Deep-sea mine tailings placement (DSTP) is an alternative to land-based mine-waste disposal, whereby mineral ore is converted into slurry and transported via a pipeline to processing plants located at the coast, with the resultant waste being discharged into very deep water offshore. Specific topographical and hydrodynamic conditions must exist if the mine tailings are to sink to the seabed and remain there.
Although not always appropriate, when compared to the capital and operational costs of on-land impoundments, this type of tailings disposal can be very economical. DSTP is therefore gaining favour in the light of catastrophic dam failures and in the face of land-availability, land-use value and land-ownership disputes, which are prevalent in some countries.
The DSTP method
Submarine tailings disposal (STD) has been utilised at over 13 coastal mining sites around the world to date (some have now ceased operations). However, most of these have involved disposal into shallow or coastal waters, resulting in severe environmental damage and tarnishing the reputation of STD as environmentally viable waste management option. ‘Deep’ STP should be distinguished by the discharge of tailings slurry into deeper waters, - well below the mixed layer and the reach of sunlight in the water column (the so-called ‘euphotic zone’), with tailings settling below a depth of 1000m or more.
Mines that have, or are still utilising DSTP include Island Copper and Kitsault Mines in Canada, Black Angel in Greenland, Cayeli Bakir in Turkey, Batu Hijau in Indonesia, and Misima, Lihir and Ramu mines in Papua New Guinea.
If DSTP methodology is engineered correctly, tailings slurry should form a turbidity current which flows coherently, with minimal dispersion, until it reaches the edge of a steepening, or more ideally an underwater ‘drop-off’. From here, the mixture continues in a gravity-assisted descent along the seafloor for as long as it remains denser than the surrounding water. The slope of the seabed must be steep enough to maintain the flow of tailings down the slope, allowing the tailings to move to deeper areas rather than accumulating at the outfall site. As the tailings slurry descends, it becomes diluted and dissipates with increasing distance from the pipeline due to entrainment of seawater and frictional losses.
For DSTP to be successful there should be very little or no risk at the deposition site of hazardous amounts of tailings ‘upwelling’ back into shallow waters, where toxic components may enter the food chain. For this reason, feasibility studies and site selection require a detailed knowledge of both the seafloor topography and the regional hydrography. The need for robust environmental baselines to be conducted early, as part of feasibility studies is of paramount importance to both risk-assessment and site-selection exercises, which must be conducted by mining companies as part of the Environmental Impact Assessment process.
Best practice for DSTP
Best practice in the application of DSTP centers on appropriate site-selection for tailings discharge. Consideration of the following environmental attributes can help to de-risk the process of deep-sea tailings placement for mines where DSTP is a viable option:
- Accessibility to the coast: tailings can be piped overland providing the topography is suitable (in some instances tailings are piped up to 150km).
- Suitable bathymetry and physical oceanography: steep-sided submarine slopes, canyons, or naturally-excised deep-water channels near to the coast.
- The pipeline discharge depth: should be greater than the maximum depth of the surface mixed layer, euphotic zone, and the upwelling zone to maximise stable deposition on the seafloor.
- Absence of upwelling or seasonal overturning: to prevent tailings re-suspension into surface waters.
- Siting in a low energy environment: to reduce the likelihood of pipe breaks and reduce the formation of subsurface tailings plumes and re-suspension of deposited tailings.
- Deep water receiving environment: should be a soft bottom depositional area.
- Low productivity environment: to reduce the potential impact on marine resources, such as fisheries/shellfish.
Given the above criteria, suitable sites for DSTP exist principally on oceanic islands and archipelagos where very deep water occurs close to shore, such as mine sites in Indonesia, the Philippines and Papua New Guinea. However, suitable sites also exist off several mainland coastlines worldwide, including Australia.