The Falconbridge Limited, Kidd Metallurgical Division undertook a study which involved the injection
of two tracer compounds into thickened tailings in 1992, and downstream pore water sampling and
analyses in 1993 and 1996. The key objective was to verify the predicted movement of the pore
water.

The research program was developed based on extensive knowledge of the site which included the
results of two MEND studies completed at the site. The actual pore water sampling and analysis
program presented a number of challenges. The greatest challenge was in collecting representative
samples of pore water. The program was modified on a number of occasions to meet these
challenges. The findings of the study were developed after expert review of the tracer monitoring
database. A key finding is that the measured pore water velocities tend to support the present
understanding of the pore water movement. The measured average horizontal velocity of the pore
water is approaching the maximum predicted horizontal velocity of 70 cm·yr-1. Pore water flow is
indicated to be predominantly downgradient and follows the tailings slope as previously modelled.
The greatest velocities are in the horizontal direction, with the average downward velocity ranging
from 8 to 16% of the average horizontal velocity.

The Falconbridge Limited, Kidd Metallurgical Division was the first to adopt the thickened tailings
disposal system conceived by Dr. Eli I. Robinsky. Thickened tailings disposal has successfully been
practiced at the site since 1973. The thickened tailings are deposed in a 1200 ha tailings
management area (TMA) which contains in the order of 105 Mt of tailings. The thickened tailings as
disposed form a flat-sloped tailings mound, also referred to as the Robinsky cone.

The Kidd TMA is one of the most extensively studied tailings impoundments in the world. Much of
this research has been focussed on determining the properties and characteristics of the thickened
sulphide tailings. The MEND project described in this report represents the third component of a
three-part study of the Kidd TMA (2.23.2c). The previous two MEND studies examined the hydrologic
and hydrogeologic properties of the TMA (2.23.2ab) and its geochemical, hydrogeological and
hydrological characteristics (2.23.2d). The previous MEND studies demonstrated that:

The thickened tailings are non-segregating and form a homogenous deposit.

• The thickened tailings exhibit a very high level of saturation with the capillary fringe extending
  upwards to near the surface.
• Sulphide oxidation is limited to exposed and unsaturated tailings in the near surface zone.
• The elevated central area of the TMA serves as the pore water recharge area, receiving
  infiltration due to precipitation and tailings discharge.
• Pore water movement is radially downslope from the central elevated area to the flat-lying
  peripheral areas of the TMA. Pore water movement predominantly follows the tailings slope
  with downward movement impeded by the underlying saturated conditions and the low
  hydraulic conductivity of the tailings and natural clay base.

The present study involved the use of tracers to verify the predicted movement of pore water within
the thickened tailings. Horizontal pore water velocities, representative of rates of salt transport due to
advection, had been estimated in MEND 2.23.2ab to range from a minimum of 0.1 cm·yr-1 to a
maximum of 70 cm·yr-1.

Three tracer injection and monitoring sites were chosen to provide tracer monitoring data in different
areas of the tailings mound. The sites were situated along the southern slope of the TMA with the
topmost station located on the apex of the tailings mound where the slope is steepest. Potassium
bromide and potassium iodide tracers were injected in the tailings in the fall of 1992. The movement
of the tracers was sampled using mini-piezometers in June 1993. Collecting samples of pore water
from shallow (e.g. < 0.5 m deep) mini-piezometers using filter paper presented a challenge. A
sampling methodology that included the addition of ~20 mL of deionized water to shallow piezometers
prior to sampling was used in September 1993. The revised method was not considered optional
because of uncertainties regarding the concentrations of tracers in samplers. In response, the pore
water sampling and analysis program was revisited and revised for the second time to include:

• The collection of tailings cores using an aluminum tube.
• The extraction of pore water from the tailings core samples.

In October 1996, the above method proved useful in sampling unsaturated tailings, but was found
unsuitable for the sampling of saturated tailings. An attempt was then made to sample the tailings
using a power auger – this method was acceptable for unsaturated tailings but was problematic in the
saturated tailings.

Pore water samples were promptly analyzed for the presence of the tracers. The tracer monitoring
database was later assessed by Barbour and Bews (1998). In summary,

• The June 1993 monitoring data indicated that the pore water horizontal velocity range from 80
  to 112 cm·yr-1. These results should not be heavily relied upon given concerns associated with
  the initial high density of the tracer solutions and diffusion effects. These concerns are
  expected to diminish with time and as the tracer plume expands.
• The October 1996 monitoring data are likely subject to the effects referred to above but to a
  lesser extent. The October 1996 data indicate that the average pore water horizontal velocity is
  in the range of 67 to 74 cm·yr-1 and as such is approaching the predicted maximum horizontal
  velocity of 70 cm·yr-1.