Dispersion of tailings in the Knabena—Kvina drainage basin, Norway, 2: Mobility of Cu and Mo in tailings-derived fluvial sediments

Abstract

Downstream of the Knaben Molybdenum Mines, tailings have been identified in sandbars and on top of the floodplains throughout the Knabena—Kvina drainage basin. The tailings and tailings-derived fluvial sediments are enriched relative to the local, pre-industrial overbank sediments by factors of 8–53 for Cu and 22–57 for Mo. Fourteen samples from the tailings pond, sandbars, and overbank sediments were analysed for Cu and Mo after a sequential extraction scheme that sought operationally to define the “bioavailable”, the “adsorbed/exchangeable”, the “oxidisable”, the “reducible”, and the “residual” fractions of the metals. In the tailings pond, most Cu was contained in the oxidisable fraction (68–78%), probably as primary chalcopyrite. Extensive oxidation may have been prevented by acid-consuming substances, buffering the system. A rapid release is expected if the buffer capacity is exceeded. In the fluvial sediments, approximately 30% of the Cu is easily soluble (bioavailable and adsorbed/exchangeable fractions), while the oxidisable fraction is reduced to 16–62%. The easily soluble Cu probably contributes to the presently high Cu concentrations in river water, while oxidation of the oxidisable fraction may contribute to the easily dissolved Cu fractions and also to Cu in river water on a longer time scale. Molybdenum in the tailings pond was mainly present in the reducible and residual fractions. The reducible fraction is probably made up of molybdate adhered to Fe and Mn oxidates or ferrimolybdite, while Mo-bearing micas and amphiboles may constitute the residual fraction. Upon fluvial transport, the residual fraction rapidly disappeared with distance from the tailings dam, suggesting that micas and amphiboles are easily disintegrated. In the downstream end of the drainage basin, the oxidisable Mo fraction prevails over the reducible, probably because liming and phosphate fertilising have ion exchanged molybdate from the oxidates. Subsequently, the dissolved molybdate may have been taken up by living plants or been adsorbed to organic matter, giving the reducible fraction. Care should be taken when liming and phosphate fertilizing these fields, although the risk of acute Mo poisoning of grazing animals is probably low since grass from the floodplains only constitutes a part of their diet.