In Australia, once an open cut coal mine closes, the final mine voids are often left to fill with water and become lakes. Mine voids can be used to store tailings; following mining, the tailings can be capped, however another option is a water cover, which is a method for controlling the oxidation of sulphides in tailings.
A miner in NSW is currently investigating the option of storing tailings in pit(s) and utilising a permanent water cover. In the initial stages of this work, various options are being considered, including tailings to differing elevations in the pit(s).
There are many factors that need to be considered and investigated to prove the concept is viable for this mine at its location, including whether the rainfall will be sufficiently high to maintain the cover, including during periods of drought.
To determine the minimum thickness of the water cover, an extensive literature review was undertaken, including identifying the critical aspects for defining a water cover thickness, methods for estimating the thickness, and collating what thicknesses have been utilised around the world and whether the water cover can be considered successful. For water covers to be effective, there should be no stirring or resuspension of the bed tailings. Resuspension would increase suspended solids and turbidity, and increase the possibility of oxidation, acid generation and the release of metals and sulphates, as well as increase salinity. This will occur if the water cover is too shallow, roughly below about 2-3 m. Australian examples were also found, such as in Victoria and Tasmania.
Numerical groundwater modelling was undertaken to predict the void inflows/outflows under differing scenarios. Water balance modelling is now being undertaken, utilising the inflows/outflows from the numerical groundwater modelling, rainfall, evaporation and rainfall runoff. The water elevation in the in-pit tailings storages for each of the scenarios will be predicted, and an assessment undertaken on whether a water cover of sufficient thickness (identified during the literature review) will develop for any of the scenarios.
Should the water cover in this NSW example be deemed feasible following these initial works, further work to prove this concept will be undertaken, which may include refining the minimum water cover thickness through the application of various methods, such as the use of a lake water balance and stratification model coupled with a water quality model, field measurements, laboratory tests and pilot studies.
Given permanent water covers for in-pit tailings storages are not well-known in Australia, and not previously used in NSW, scientific rigour is imperative to obtain regulatory approval. Should regulatory approval be obtained, it is expected in-pit tailings storages with a permanent water cover will become more common in Australia.