THE LONG-TERM FATE AND TRANSPORT OF ARSENIC IN A DIFFUSION-DOMINATED IN-PIT MINE TAILINGS FACILITY

One of the most important environmental issues facing the mining industry is the long-term migration of arsenic from its mine wastes to the underlying groundwater system. Thus, decommissioning of these mine wastes requires that the long-term impact for arsenic on surface waters and groundwaters be understood. Arsenic-rich uranium mine tailings from the Rabbit Lake in-pit tailings management facility (RLITMF) in northern Saskatchewan, Canada were investigated to determine the long-term impact of arsenic on the regional groundwater regime. The tailings in the RLITMF are 425 m long x 300 m wide x 100 m depth at its center and were deposited in layers between 1985 at its base and 2004 at its top. Solute transport in these fine-grained tailings is dominated by diffusion. Contained within the layers is approximately 23,000 tonnes of arsenic. Because the layers of tailings have varying chemical characteristics (controlled by the ore being milled at the time), the total arsenic concentrations in the layers and their associated pore fluids range from 56 to 9,871 ƒÝg/g and 0.24 to 140 mg/l, respectively. Associated studies show that the arsenic in the tailings is strongly attenuated by adsorption to 2-line ferrihydrite through inner sphere bidentate linkages. Single reservoir diffusion cell testing shows that the effective diffusion coefficient for arsenic in the tailings is 4.5 x 10-6 cm2/s and that the Kd is 5 cm2/g. Based on the results obtained from field and laboratory-based studies the diffusion and attenuation of the arsenic in the RLITMF was modelled using a one-dimensional geochemical reactive transport model and a three-dimensional transport model. These results will allow us to determine the long-term migration of arsenic both within this man-made aquitard system and its impact on the regional groundwater over the next millenia.