Reclamation of acid-generating waste rock by in-pit backfilling and sealing An evaluation of the Kimheden mine site, northern Sweden

Abstract: The notion of mine site reclamation is very recent compared to the history of mining. In the past, mine sites were left as is following termination of the operations, but modern regulations require anticipation of long-term contamination of the surrounding environment and return of the land to a sustainable use. A primary concern for mines used to extract sulphide-rich ores is the generation of acid mine drainage (AMD), a low-pH and metal-rich solution formed when sulphidic mining wastes and mining surfaces come into contact with oxygen and water. AMD may be responsible for the contamination of watercourses and other receiving environments far downstream of the site. Thus, reclamation at such sites will usually involve measures intended to prevent or mitigate its generation. Despite their relatively recent introduction, the increasing time that has passed since the first prevention and mitigation programmes were applied at mine sites, some two to three decades ago, provides an invaluable opportunityto assess their long-term effects on the abatement of AMD. Reclamation at the Kimheden copper mine in Västerbotten, northern Sweden, involved the progressive backfilling of two small open pits with waste rock and application of a dry cover in 1996, in order to reduce the influx of atmospheric oxygen into the waste. The objective of the studies this thesis is based upon, performed in 2009 – 2014, was to evaluate the effects of these reclamation measures on the abatement of Cu and Zn-rich AMD and to identify potential inadequacies in them, using geochemical, geophysical and hydrogeological methods. The results show that despite large reductions in Cu and Zn concentrations in the receiving stream following reclamation, its water quality has remained in a steady state for about a decade and is still not considered satisfactory for discharge into the natural environment. Steady-state moderately high concentrations in the stream, together with a relatively short turnover time for water in contact with the waste rock, indicate that sulphide oxidation is continuing in the backfill despite the cover. Hydrogeochemical modelling suggests that the rate of oxygen consumption by sulphide oxidation in the waste rock is higher than the expected rate of diffusion through the dry cover. Substantial ingress of oxygen into the waste was also corroborated by stable isotope measurements and direct measurements of dissolved oxygen concentrations in the groundwater of the backfill. According to sulphate isotope distributions in the mine drainage, oxidation of pyrite by Fe(III) constantly rejuvenated by oxidation of Fe(II) with oxygen is suspected to be an important process in the covered backfill. Potential sources of oxygen transport into the waste have been explored. Mapping of the groundwater table in one of the backfilled open pits showed that up to 40 % of the waste rock is unsaturated during baseflow, providing pathways for oxygen to enter through unsaturated fractures in the pit walls. Two sources of deterioration of the dry cover were identified during geophysical surveys: seepage of drainage water from the backfilled waste upwards into the dry cover and insufficient thickness of the cover in some areas. Geophysical and hydrogeochemical studies showed that the collection ditch constructed to divert the contaminated water to a liming treatment station fails to retain a large fraction of the drainage. Based on the results obtained, the sustainability of current approaches for AMD prevention and mitigation is discussed, and possible strategies for improving backfilling and sealing measures at similar sites are proposed. Keywords Acid mine drainage or acid and metalliferous drainage (AMD) or acid rock drainage (ARD); mine site reclamation; open pit; sulphidic waste rock; backfilling; dry cover; performance; unsaturated.