Effectiveness of reclamation by backfilling and sealing at Kimheden open-pit mine, northern Sweden

Abstract: Reclamation of mine sites is a very recent concept on the scale of mining history. It involves the prevention or mitigation of the environmental impacts from mining, and the establishment of sustainable post-mining uses of the land. Many methods have been tested to reduce the contamination from mining waste, but their actual performance depends highly on the nature of the site. In this thesis, the effectiveness of reclamation is investigated at Kimheden open-pit copper mine in Västerbotten, northern Sweden. Sulphide-containing waste rock left from the extraction of the ore was originally dumped on the surface close to the two open pits at Kimheden. The contact of sulphides with water and oxygen induced the generation of copper- and zinc-rich acid mine drainage. Therefore, reclamation of the mine involved backfilling of the mine waste into the open pits and covering with a till composite dry cover including a sealing layer, in order to reduce the oxygen ingress to the waste. Geochemical and geophysical studies conducted in 2009-2010, complemented by monitoring data from the mining company, were interpreted to assess the success of the reclamation. Long-term annual monitoring of the contaminated drainage by the mining company showed that concentrations of copper and zinc added together have decreased by more than 87% since the completion of reclamation in 1996. However, the decrease occurred rapidly, and metal concentrations in the last ten years have remained stable at values that are still not satisfactory for discharge into the environment. Furthermore, pH in the drainage increased only slightly after reclamation. In 2009, seepage water from one of the pits exhibited a pH of 3.0, and copper, zinc and aluminium concentrations of 3 mg/L, 0.3 mg/L and 20 mg/L, respectively. The partial success of the reclamation in mitigating the acid mine drainage may be explained by persistent oxidation of the backfilled mine waste in spite of the application of the dry cover. This would imply that oxygen gas or oxygenated water is still accessing the waste rock, either through the dry cover, or through fractures in the pit walls. These hypotheses were investigated in the field with two different geophysical methods, ground penetrating radar and direct current resistivity. With ground penetrating radar, good-quality image of the sealing layer could be obtained at one of the two backfilled pits, and no evident sign of disturbance in the layer could be found. Resistivity surveys succeeded to image the drainage from the backfill, and suggested that it was seeping out through the dry cover, with a risk of erosion of the sealing layer. Water discharge measurements and resistivity data indicated that the current channelling of the mine drainage to a treatment pond is inadequate, since a large proportion of the contaminated water disappears as groundwater. The interpretation of the resistivity profiles supported by archive data suggested that fractures in the bedrock close to the pits could explain the ingress of oxygen gas or oxygenatedwater through the pit walls, and would be the primary reason for on-going sulphide oxidation.The results of these studies may help to improve the reclamation of the mine, and in a broader perspective, provide information for reclamation at other sites.