Backfilling in water management

Samrit Luoma1 & Kaisa Turunen2, 1Geological Survey of Finland, P.O. 96, FI-02151 Espoo, FINLAND, Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, FINLAND, e-mail: samrit.luoma(at)gtk.fi, kaisa.turunen(at)gtk.fi

Backfilling of open voids

Not all mine pits are kept open, or are flooded. Alternative uses might include soil reconstruction, forestry, recreation, agriculture, or wildlife habitats (Kennedy 2002, Wolkersdorfer 2008). In some cases the best way to reduce the potential of water contamination after mine closure is backfilling of open voids and shafts. In the respect of water management, the backfilling is usually carried out to protect the aquifers from contamination through reducing the inflow and outflow of groundwater and surface waters in underground mines. The mine backfilling is typically carried out by mixing a suitable particulate solid material, e.g. mine waste material or combination of material (waste + additives) which is then used to fill void openings created by mining. Nowadays backfilling is carried out gradually during mining operations, but it is also a valid method when remediating old mine sites to reduce the local environmental impact. Backfilling material can be waste rock, tailings or water treatment sludge, and to increase strength it may include cement or other modifiers. After backfilling groundwater will eventually rebound until it reaches its equilibrium of the pre-mining groundwater table. Within a backfilled open pit, several zones can be identified in terms of oxygen abundance. The saturated anoxic zone is located below the groundwater table. Wastes with very high acid-generating capacities may require addition of neutralizing agents prior to or during backfilling (Lottermoser 2007, Masniyom 2009).

Any backfill material placed below the water table will become part of the aquifer (Lewis-Russ 1997, Siegel 1997) and water-rock reactions may lead to the mobilization of contaminants into groundwater. The impact depends on the mineralogical and geochemical characteristics of the mine wastes used in the backfill and their permeability as well as on the composition and reactivity of the wall rocks, fractures and the contact time (INAP 2009).

During the mine closure the backfilling is made to ensure ground stability and to decrease environmental impact while reducing oxidation of the waste. However backfilling may also bear several problems, for example, although the backfilling reduces the contact surface of wastes with the atmosphere and waters, the chemical and mineralogical changes within the processed ore and waste rocks may still continue in the open pit. This is due to e.g. the very fine grain size of the reactive ore particles and the pore waters with reactive process chemicals and other dissolved elements and compounds in the tailings (Lottermoser 2007). Moreover, if water is flowing through reactive, permeable and soluble materials, the increased surface area of the waste coupled with its reactivity will result in groundwater enriched in various components. For example, once the oxidized sulphidic waste is disposed into an open pit or a flooded pit, soluble secondary minerals may dissolve in the pore water. This may lead to the release of metals, metalloids, and salts to the groundwater or to the overlying water column. Thus, the disposed sulphidic tailings in open pits or underground workings should be kept below the post-mining water table to preclude any access of atmospheric oxygen to sulphidic tailings. (Lewis-Russ 1997, Siegel 1997). Groundwater outflow from backfilled cavities may require treatment before discharge to the environment. The monitoring should be continued at least until the oxygen and water levels are balanced. More detailed description of backfilling in underground mine and open pits are presented in Closedure Wastes and waste facilities page:

References

INAP 2009. The GARDGuide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP).http://www.gardguide.com/ modified 2014. Accessed 17.7.2014.

Kennedy, C. 2002. Alternatives for the Reclamation of Surface Mined Lands, In: Mudroch, A., Stottmeister, U., Kennedy, C. & Klapper, H.: Remediation of Abandoned Surface Coal Mining Sites. Heidelberg, Springer, 37–56.

Lewis-Russ, A. 1997. Ground water quality. In: Marcus, J. J., (ed.): Mining environmental handbook: effects of mining on the environment and American environmental controls on mining. Imperial College Press, London, 162–165.

Lottermoser, B.G. 2007. Mine wastes – Characterization, Treatment, Environmental Impacts 2nd ed. Springer. 304 p.

Masniyom, M. 2009. Systematic Selection and Application of Backfill in Underground Mines. Der Fakultät für Geowissenschaften, Geotechnik und Bergbau der Technischen Universität Bergakademie Freiberg genehmigte. Dissertation zur Erlangung des akademischen Grades Doctor-Ingenieur. 168 p.

Siegel, J. 1997. Ground water quantity. In: Marcus, J. J., (ed.) Mining environmental handbook: effects of mining on the environment and American environmental controls on mining. Imperial College Press, London, 165–168.

Wolkersdorfer, C. 2008. Water Management at Abandoned Flooded Underground Mines. Fundamentals, Tracer Tests, Modelling, Water Treatment. Springer. 465 p.