DISPERSION OF TAILINGS IN THE KNABEANA-KVINA DRAINAGE-BASIN, NORWAY .2. MOBILITY OF CU AND MO IN TAILINGS-DERIVED FLUVIAL SEDIMENTS

Downstream of the Knaben Molybdenum Mines, tailings have been identifi ed in sandbars and on top of the floodplains throughout the Knabeana-K vina drainage basin. The tailings and tailings-derived fluvial sedimen ts are enriched relative to the local, pre-industrial overbank sedimen ts by factors of 8-53 for Cu and 22-57 for Mo. Fourteen samples from t he tailings pond, sandbars, and overbank sediments were analysed for C u and Mo after a sequential extraction scheme that sought operationall y to define the ''bioavailable'', the ''adsorbed/exchangeable'', the ' 'oxidisable'', the ''reducible'', and the ''residual'' fractions of th e metals. In the tailings pond, most Cu was contained in the oxidisabl e fraction (68-78%), probably as primary chalcopyrite. Extensive oxida tion may have been prevented by acid-consuming substances, buffering t he system. A rapid release is expected if the buffer capacity is excee ded. In the fluvial sediments, approximately 30% of the Cu is easily s oluble (bioavailable and adsorbed/exchangeable fractions), while the o xidisable fraction is reduced to 16-62%. The easily soluble Cu probabl y contributes to the presently high Cu concentrations in river water, while oxidation of the oxidisable fraction may contribute to the easil y dissolved Cu fractions and also to Cu in river water on a longer tim e scale. Molybdenum in the tailings pond was mainly present in the red ucible and residual fractions. The reducible fraction is probably made up of molybdate adhered to Fe and Mn oxidates or ferrimolybdite, whil e Mo-bearing micas and amphiboles may constitute the residual fraction . Upon fluvial transport, the residual fraction rapidly disappeared wi th distance from the tailings dam, suggesting that micas and amphibole s are easily disintegrated. In the downstream end of the drainage basi n, the oxidisable Mo fraction prevails over the reducible, probably be cause liming and phosphate fertilising have ion exchanged molybdate fr om the oxidates. Subsequently, the dissolved molybdate may have been t aken up by living plants or been adsorbed to organic matter, giving th e reducible fraction. Care should be taken when liming and phosphate f ertilizing these fields, although the risk of acute Mo poisoning of gr azing animals is probably low since grass from the floodplains only co nstitutes a part of their diet.