The study was initiated in 1992 to address uncertainties in the design of water covers
for decommissioning oxidized tailings. Laboratory column experiments and field cell
tests were carried out with tailings from the Mattabi Mine (Noranda Inc.) site near
Ignace, Ontario. Some experiments involved placement of attenuation layers made of
sand or peat at the tailings-water interface. In contrast to other studies, tailings were
not amended with alkaline material, either prior or after flooding.

The benefits sought by implementing a water cover over oxidized tailings include: (1)
limiting further oxidation of the tailings, (2) providing sufficient improvement of the
water cover quality to permit discharge of runoff to the surrounding environment
without the need for treatment after a transition period (i.e., the period during which
previous oxidation products are flushed out of the water cover), and (3) reducing
contaminant loads and treatment costs associated with seepage from the tailings over
time.

Flooding oxidized tailings without prior installation of an attenuation layer resulted in
the release of metals and sulphate from the pore water solution and soluble mineral
phases to the water cover. In the columns, average total iron, sulphate and zinc
concentrations in the water cover reached respectively 257 mg/L, 927 mg/L and 20.2
mg/L approximately one year after flooding. Corresponding concentrations in the field
cell water cover one year after the flooding of July 1992 were 466 mg/L, 2850 mg/L
and 3.8 mg/L, respectively. Differences between concentrations measured in the
laboratory and in the field tests were likely due to differences in initial pore water
compositions and soluble mineral contents between the columns and the test cell.

Because the pore water of Mattabi tailings was rich in ferrous iron, the establishment of
a water cover was accompanied by the precipitation of a thin layer of hydrous ferric
oxide precipitate on the surface of the tailings. This was due to the oxidation of ferrous
iron by dissolved oxygen and subsequent hydrolysis of ferric iron.

Over time, contaminant concentrations in the water cover decreased as a result of 1)
dilution of the water cover by addition of deionised water (laboratory columns) or rain
and snowmelt (field test cell), 2) flushing of solutes by water infiltration from the cover
to the tailings, and 3) removal of some metals by precipitation and sorption on a
hydrous ferric oxide precipitate layer that formed at the water/tailings interface. In the
field test cell, the water cover met regulatory discharge limits two years after flooding:
As<0.5 mg/L, Cu<0.3 mg/L, Fe < 3 mg/L, Pb < 0.2 mg/L and Zn < 0.5 g/L. However,
the water cover pH remained lower than the minimum of 6.0. It is estimated that about
1855 mm of water infiltrated during this period. In the laboratory column water covers,
the zinc concentration decreased much more slowly than in the field test cell and was
still ~6 mg/L after 620 days of simulated infiltration, which translates into ~955
equivalent field days given average field infiltration rates.

The persistence of elevated zinc concentrations in the laboratory column water covers
is likely explained by geochemical equilibrium of the overlying water with a
Zn-containing solid phase in the hydrous ferric oxide layer at the tailings/water
interface. Hence, the hydrous ferric oxide precipitate, although contributing to the
decrease of zinc concentrations in the column water cover during the first year of
testing, later acted as a source of soluble zinc as metal concentrations in the water
cover and pore water decreased. In the field cell, the zinc content in the precipitate
layer (0.22%) was less than in the laboratory columns (1.28%), presumably because of
lower initial pore water concentrations. As a result, zinc did not leach as much, and
discharge limits were met in the water cover two years after first flooding the cell, as
pointed out above. Hence, the geochemical characteristics of the oxidized tailings
(pore water composition, soluble minerals) have a large influence on the time required
to achieve discharge limits in the water cover.

Attenuation layers made of sand or peat were only tested in the laboratory columns.
Fluxes of metals from the tailings to the water cover were greatly reduced by placing
an attenuation layer at the tailings-water interface. This was primarily because diffusion
of metals from the oxidized tailings through the attenuation layer and to the water cover
is a very slow process. Moreover, the attenuation layer also imposes a diffusion control
on the availability of dissolved oxygen to the tailings, and therefore limits further tailings
oxidation more effectively than a simple water cover.

When a sand layer was used, the water cover met discharge limits during the entire
duration of the tests. The average total iron, sulphate, and zinc concentrations in the
water cover were below 0.3mg/L, 50 mg/L, and 0.03 mg/L approximately half a year
after flooding. The peat layer was less effective because of its own leachable zinc
content: after 163 days of test, the zinc concentration was still 1.5 g/L in one of the
columns. Hence, it is important to ensure that material used to build the attenuation
layer has low contaminant levels so that it does not contribute to the contamination of
the water cover. A careful and detailed characterization of this material is therefore
very important. In general, peat obtained around the vicinity of mining sites does not
appear to be a good candidate for building attenuation layers, as it is often a source of
iron and other metals in water cover applications.

Seepage water quality was only measured in the laboratory column tests. Seepage
concentrations were initially very high (similar to pore water concentrations) and
remained stable for all columns until the displacement front reached the bottom of the
columns (at about 0.7 pore volumes), after which they decreased over time as
infiltrating water flushed the contaminated pore water. The presence of a water cover
accelerated the decrease in seepage concentrations when compared with tailings left
exposed to the atmosphere, probably as a result of reducing further tailings oxidation.
Long-term maintenance of water covers generally requires that seepage losses from
the facility be minimal. In this case, the contaminated pore water will remain in place for
many years, and seepage treatment, if part of the closure plan, can be expected to be
long lasting.

If the seepage rate is high, as was the case in this study, large volumes of seepage will
need to be intercepted and treated. Economic considerations suggest that at sites with
high seepage rates, a water cover may be suitable only if it can be supplemented with
gravity-fed fresh water from a nearby source to the cover, and thus compensate for
seepage losses. In this situation, treatment costs for the seepage will increase after
flooding due to higher infiltration rates. Although the quality of seepage will increase
over time, treatment may still be required in the very long term since solute
concentrations may reach near-asymptotic values that are still above discharge limits,
as was the case in some of the tests. Hence, the establishment of a water cover over
oxidized tailings is not recommended at sites with significant seepage losses.