Hydrogeological Analysis of Monolayer Covers for the Reclamation of Acid Generating Tailings - Column Tests Results and Interpretation.

Mine tailings, produced during ore treatment, are usually stored in surface disposal sites. Tailings containing iron sulfides may generate acid mine drainage (AMD) when put in contact with oxygen and water. The resulting acidic effluent often contains high concentrations of dissolved metallic ions and sulfates and may lead to very serious environmental issues. Considering that both oxygen and water are required for the oxidation reaction to occur, reclamation methods mainly tend to restrain the access of one of these components to the sulfide minerals.

This study focuses on two old acid generating tailings sites, located in Quebec (Canada). Both are partially oxidized so the pore water is already acidic. Monolayer covers, made of till or non acid-generating tailings, are being used to control DMA generation on these two sites. The goal of this investigation is to assess the behaviour of the tailings-cover systems, to determine the efficiency of the covers, and, if needed, to provide alternative reclamation methods.

To this end, samples were collected in situ (oxidized and non-oxidized tailings, and cover materials) and characterized in the laboratory. Large columns (230 centimeters tall, 15 centimeters in diameter) have been set up to study the hydrogeological and geochemical behaviour of the tailings and their covers, considering various depths from the water table. The column configurations were selected to reproduce some of the existing site conditions and to provide representative results for long term analyses. Monthly wetting and drying cycles were repeated to simulate climatic conditions. TDR probes, tensiometers and oxygen sensors were used to monitor, respectively, the water content, suction and oxygen concentration in the columns during the drainage cycles. The effluent was also regularly sampled for chemical analysis.

The main laboratory results, and in situ measurements, will be introduced in the presentation. The data were used to validate numerical models constructed with Vadose/W (GeoSlope Inc). Sensitivity analyses were conducted to evaluate the impact of various factors such as water-table depth, cover thickness and material properties. Alternative reclamation methods were also investigated, including covers with capillary barrier effects. The results will be presented and discussed.