English

Abstract

To assess the mobility of metals and consider the geochemical consequences of remediation for exposed Ni-Cu tailings at INCO Ltd., Thompson, Manitoba, Canada, the tailings were described visually, characterized mineralogically, and subjected to sequential extraction and hydraulic conductivity tests. Exposure of the tailings has resulted in the release of metals from sulfides, the acidification of surface and shallow ground waters within the tailings body, and the formation of cemented hardpan layers. Sequential extraction from the solid tailings showed most of the Cu and Zn to be adsorbed by secondary Fe- minerals, while ~15% of the total Ni concentration occurs as water-soluble sulfates in the vadose zone. A rise in water level would result in the dissolution of soluble minerals and the remobilization of metals from the exposed tailings. The formation and thickness of the cemented hardpan layers on the surface depend on the elevation of the water table and the degree of capillary action as this controls the moisture content of the pore space and hence sulfide oxidation. According to geotechnical measurements, the effect of cementation on reducing water flow through the hardpans to the interior of the tailings is insignificant, but the hardpan could act as a diffusive barrier for oxygen. In the hardpans, from 30 to 50% of sulfides are still not oxidized, and are, therefore, able to generate acid. The high acidity of the water at the surface of the exposed tailings prevents metals from being effectively co-precipitated by the secondary Fe-minerals, jarosite and schwertmannite. Thus, capping exposed tailings with acid neutralizing material is recommended.