Reactive transport modelling of the hydro-geochemical behaviour of partially oxidized acid-generating mine tailings with a monolayer cover

The efficiency of a monolayer cover to prevent acid mine drainage (AMD) generation from two pre-oxidized tailings impoundments was assessed using the MIN3P code, a finite volume model for coupled groundwater flow, oxygen diffusion and multi-component reactive transport. Numerical simulations were validated using large column tests set up in the laboratory and monitored during 19 wetting and drying cycles, over approximately two years. Results indicate that a monolayer cover made of either non acid-generating tailings or a till was not able to prevent sulfide oxidation in the underlying reactive tailings, both for the conditions applied in the laboratory and under conditions more representative of field observations. The efficiency of the simulated monolayer cover was highly dependent on the position of the water table. Despite improved water quality, the reactive tailings nonetheless continued to oxidize and generate AMD, even when the water table was close to the surface of the tailings (1 m depth). The results from this study indicate that the efficiency criteria for cover systems on fresh unoxidized tailings may not be directly applicable for pre-oxidized tailings. The paper also presents a few modifications to the MIN3P code that were successfully applied to simulate the hydrogeological and geochemical response of pre-oxidized, already acidic and highly contaminated tailings.