A waste rock pile at the Heath Steele Division of Noranda Mining and Exploration Inc.
was covered with a composite soil cover to test the ability of the cover to limit the rate
of sulphide mineral oxidation. The experimental waste rock pile was constructed in
1989. Measurements of pore-gas oxygen, temperature and leachate water quality were
conducted for two years, with the composite soil cover placed on the pile in 1991. The
cover was designed to impede the ingress of water and oxygen to the pile. After
construction of the cover, monitoring continued for another five years to assess the
effectiveness of the cover. Additional measurements during the post-construction
period included cover water content, soil suction, hydraulic conductivity and infiltration.

Results showed reductions in gaseous oxygen concentrations in the waste rock pile
after the cover was built, indicating reduced oxidation rates. Similarly, temperatures in
the pile have decreased, now appearing to be controlled primarily by climatic
conditions rather than sulphide oxidation rates. These findings indicate that oxidation
rates are being controlled by the cover.

Since placement of the cover, concentrations of metals and sulphate in leachate
collected at the base of the pile appear to have shown a gradual decline, but have also
shown an annual fluctuation that increases in the summer and decreases in the fall
and winter. The improvement in porewater quality is expected to continue to be gradual
since the porewater flushing rate is low. A rough estimate of the flushing rate was
calculated at about 30 years for one pile pore volume, and many pore volumes are
needed to dissolve and flush products of sulphide mineral oxidation that precipitated
prior to placement of the cover.

Effluent loadings decreased immediately following construction of the cover due to
reduced infiltration, and since then, declining porewater concentrations have reduced
effluent loadings. These findings indicate a positive performance of the cover for
limiting sulphide mineral oxidation in waste rock. Based on improved loadings, a
savings in the cost of lime was calculated as $196/yr per 1000 tonnes of waste rock;
94% of the savings was observed shortly after construction of the cover due to reduced
flushing flows through the cover. Other benefits for treatment include a low volume of
flow to be treated and effluent water quality consistency.

The results of five years of monitoring indicate that composite soil covers on waste
rock piles are effective in limiting the rate of sulphide oxidation. For a specific
application, the cost and savings, as well as long term stability, need to be evaluated.
Four recommendations for future cooperative research are made. The two most
important are to continue monitoring of the cover performance and to evaluate the
apparent dewatering of part of the cover.