MEND Report 2.31.1c
February 1998

SUMMARY

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.

Last Modified: 2003-11-26		Important Notices

Français | Contact Us | Help | Search | Canada Site
Home | What's New | CANMET-MMSL | MMS Site | NRCan Site