Future scenarios for mine voids from open cut mining in the Hunter Valley

A void left by coal mining that is now used for crayfish farming, Western Australia (Photo: W. Timms)

Research by a team from the Australian Centre for Sustainable Mining Practices (ACSMP) at UNSW Australia is looking at future scenarios for mine voids that are left by open cut mines in the Hunter Valley

Alexandra Fegan, Dr Wendy Timms and Dr Simit Raval have summarised their research progress so far.  

Final voids of open cut coal mines are a potential issue for water resources and communities, particularly as the number, depth and size of voids increases. A review of available information on mine voids within the Hunter Valley indicated there are currently plans for 30 final voids, with a combined footprint of 3,840 hectares (~38 km2 or 0.18% of the total region). Reported plans for final void use was varied: backfilled and rehabilitation to a stable landform (n=6), coal and tailings placement (n=7), water storages (n=6), while others were yet to be decided. Mine void areas are small compared with 19,570 hectares of mining disturbed land and 8,188 hectares of rehabilitated mining land in this region (Summerhayes, 2011).

Based on this review, mine void models (A to F) were designed to typify a range of realistic site conditions (eg. A was 300 ha, 200 m depth; B was 40 ha and 250 m depth). An indicatication of the feasibility of backfilling each void type was evaluated with a semi-quantitative approach. It was found that backfill might be economically feasible only for void types B and E, with various assumptions including a strip ratio of 4:1 (waste:ore) and the number and type of earth moving equipment deployed. However, backfill of other void types was unlikely to be feasible, given a back fill time >5 years during mine rehabilitation. Thus, alternative beneficial uses of many open final voids are required.

Numerical models (n=18) for water level and salinity of open voids over 500 years were then developed in Goldsim software, taking into account evaporation losses and mass balance of water and salt on a monthly time step. For each void type (A to F), equilibrium water levels and salinity were modelled for 3 scenarios -  0 and 4 ML/day groundwater inflow, and a groundwater in flow rate to achieve an equilibrium water level near the top of the void. Preliminary results will be presented for water salinity and level trends for the 18 models. Results indicated water salinity at 500 years was fresh (n=6), brackish water (n=9), moderately saline (n=2), and seawater salinity (n=1), and thus with a range of benefical uses for the water. These results extend and confirm model results by Hancock et al. (2005), though both approaches have several limitations. Final voids that could be brackish or saline should be designed as groundwater sinks (no flow away from the void) with negligible risk of overflow from the top of the void. Further work is recommended to evaluate future scenarios for voids on a case by case basis, including for water storage, aquaculture, pumped hydro storage, and the possibility of wetlands managed for carbon sequestration.

For further information contact w.timms@unsw.edu.au