MEND 2.23.2ab
Octobre 1993

Summary

The Kidd Creek Division of Falconbridge Ltd. has been using the Thickened Tailings Disposal (TM) method at its metallurgical site near Timmins, Ontario. In 1991, Noranda Technology Centre was requested to examine the environmental situation at the site. The objective of the two-year study was to measure the hydraulic properties of the tailings site to analyze their effects on the long-term impacts to the environment.

The use of the TM method for tailings disposal was originally proposed by Robinski (1975), for specific advantages such as low initial capital investment, low operational costs, good storage capacity per unit surface, and the elimination of high perimeter dams, slime ponds and decant systems. The method consists in the thickening of the tailings slurry which is discharged in an elevated central spigot line producing a large cone-shaped deposit. Currently, the tailings deposit at Kidd covers a surface of 1215 ha and resembles a broad, gently sloping (± 1 %) cone. The tailings contain approximately 5% sulphur, and have been determined to be a strong net acid producer using B.C. Research Initial Test and Confirmation Tests.

Recent reports and publications by Robinski (1990) and Robinski et al. (1991) suggested that the TTD scheme has the potential to reduce acid generation and seepage by creating a homogeneous tailings mass of low hydraulic conductivity and high moisture retention characteristics in which oxygen entry and the resulting tailings oxidation are very limited.

NTC concentrated on the field evaluation of the principal components of the site hydrology to obtain a verification of the high moisture retention of the TTD deposit. The field program included measurements of moisture content, hydraulic head, water table elevation and hydraulic conductivity. Precipitation, pan evaporation and evaporation from tailings were monitored during 1991-1992 and a monthly water balance was prepared. Porewater was sampled and analyzed for major metal and ion concentrations.

The magnitude of the water balance components during average conditions -- runoff 42% of precipitation, evaporation 51 % and infiltration 7 % -- were similar to those predicted by the HELP model. The water balance for extreme dry conditions predicted an infiltration deficit that results in surface de-watering at the end of the year that is similar to the de-watered condition observed during the summer of 1992 (a year with average rainfall and evaporation). A year of above normal precipitation will likely replenish the deficit. Normally, the tailings are saturated to the surface after spring snowmelt, and summer de-watering is replenished during the fall and with the following spring snowmelt.

Hydraulic gradients suggest that pore water in the saturated zone tends to move downward near the centre of the cone, and upward (and exfiltrate) along the slope of the tailings. Near-surface hydraulic gradients on the upper part of the cone indicate that upward flow dominates during the period of summer water table drawdown, and that downward flow dominates during the subsequent recharge period. Average linear pore water velocities in the tailings are very low (12 cm/y).

The thickness of the capillary fringe (the saturated tailings above the water table) was observed to be 4 m at the top of the tailings cone. A maximum thickness to 5 m to 6 m is predicted from published drainage curves of the tailings. The 5 m contour on the depth to water table contour map, therefore, delineates the area of enhanced surface Wags oxidation.

Over most of the tailings, the surface is observed to dry during the summer. Sulphide oxidation is observed at the surface and at depth along shrinkage cracks. The analysis of porewater indicates that sulphide oxidation is occurring in the tailings mass. De-watering during a draught year is expected to have little impact on long-term saturation of the tailings. However during these periods, oxidation is promoted deeper in the tailings. Considering the tailings cone at present, then following closure, tailings saturation and water table position are expected to resemble that presently observed in areas of the cone where deposition is not active. Release of contaminants from the tailings general mass to the environment should remain at present rates because porewater velocities are slow, and because of the sustained near-surface saturated conditions.

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