Metals released from mine waste deposits : Redistribution and fluxes through geological barriers

Abstract: The redistribution of metals from point sources (sulphidic mine waste) is presented in this thesis. The impact on surface water systems is determined from water quality and sediment data. It is generally found that lakes act as sinks for the metals, to a different extent for eachmetal. Other (diffuse) sources give increasing contributions to the metal concentrations in the surface waters downstream the point source. Accumulation is attributed to adsorption of metals on suspended solids and subsequent sedimentation. Differences in attenuation of metals in the aqueous phase (cadmium, copper, lead and zinc) are interpreted as the result of differences in sorption, determined by the speciation of metals in solution as well as surface properties of the solid phases. The mobilization of elements from the sediment/water interphase is discussed as well as the importance of humic and fulvic acids in these processes.The chemical composition of the ground water is discussed. It is found, that weathering of sulphides is possible even in deep groundwaters. Metal concentrations could be determined by secondary solubility limiting phases, e.g. metal carbonates.A natural clay barrier, exposed to metal and acid releases from overlying tailings, is studied. The migrations of metals is inversely related to pH. Clay and similar materials are pt;oposed as sealing layers in waste deposits to prevent continued weathering ami metal releases.Weathering rate and metal release is evaluated ~n two engineered mine waste deposits (Bersbo and Ranstad). In Bersbo only measurements in the surface water run-off are available. The release of zinc, cadmium and copper is generally of the same magnitude as before remedial actions. The increase in the release of (ferrous) iron could be an indication that ferric iron serves as an oxidizing agent instead of oxygen for some of the sulphidic mine waste. Oxidation to the ferric state and subsequent hydrolysis and acidification could affect the mobility of metals from the secondary sources in e.g. lake sediments.In Ranstad, weathering of waste material and amounts and quality of the percolating water was measured. The weathering rate is reduced by about 3 orders of magnitude, and the release of metals further retarded due to the high pH in the pore water.