The current BHP Billiton Olympic Dam operation comprises underground workings and a number of surface facilities, including the tailings storage facility (TSF) and processing plant. Mined out stopes are backfilled using cement amended tailings and aggregate fill. At the end of mining, dewatering will cease and the underground workings will re-flood. To support development of closure plans for the mine site, SRK assessed the potential impacts the operation could have on the regional hydrogeology (comprising the bedrock and Andamooka Limestone aquifers) and groundwater quality in the intermediate and long term post closure.
The approach combined hydrogeological modelling (using a FEFLOW platform*) with modelling for potential solute release from contaminant sources. The two primary sources identified were the: (i) above ground TSF, which releases percolate to the underlying groundwater table, and (ii) backfill and wall rocks within the underground workings.
The underground workings are connected to the overlying Andamooka Limestone aquifer via raise bores.
The model was used to assess the current extent of groundwater drawdown and calibrated against monitoring bore observations. The model was then used to predict the life-of-mine drawdown (3 to 11km) and the post-closure rebound, including the times required to re-flood the underground workings (several centuries). The model was also used to predict the groundwater mounding during tailings deposition (operational) and times for the mound to dissipate following closure (decades).
The TSF source was based on geochemically characterising existing tailings products and field evidence. These studies indicated that the percolate is neutralised within underlying soils and sediments, and that most metals and trace elements are attenuated, either within the tailings or immediately below the TSF.
The underground source comprised exposed (sulfidic) wall rocks and the cement-amended tailings backfill. Models were developed to describe the geochemical evolution of both components. The water quality predicted for the underground source components was similar to that observed for the neutralised TSF percolate. However, the flow from the underground workings is significantly lower than from the TSF, and in comparison, the underground workings represent a minor source.
The overall assessment showed that:
- Hydrogeological impacts would be acceptable within the near vicinity of the mine workings, with predicted water level changes at environmental and third party boreholes to be less than 3m.
- In the long term (post closure), no significant impacts on baseline groundwater quality are predicted at the lease boundary.
*Hydrogeological modelling was undertaken by Sylvie Ogier-Halim.
