Environmental impact of alum shale mining in Kvarntorp, Närke, Sweden

Due to scarcity of imported liquid fuel during World War II, alum shale was mined for oil production in the Kvarntorp area 1942-1966. The shale contains both organic matter (kerogen) and elevated concentrations of trace elements such as molybdenum, nickel, uranium and vanadium. Today there are several pit lakes in the area and a 100-meter-high waste deposit, Kvarntorpshögen, consisting mostly of crushed and burned shale but also some unburned crushed shale and lime waste. The aim of this study was to get a better understanding of the environmental impact of alum shale mining with focus on trace metal release. During the production era, the surroundings were highly affected by both sulphur rich flue gas emissions and bad water quality in downstream waters. The former mining activities show impact also today, with higher concentrations in downstream water than upstream the area. Analyses and leaching tests of solid samples have shown pyrite weathering in shale and unburned shale waste with release of for example nickel and uranium. Analyses of groundwater in eleven wells around the deposit show ongoing leaching of both shale waste generating a circumneutral leachate and unprocessed shale leading to acidic leachates. All pit lakes in contact with alum shale waste or the shale horizon show elevated sulphate concentrations indicating pyrite weathering, although only one is still acidic today. Also Norrtorpssjön was acidic until a pH rise due to adjacent dumping of alkaline waste. The pH increase was followed by a decrease in aluminium, cobalt, magnesium and nickel. Surface water analysis show that the waste deposit is estimated to contribute with less than a fifth of the mass transport whereas the western pit lakes contribute with the largest part