Metal concentrations and concomitant metal mobility in unsaturated mine and mill wastes

The shallow sediments in the Smelterville Flats portion of the Coeur d'Alene River Valley in north Idaho consist of a heterogeneous mixture of mine wastes and alluvium reworked during flood events. The wastes cover an area of approximately eight square kilometres. Atomic Absorption Spectroscopy and Inductively-Coupled Plasma analyses on mine waste-sediment samples collected to the depth of the water table show maximum metal concentrations of about 20% Fe, 12% Pb, 10% Zn, 3% Mn, 2.8% Al, 0.13% Cu, 0.1% Cd, 370 ppm As, 170 ppm Ag, 150 ppm Sb, and 70 ppm Ni. The positive correlation between Fe, Ca, and Mg concentrations and sample pH suggests that the carbonate gangue minerals (siderite, ferrodolomite, and ankerite) control the pH. Other metals that exhibit a positive correlation with sample pH also may be present in the carbonate minerals. Identification of factors that govern metal mobility suggests that: (a) the pH of the water in the uppermost aquifer plays a minor role in controlling the solubility of most metals in the solid waste samples; b) the carbonate gangue minerals buffer the ground water pH and hence the solubility of metal sulfides; c) solubility of the carbonate gangue minerals decreases under anaerobic conditions, thereby allowing the ground water pH to decrease; d) solubility of Pb, Zn, and Cd correlates directly to anaerobic conditions as opposed to aerobic conditions.