MEND Report 2.15.1a
June 2000

EXECUTIVE SUMMARY

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.

Last Modified: 2003-11-26		Important Notices

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