Element (Ag, Cd, Cu, Pb, Sb, Tl and Zn), element ratio and lead isotope profiles in a sediment affected by a mining operation episode during the late 19th century

Mining operations at Mårsätter in 1877–81 resulted in increased metal loading to a small lake, notably as sulphidic tailings. The event is taken as an opportunity to study the present environmental impact of a historical single major metal release. Lake water and four sediment cores were sampled and analysed for principal and trace elements in solid and aqueous phases as well as general hydrochemical conditions. Chronologies were determined from ²⁰⁶Pb/²⁰⁷Pb ratios and historical records. Ordinary sedimentation after the event has lead to that the tailings are found as a distinct layer at a depth of 18–22 cm in the sediment. The layer is characterized by elevated metal concentrations in the solid and pore water phases, respectively, circum neutral pH and sulphate concentrations below detection. Geochemical modelling indicated a preference for carbonate equilibrium in the waste while sulphides prevailed above it. It is concluded that the growth of the ordinary sediment on top of the waste has lead to a physicochemical barrier that seals of the waste from the overlying sediment. Chemical or physical rupture of the barrier will release the metals to downstream regions. According to the chronologies at least three sources have contributed to the present elevated levels of metals, in additions to the release of tailings. Copper from a historical blast furnace prior to the event at Mårsätter, transport from mineralized parts of the watershed and release of contaminated water from present mining operations maintain elevated levels of notably zinc, silver, cadmium and lead. At present less than 10% of the lead content at the sediment/water interface comes from atmospheric deposition. Increased levels of antimony and thallium were not observed prior to ca 1950.