Päivi M. Kauppila & M.L Räisänen, Geological Survey of Finland, P.O. Box 1237, FI-70211 Finland; e-mail: paivi.kauppila(at)gtk.fi
Subaqueous disposal of mining waste or covering mine wastes with an adequately thick layer of water is nowadays considered as one of the best remediation methods for reactive mine wastes to prevent or reduce the oxidation of iron sulphides and the subsequent leaching of harmful substances (Tremblay & Hogan 2001, EC 2009, INAP 2009, Eriksson et al. 2001). Efficiency of the water cover is based on the facts that the maximum concentration of dissolved oxygen in water is around 30 times less than in the atmosphere, and the transfer of oxygen trough water is almost 10,000 times slower than in the air, which reduce the rate of sulphide oxidation (Tremblay & Hogan 2001) In addition, the water covers also promote the sulphide reduction by bacteria and precipitation of metal hydroxides. Under water, sediment layers are developed on top of the waste material which further prevents the reactions of the waste and the overlying waters. (INAP 2009)
Water covers are best suited for climates with a positive water balance. Other requirements include long-term physical stability of the waste facilities and outlet structures. The thickness of the required water layer depends on the extent of the area to be covered, as well as the windiness in the area and the depth of the vertical blending of water caused by wind. It should be deep enough to prevent resuspension by wind and wave action. (INAP 2009)
Water covers typically have low maintenance requirements. Another benefit is that they simultaneously prevent dusting of the waste materials. (Höglund et al. 2004)
Table 1 presents a summary of the principles of different water cover technologies, and the following articles evaluate closure technologies exploiting the use of water in more detail. These technologies include e.g.
Table 1. Summary of the principles of the various water cover methods, their suitability for different waste types and requirements for water collection and treatment (EC 2009, INAP 2009, Kauppila et al. 2013).
|Rehabilitation method for waste areas
||Principle of the method and suitability for different waste types
||Collection and treatment of water
||The diffusion of oxygen in water is 30 times smaller than in air, which slows down the travel of oxygen into the waste. In shallow water cover (<2 metres) oxygen diffusion can be further prevented by covering the waste with fine-grained till1). This method is suitable for acid-forming tailings impoundments that have impermeable basal and dam structures or for backfilled acid-generating waste rock or the mixed backfilling of tailings with waste rock. In pit backfilling, the bedrock groundwater and surface water raining into the pit will form the water cover. Bedrock groundwater contamination can be mitigated by filling the gaps between rock walls and waste with fine-grained filter filling (rock powder/sulphide-free tailings). Pit backfilling is not suitable if oxidised groundwater accesses the pit via rock fractures.
||The water level of the waste impoundment with water cover must be adjusted and an overflow channel or discharge pipe must be constructed for the discharging of water; possible deteriorated water must be conducted for treatment; active or passive treatment
|Pit backfilling and water cover
|Wet cover, partial wet cover (wetland cover)
||The waste area is shaped in its centre to become basin-like for the collection of rain water and thaw water from snow. In the centre of the impoundment, the water level is close to the ground level and above. The water level of the waste area falls when moving closer to the edges. Suitable for poorly acid-generating, or used with paste cover for an acid-generating tailings impoundment that is located in a valley2). This method requires the reinforcement of edge slopes/dams in order to endure the annual fluctuations in water level and a waterproof or partially waterproof basal structure.
||Rain water is allowed to infiltrate into the pile. The water seeping from the pile is conducted for treatment; either active or passive treatment.
|Flooding of the waste area
||Surface water or groundwater is conducted to the waste area, as a result of which the water level will rise to the surface of the waste. This assists with the waste remaining saturated in water. Suitable for waste areas with acid-forming tailings, where the groundwater/surface water can be conducted in a controlled manner via the boundary areas and base to the waste area3).
||The water level of the flooded waste impoundment must be adjusted and an overflow channel must be constructed for the discharging of water; possible deteriorated water must be conducted for treatment; active or passive treatment
1) Ljungberg et al. 1997, Eriksson et al. 2001; 2) Räisänen 2005, Heikkinen et al. 2009; 3) Alakangas et al. 2006
Alakangas, L., Lundberg, A., Corrége, O. & Öhlander, B. 2006. Changes of groundwater quality in sulphide-bearing mine-tailings after remediation at Kristineberg, northern Sweden. Manuscript. In: L. Alakangas. Sulphide oxidation, oxygen diffusion and metal mobility in sulphide-bearing mine tailings in northern Sweden. Doctoral Thesis 2006:27. Luleå University of Technology, Department of Chemical Engineering and Geosciences, Division of Applied Geology. ISSN: 1402-1544 ISRN: LTUDt—06/27—SE.
EC 2009. Reference document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities. January 2009. European Commission. 557 p. http://eippcb.jrc.ec.europa.eu/reference/BREF/mmr_adopted_0109.pdf Accessed 28th November 2013
Eriksson, N., Lindvall, M. & Sandberg, M. 2001. A quantitative evaluation of effectiveness of the water cover at the Stekenjokk tailings pond in northern Sweden: Eight years of follow-up. Securing the Future, International Conference on Mining and the Environment, Skellefteå June 25 – July 1, 2001. Proceedings, Volume 1: 216-227.
Heikkinen, P.M., Räisänen, M.L. & Johnson, R.H. 2009. Geochemical characterization of seepage and drainage water quality from two sulphide mine tailings impoundments: Acid mine drainage vs. neutral mine drainage. Mine Water and the Environment 28, 30-49.
Höglund, L.O. (Ed.), Herbert, R. (Ed.), Lövgren, L, Öhlander, B., Neretnieks, I., Moreno, L., Malmström, M., Elander, P., Lindvall, M. & Lindström, B. 2004. MiMi – Performance assessment Main report. Mimi 2003:3. ISBN: 91-89350-27-8. 345 p.
INAP 2009. The GARDGuide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com/
Kauppila, P., Räisänen, M.L &, Myllyoja, S. (Eds) 2013. Best Environmental Practices in Metal Ore Mining. The Finnish Environment 29en/2011. Helsinki, Finnish Environment Institute. ISBN: 978-952-11-3942-0. 219 p.
Ljungberg, J., Lindvall, M., Holstrom, H. & Öhlander, B. 1997. Geochemical field study of flooded mine tailings in Stekenjokk, northern Sweden. Proceedings of 4th International Conference on Acid Rock drainage. Vancouver, B. C., Canada, 31 May – 6 June 1997: 1401-1417.
Räisänen, M.L. 2005. A sustainable use of magnesite tailings as a cover for acid generating wastes – A case study of an old copper mine. Julk. Securing the future – International Conference on Mining and the Environment Metals and Energy Recovery, June 27 – July 1 2005 Skellefteå. Proceedings, vol. 2: 826-636.
Trembley, G.A. & Hogan, C.M. 2001. MEND Manual, Volume 4. Prevention and Control. MEND Report 5.4.2d.