Päivi M. Kauppila & Marja L. Räisänen, Geological Survey of Finland, PO Box 1237, 70211 Kuopio, Finland; e-mail: paivi.kauppila(at)gtk.fi
The dry cover can be simple or complex, comprising a single soil layer or a layered structure of numerous soils, waste materials and/or synthetic materials. The requirements for the cover structure are specified according to the objectives set for rehabilitation. (MEND 2012, Kauppila et al. 2013)
Depending on the closure objective the dry cover shall:
- isolate the waste,
- provide a growth medium for vegatation,
- retain water for use by plants (growth medium for plants),
- conduct water well for collection of surface water (covered drains on surface section),
- reduce the quantity of water accessing the waste (impermeable layer)
- limit the travel of oxygen through the cover and into the waste material,
- cover structure that consumes oxygen (e.g. wetland),
- cover structure made from water retaining fines, which slows down the travel of oxygen downwards (paste cover),
- control erosion of waste material,
- increase the alkalinity of water seeping through the cover; the alkaline layer can also act as a chemical protective layer that weakens the interaction reactions of waste material and cover material, or
- prevent the capillary rising in saline waste water and consequent precipitation to top cover structures (coarse cover structure, lowest) (O’Kane & Wels 2003, Lottermoser 2007, INAP 2009, MEND 2012).
Typically, the main objective of the use of a cover system at mine sites is to prevent spreading of harmful effluents from reactive mine wastes into the receiving environment (O’Kane & Wels 2003, MEND 2012).
Table 1 summarizes the principles of selected dry cover methods, and the following pages present more detailed evaluations of the properties, performance and applications of various dry cover structures:
- Single-layer cover
- Multi-layer cover
Table 1. Exmples of the principles of various dry 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 stockpiling area is covered with mineral soil/till containing organic matter and a growth medium (layer thickness 0.5 – 1 metre), which enables vegetation of the area (grass + tree stand). This method is suitable for the landscaping of a non-acid-generating waste area (waste rock, tailings) and for waste areas where the long-term solubility of potentially harmful substances is minimal or manageable by conducting the water for treatment.
||Clean surface water is separated from possible deteriorated seepage from the waste area. Deteriorated water is conducted for treatment (infiltration field/constructed wetlands).
|Multi-layered cover including a layer that prevents the travel of oxygen and retains water
||The dry cover is made as a multi-layered cover used to reduce the travel of oxygen into the pile. This method is suitable for the rehabilitation of waste rock that is non-acid-generating or poorly acid-generating (including harmful metals/metalloids).
||Clean surface water is conducted separate from the drainage and exit seepage of the pile. Seepage is treated using either active or passive treatment methods.
|Multi-layered cover including a layer that slows down the travel of oxygen and increases the consumption of oxygen
||The cover is made as a multi-layered cover used to prevent the travel of oxygen into the pile. The cover has a layer containing organic matter that consumes oxygen, which prevents oxygen from accessing waste containing sulphides. The oxygen-consuming layer can be a basin-shaped wetland depression, where the decomposition reactions of plants consume oxygen. This method is suitable for the rehabilitation of acid-forming tailings (includes harmful metals/metalloids).
|Cover with carbonate-containing tailings/fine-grained waste that increases neutralisation and slows down the travel of oxygen (paste cover)
||The waste area is covered with carbonate-containing, fine-grained tailings, or rock or mineral powder (Mg and/Ca silicate ± carbonate), that retains rainwater and that has alkaline gravitational water1). Cover structure [paste cover ≥ (1.5–2) m] decelerates oxygen diffusion and enhances the neutralising capacity of the waste. Suitable for the rehabilitation of acid-forming tailings.
||Rain water is allowed to infiltrate into the pile. The water seeping from the pile is conducted for treatment; either active or passive treatment.
|Impermeable cover (includes synthetic materials)
||The cover structure includes a cover layer that prevents water from percolating (HDPE lining + /– bentonite lining). A synthetic cover structure requires an upper (sunlight protection, growth medium) and a lower protective layer (prevention of point loading, prevents interaction) and careful sealing. The waterproof layer prevents oxygen from accessing the pile. This cover structure is suitable for the rehabilitation of acid-forming tailings areas and mineral precipitate sludge ponds. The suitability for using bentonite lining requires that the structure will not dry (desiccation cracks) and no cation exchange reactions occur2).
||The clean surface water is directed away from the stockpiling area using channels; spreading of tree stand prevented. Deteriorated seepage from edge slopes and dams are conducted for treatment. Active and/or passive treatment.
1) Räisänen & Juntunen 2004, Räisänen 2005; 2) INAP 2009
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
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.
Lottermoser, B.G. 2007. Mine wastes – Characterization, Treatment, Environmental Impacts 2nd ed. Springer. 304 p.
MEND 2012. Cold Regions Cover Systems Design Technical Guidance Document. MEND Report 1.61.5c. http://mvlwb.com/sites/default/files/mvlwb/documents/Cold%20Regions%20Cover%20System%20Design%20Technical%20Guidance%20Document%20(MEND%20Report%201.16.5c).pdf
O’Kane, M. & Wels, C. 2003. Mine Waste Cover System Design – Linking Predicted Performance to Groundwater and Surface Water Impacts. Proceedings of 6 th International Conference for Acid Rock Drainage, Cairns, Qld., Australia, July 2003. http://www.okc-sk.com/wp-content/uploads/2012/02/MineWaste.pdf Accessed 28th November 2013.