An alternative: Deep-sea tailings placement
Tracy Shimmield | Australian Mining
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.
International Protocol and National Policy Development
Australia regulates the disposal of waste at sea under the Environment Protection Act 1981 (the Sea Dumping Act) by:
- Prohibiting ocean disposal of waste considered too harmful to be released in the marine environment, and
- Regulating permitted waste disposal to ensure environmental impacts are minimised.
The Sea Dumping Act also fulfils Australia’s international obligations under the London Convention and Protocol: The Convention on the Prevention of Marine Pollution by Dumping of Waste and Other Matter, 1972 (London Convention) and its updated version, the 1996 Protocol to the Convention on the Prevention of Marine Pollution by Dumping of Waste and Other Matter, (London Protocol) are the primary international instruments to protect the world’s oceans from pollution. There are currently 42 parties to the London Protocol. This includes Australia, New Zealand and in 2012 the Philippines, but not yet Indonesia and Papua New Guinea (PNG is a signatory of the London Convention however).
Unlike the London Convention, which lists materials that may not be dumped, the London Protocol prohibits all dumping, except for certain wastes named on a “reverse list”.
The Protocol does allow for marine disposal of ‘inert, inorganic geological material,’ but this does not include mine waste, as tailings have not been shown to be ‘inert’.
Technically however, neither the London Convention nor Protocol deals with discharges from land, only with ‘dumping’ at sea.
For this reason, government authorities must themselves evaluate tailings management alternatives, setting-out the terms of any permits to discharge tailings into the marine environment. Since environmental legislation, regulations, and permitting processes vary from country to country, this means that different decision-making processes, reviewing different levels of scientific evidence exist across the permitting process worldwide.
SRSL has undertaken a number of environmental impact studies of DSTP, which include Misima (now closed) and Lihir (operational) mines in Papua New Guinea (2007-2010), as well as several environmental baseline surveys of Basamuk (2008-2012), the site of the now operational Ramu Nickel mine processing plant.
The aim of these projects has been to investigate the effects of DSTP in a bid to sustain PNG’s economic performance through mineral production and exports, alleviate poverty, increase employment opportunities and mitigate mine-induced environmental impacts.
In so doing, SRSL was funded by the European Commission (8th European Development Fund, 2007-2010) to produce ‘best-practice’ guidelines for DSTP on behalf of the Department of Environment and Conservation and the Mineral Resource Authority in PNG. The general guidelines that were produced by SRSL in 2010 have since been accepted by the PNG government and are presently being included as regulation within PNG’s legislation. The International Marine Organisation (IMO) and the Scientific Group of the London Protocol have also ‘acknowledged’ the guidelines.
Environmental Impacts and Site-Specific Guidelines
The potential environmental impacts of DSTP are irrefutably significant but at the same time extremely site-dependent; the result of complex and interacting biogeochemical, ecological, topographical and oceanographic conditions. Under some of these conditions, DSTP may be the waste management option with the least impact out of several alternative tailings placement strategies available. In other situations DSTP would be environmentally irresponsible. For example, DSTP operations in areas experiencing oceanographic upwelling have the potential to impact shallow coastal waters, reefs and fisheries.
As with land-based tailings storage, the principal environmental impact of deep-sea tailings discharge is the alteration of the physical environment at the location where the tailings are deposited (smothering organisms residing within the trajectory of the tailings density plume and inhabiting the final deposition area). In the deep sea, secondary effects relate to the toxicity of metals and process chemicals for deep-sea organisms, and the progressive concentration of these toxins up food-chain. For this reason, chemical and biological characterisation of sample mine tailings and their potential impacts on water, sediment quality, biological resources and ecosystems are fundamental aspects of the environmental impact assessment process for DSTP.
Independent scientific studies of current DSTP practices play an important role in supporting regulatory authorities in the production of site-specific guidelines, to be imposed upon DSTP operations at the time of permitting. In recent years, SRSL has authored independent site-specific guidelines relating to a number of mines. These guidelines have enabled regulatory bodies to tailor their permitting processes to the unique character of a proposed site and its specific marine environment, through stipulating the most relevant Environmental Monitoring Program (EMP) requirements.
As DSTP continues to be practiced, a growing body of scientific knowledge on the physical and biogeochemical effects of tailings in the marine environment is emerging. Several long-term monitoring studies, including those undertaken by SRSL, reveal recolonisation of mine tailings deposits on the ocean floor in relatively short timescales after mine closure (one to ten years, depending upon local conditions). Full ‘recovery’ however, has not been demonstrated, even after 10 years. This is largely due to different species and community structures in re-colonised areas compared to un-impacted sites.
Marine scientists at SRSL continue to analyse the substantial data set resulting from surveys conducted in PNG and are set to publish new findings later this year addressing long-term environmental impacts of DSTP; information which is expected to be of significant interest to the mining industry.
The fundamental objective of any waste management strategy should be the safe, stable, and economical storage of tailings, while presenting negligible public health, safety and social impacts, and minimal environmental damage.
The case for DSTP as a disposal option can only be justified following full analysis and risk-assessment of all disposal options available. Nevertheless, greater awareness of potential environmental impacts and better informed site-selection by mining companies can help reduce risk. Combining this best practice approach with implementation of site-specific guidelines by regulatory authorities should drive improvements in DSTP standards worldwide.