Hydrological and geomorphic processes
The Alligator Rivers Region has rich uranium resources. Located in the ARR are small scale abandoned mines (South Alligator River Valley), the non-operational Nabarlek ex-mine site, the operating ERA Ranger mine and Jabiluka Project Area in the Jabiluka Mineral Lease.
ERA Ranger mine (photo: D Moliere)
On the completion of mining, disturbed land areas need to be rehabilitated, that is, returned to a condition where stable ecosystems can develop and the land can be used by future land users. After the uranium is mined, the remaining landform may comprise tailings storage sites, a dump where waste rock has been deposited and mined-out pits. Tailings are a slurry of ground-up rock and other waste that remains after uranium is removed. The tailings may contain low levels of radioactivity. Sites must be left so that risks of physical injury to the public and release of radioactive material to the environment through erosion or weathering will be very small.
A post-mining rehabilitated landform needs to be sufficiently stable to encapsulate waste material, tailings and/or low grade mineralised rock. This means that erosion should not result in gullying and exposure of tailings and other waste material to the environment within a defined time period (Post-mining landform stability)
Once the integrity of the encapsulating structure has been assessed it is necessary to determine how much sediment could be delivered from the slopes of the landform through natural catchments into downstream-receiving waterways (Sediment/contaminant delivery from mine sites to stream systems).
Jabiluka Project Area with Magela Creek floodplain in the distance and tree fall from Cyclone Monica (Photo: Wayne Erskine – University of Newcastle)
The addition of sediment to stream systems from erosion of the rehabilitated landform could elevate stream sediment concentrations above accepted water quality guidelines. If the impact is unacceptable, control strategies should be implemented on the mine site so that an acceptable outcome is achieved (Impact on downstream water quality). Finally, the rehabilitated landform must be self-sustaining so that its integrity is retained in the future with little maintenance and should be revegetated and shaped so that it blends with natural surroundings.