arise from contaminated groundwater and/or surface water from mine-site runoff or leachate.

Conceptual cross-section of Magela Creek in the dry season.

The project conducted greenhouse trials to test the sensitivity of common riparian tree species to a wide range of magnesium sulfate concentrations, including three concentrations above the modelled peak magnesium load. The highest concentration tested was nearly three times the peak modelled magnesium load.

The University of Western Australia project team conducted initial trials on two targeted species, Melaleuca viridiflora (broad-leaved paperbark) and Alphitonia excelsa (red ash). The Charles Darwin University team then subsequently tested a further four species – Syzygium armstrongii (small white bush apple), S. forte (white bush apple), Lophopetalum arnhemicum and Carallia brachiata (bush currant). A final phase included testing two additional species (Pandanus aquaticus [river pandanus] and Barringtonia acutangula [river mangrove]) and a retesting of A. excelsa at narrower range of treatment concentrations. The project team identified these species, which are all used by local Traditional Owners, in conjunction with staff from the Australian Government’s Supervising Scientist Branch and Kakadu Native Plants Pty Ltd.

These findings all suggest that these riparian species are unlikely to be severely impacted by magnesium sulfate at (and even greater than) modelled concentrations, as no significant growth impacts were observed up to the highest concentration treatment across any of the species tested.