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.