MEND Report 2.23.3
May 1995

Executive Summary

In 1992, Noranda Technology Centre undertook a hydrogeological investigation of the Fault Lake tailings site. The tailings have been deposited in a kettle lake formed within glacial outwash sand and gravel. The site is unique in that, theoretically, a "porous envelope effect" may occur. If this is the case, flow through the tailings mass is low enough, relative to the surrounding, more permeable till, that impact to the ground water by tailings oxidation is insignificant at the regional scale. The specific objectives of the investigation were to analyze the chemical and physical hydrogeology of the site, to delineate areas affected by acid mine drainage generated from the tailings, and to verify the presence of the porous envelope effect.

The hydraulic conductivity (K) of the Fault Lake tailings measured 1.2 x 10^-5 cm/s at a mid-level depth in the tailings and 3.6 x 10^-6 cm/s in the deepest part of the tailings. Comparatively, the measurements of K for the glacial sediments averaged 1.6 x 10^-3 cm/s. This is a two order of magnitude contrast in hydraulic conductivity. Flow modelling indicated that this is sufficient to route most regionally flowing groundwater around the tailings.

During the spring and fall, ponding occurs at the north dam, south dam and various berms. The water slowly infiltrates into the tailings and evaporates from the ponds. During the summer months, extensive ponding has not been observed. The water level in the tailings is perched higher and fluctuates greater than the regional water level. Regional groundwater flows to the northeast from the tailings dam at a velocity of about 2 m/yr. Groundwater flowing from the southerly dam goes south. Because the groundwater velocity is controlled by the hydraulic gradient, the velocities could have been higher during tailings disposal.

The tailings are characterized by two layers due to the disposal of different types of tailings: Layer 1 is pyrrhotite rich and Layer 2 is pyrrhotite poor. Layer 1 is centrally located on the tailings and in close proximity of the northerly spigot position. In the centre of Layer 1 pyrrhotite was identified to a maximum depth of 9 m, but was at highest composition in the upper 3 m where it is near 50%. Layer 2 is below Layer 1 in the centre of the tailings. In the southerly portion of the tailings Layer 1 pinches out.

Mineralogical analysis and acid-base accounting of the tailings indicated that carbonate mineral reserves are available for short-term neutralization of acid during the first stage of oxidation when rates are high, and silicate mineral reserves are abundant for long-term buffering. The neutralization potential of the tailings plays an important role for the attenuation of acidity and metals from sulphide oxidation, which were detected but have been attenuating in the tailings deposit.

Sulphide oxidation has been at its highest rate since deposition discontinued in 1978, yet little impact of sulphide oxidation was observed in the groundwater of the surrounding till. Sulphide oxidation products leaching from the tailings appear to be alleviated by the porous envelope effect. Several favourable factors contribute to create the porous envelope effect and to limit the observed metal concentrations downgradient of the tailings:

(1) the hydraulic conductivity contrast between the tailings and the surrounding sediments;

(2) the limited infiltration through the surface of the tailings;

(3) the dilution of metals flushed from the tailings by water flowing around and below the tailings; and

(4) the chemical attenuation of metals, which likely plays a large role both inside the tailings mass and in the surrounding sediments.

The porous envelope effect could probably be present at other locations near mine sites. Tailings deposition could possibly be done at these sites with little effect on groundwater quality, pending that thorough site evaluations are performed and that appropriate control is done at the time of deposition.

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