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:
- the hydraulic conductivity contrast between the tailings and the surrounding
sediments; - the limited infiltration through the surface of the tailings;
- the dilution of metals flushed from the tailings by water flowing around and below
the tailings; and - 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.
