Abstract
At Kimheden, a small copper mine in northern Sweden, reclamation of the two open pits was investigated using ground penetrating radar and geoelectrical multiple-gradient array measurements. The pits had been backfilled with waste rock, with a dry cover being applied on top in 1996 in order to reduce the influx of oxygen to the sulphidic mine waste and the subsequent production of acid mine drainage. The dry cover consists of a sealing layer of clayey till and a protective layer of unsorted till. As geochemical sampling in the drainage from the pits had previously revealed the continued release of contaminating oxidation products, the purpose of the geophysical survey undertaken in 2010 was to identify deficiencies in the cover or other pathways for oxygen to reach the waste rock. The radar images did not reveal any damage in the sealing layer but risks of deterioration of the cover in the long term were identified with both the radar and geoelectrical data. The radar localised regions of thinner protective layer where the sealing layer could be exposed to frost action. The geoelectrical measurements indicated the existence of seepage through the dry cover that presented a risk of erosion of the sealing layer. 2-D inversion of geoelectrical data also imaged some pathways of groundwater around the main pit. The results from the geophysical investigations were used together with other site data in order to show that both deficiencies in the cover and superficial fractures in the pit walls may explain an ongoing influx of oxygen to the mine waste.
Highlights
Solutions to control contaminated drainage from mines have been the subject of intense discussions over the last few decades
The pits had been backfilled with waste rock, with a dry cover being applied on top in 1996 in order to reduce the influx of oxygen to the sulphidic mine waste and the subsequent production of acid mine drainage
The model obtained at backfilled open pit 2 is supported by the thickness of the protective layer observed at the four locations where sample pits were excavated (Fig. 4b)
Summary
Solutions to control contaminated drainage from mines have been the subject of intense discussions over the last few decades. Prevention and mitigation methods at the source have been promoted as an economic and practical alternative to treatment of the polluted water. In this approach, efforts are concentrated on limiting the reactions that generate contaminants and the subsequent leaching and transport of the reaction products (INAP 2009), rather than treating the contaminants in the drainage. Prevention and mitigation, in this case, generally includes the reduction of water and/or oxygen contact with the sulphidic mining residues. Different measures may be applied, including diversion of surface water from reactive areas or other types of water management, conditioning of the tailings through e.g. compaction or desulphurisation, disposal of the waste under water or various types of soil covers to limit oxygen ingress (INAP 2009)
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