Abstract
Mining companies in North America have been extracting iron ore for over 300 years, supplying feed for steel production both domestically and globally. Aspects of the North American regulatory environment, along with site-specific geochemical characteristics and the physical scale of iron mining, present unique opportunities and constraints for constituent management via geochemically based source control strategies. A progressive series of investigations were conducted to characterize sources of sulphate, alkalinity, and total dissolved solids (TDS) observed in surface water surrounding the tailings storage facility (TSF) at an iron mine. Sulphide mineral abundance in the tailings is low (S < 0.11 weight percent) and the majority of tailings are sequestered within the pond, limiting their oxidation potential. However, oxidation of minor sulphides present in outer TSF embankments, and subsequent dissolution of mixed cation carbonate minerals, were identified as significant sources of sulphate, alkalinity, and TDS to the system. Although constituent concentrations in tailings porewater are generally low compared to other metal mines, site-specific environmental objectives created a need to identify opportunities for further reductions in constituent loading from the tailings. A field investigation of TSF pore gas, temperature, and water chemistry was conducted, accompanied by reactive transport modeling. Results indicated that drivers of oxygen consumption included both oxidation of sulphide minerals and ferrous iron released during dissolution of mixed cation carbonates. Additionally, source control (in the form of a low oxygen diffusivity layer) has the potential to decrease sulphate in water reporting to the toe of the embankment.