Hydrogeological Studies of the Reclamation Measures at the Kimheden Open-Pit Mine, Northern Sweden

Abstract: Open pit copper mining was performed at Kimheden four decades ago. The waste rock was stored in heaps next to the two open pits, which lead to the formation of acid mine drainage. Therefore, the waste rock was gradually backfilled to the open pits and allowed to water saturate to the natural water level. A ditch system was constructed to reduce the water inflows into the pit and to collect mine water for treatment. Additionally a dry cover was applied after the final backfilling. Mine water has been monitored since 1983, but does not show satisfactory low metal concentrations for a release into the environment. This study focuses on the hydrogeology of the pit, which generates the larger contaminant load and also on the collection ditch, with particular attention on water flow paths. Methods applied for this study were water level measurements, slug tests, mapping of the electrical conductivity in seepages, site water balance, and water quality analysis. Compared to the surrounding bedrock and the overlying natural till the hydraulic conductivity is quite high within the pit. It ranges over two orders of magnitude and is inhomogenously distributed. The data suggests two major sources of inflows from fractures in the bedrock into the pit. The largest volumes of mine water appear to flow out from the pit at its southern end and from a protrusion close to its center. The latter outflows additionally seem to contain the highest contaminant concentrations. The indicated outflow locations match those that were simulated by a digital flow model. The investigation of the collection ditch has shown that there is a high probability of vertical exchange fluxes between the natural till and the underlying bedrock, and that downward fluxes seem to dominate over upward ones, suggesting the infiltration of surface water into the ground. According to the water balance, large volumes of mine water are flowing underneath the shallow collection ditch. Seasonal variations in the water level of the pit cause different water saturation degrees of the waste rock and a wash-out effect during spring flood. The dry cover creates confined conditions in the southern part of the pit, which extend to artesian conditions during spring flood. This might lead to a disturbance of the cover’s integrity by up-pressing water, but could also prevent a possible overflowing of the pit during high water level conditions. Some suggestions for future reclamation measures at the site are presented within this work.