MEND Report 2.23.2d
October 1995

Summary

A geochemical. hydrogeological and hydrological study of the Kidd Creek tailings impoundment was initiated in August 1991 at the request of Falconbridge Limited, Kidd Creek Division. This study is part of a larger investigation which is intended to aid in the development of a comprehensive, long-term environmental management program for the Kidd Creek tailings. The goals of the Waterloo Center for Groundwater Research study were to characterize the hydrogeological flow system, the geochemical interactions between the tailings pore water, pore gas and solids and to determine the influence of discharging tailings pore water on the quality of run off from the tailings surface during storm events. The results of the research could then be used to aid in predicting the future effluent quality.

The elevated central area of the tailings cone is a groundwater recharge area where precipitation infiltrates and moves downward to replace pore water that flows away from this area. Recharge rates vary from a maximum near the apex of the tailings cone and decline outward toward the perimeter road. The dominant pore-water flow direction is radially outward from the center of the impoundment to discharge in the flat-lying peripheral areas of the impoundment. Some pore-water flow occurs downward and inward toward the spigot road in the center of the impoundment. This road is constructed of more permeable material than the tailings and is a drain for the elevated central tailings.

The current practice of co-disposing natrojarosite with sulfide-rich tailings at Kidd Creek introduces the natrojarosite to a neutral-pH and low-EH environment in which it is thermodynamically unstable. Geochemical modeling suggests that conditions favoring natrojarosite dissolution are present throughout most of the tailings impoundment. Results of this study indicate that the natrojarosite is dissolving, causing the release of Na, K, Mg, Mn, Fe, Zn, Pb, As, HC0₃ and S0₄ to the pore water. Mineralogical studies indicate that a significant mass of natrojarosite remains in the tailings representing a long term source of contamination. Increased Fe²⁺ concentrations in the pore water may cause acid drainage if seepage occurs around the perimeter of the tailings impoundment.

The effects of natrojarosite dissolution on the pore-water composition can be distinguished from the effects of sulfide oxidation. Natrojarosite dissolution increases the pore-water concentrations of Na, K, Fe, Pb, As and S0₄ directly, and increases the concentration of Mg, Mn, Fe and HCO₃ indirectly through carbonate-mineral dissolution. Increases in Zn concentration result from natrojarosite disposal due primarily to the release of Zn retained within the aqueous phase of the natrojarosite residue. Sulfide oxidation generates low-pH conditions in the pore water near the surface and further increases the concentrations of Mg, Mn, Fe, Zn, Pb, As and S0₄, as well as increasing the concentrations of Al, Cd, Co, Cr. Cu. and Ni.

Sulfide oxidation also causes the dissolution of carbonate minerals, thereby initially increasing the pore-water concentration of HC0₃; continued oxidation, however, will consume the carbonate-mineral acid-neutralization capacity of the tailings and will subsequently deplete the pore-water alkalinity. Sulfide oxidation has been limited by continuous tailings deposition on most of the main tailings cone. As a result, there is no discernable depletion of sulfur at the surface. Due to variability in the initial carbonate content of the tailings, there is no distinguishable depletion of the carbonate-mineral content.

Assuming there are no changes to the present tailings surface due to erosion or reclamation operations, sulfide oxidation modeling suggests that the most intense sulfide oxidation will occur in the first 20 years that the tailings are exposed to the atmosphere. Oxidation rates and resultant Fe- and SO₄-loading rates will decline after that period as the process becomes limited by O₂ diffusion into the tailings pore spaces. The products of sulfide oxidation reactions (dominantly aqueous Fe(II) and SO₄), produced during that 20 year period, will move through the tailings with the pore water. The residence time for the reaction products in the pore water, prior to discharging, will range from 0 to 1000's of years.

Hydrological studies were conducted to determine the amount of low quality tailings pore water contributed to the surface run off from the impoundment during storm events. The maximum measured pore-water contribution to the run off was 23.5% during a moderate intensity, long duration rainfall event. The long duration rainfall events which cause the water table to rise throughout the tailings impoundment represent the greatest potential for contributing low-quality pore water to the surface-water effluent.

Last Modified: 2003-11-26		Important Notices

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