MEND Report 2.21.1
March 1992

EXECUTIVE SUMMARY

This final report presents the results of an investigation conducted to develop laboratory and computer methodologies for evaluating the effectiveness of engineered soil covers for reactive sulphide tailings. The study was undertaken under a Supply and Services Canada (DSS) contract awarded to Noranda Technology Centre (NTC) in January 1989. Funding for the project was provided by Environment Canada, Canada Centre for Mineral and Energy Technology (CANMET), DSS and Noranda Inc. NTC subcontracted the initial fundamental aspects of the study to the University of Waterloo. NTC focused on the more-applied aspects of the work and later adopted some of the methods developed at Waterloo in the investigation of candidate natural soils from two active Canadian mine sites.

The transport of oxygen through tailings covers was shown to be mainly by molecular diffusion resulting from concentration gradients established between the atmosphere and soil void spaces. The rate of diffusion is controlled by the diffusion coefficient which is, in turn, dependent on the grain size, moisture content, porosity and other physical properties such as tortuosity or the flow path length. Oxygen diffusion coefficients, determined for one of the candidate soils, were observed to decrease with increasing water contents or degrees of water saturation. This trend was consistent with theoretical predictions and agreed with findings reported by previous workers. In the laboratory tests reported here, it was acknowledged that oxygen uptake by natural soils moulded at high moisture contents could be significant as a result of increased activity of soil micro-organisms under moist conditions. It was recommended that this factor be given serious consideration in any investigation.

Three laboratory columns were designed and constructed, and suitable instrumentations were adapted to provide reliable methodologies for soil cover evaluation. The developed apparatus were; a) a column for determining the diffusion coefficient of oxygen, b) a column for evaluating the hydraulic behaviour (moisture retention and drainage characteristics) of potential cover materials, and c) a demonstration column for estimating the low oxygen fluxes expected from saturated soil covers.

Experimental and theoretical pressure head profiles observed in a two-layered system (candidate till over sand layer) indicated drainage of the sand layer to residual saturation without a measurable drainage and therefore loss of moisture from the till layer. Experimental testing was extended to at least 100 days in the demonstration column. Similar trends in moisture contents were observed. The results suggested that, in the absence of other extraneous influences such as root penetration and cracking resulting from freezing, similar performance could be expected in the field. At most Canadian mine sites, 100 days will, most probably, be the maximum dry period when no recharge to the cover by infiltration would occur.

Estimates of oxygen concentration at the base of the demonstration column showed that only 600 ppm could be expected in a year. In terms of fluxes, this would amount to 6.5 x 10^-3 moles/m²/a of oxygen and an acid flux of 0.338 g/m²/a of sulphuric acid (H₂S0₄). In comparison, maximum acid flux estimated in uncovered tailings in laboratory lysimeter tests at NTC (outside this project) was typically of the order of 1200 g/m²/a of H₂S0₄.

Practical concepts were proposed for designing high performance soil covers. Critical material properties required for design and prediction of performance were identified to include the diffusion coefficient, moisture drainage characteristics and long term critical moisture contents under dry conditions. It was concluded, from limited laboratory testing, that freezing and thawing cycles could play an important role in soil cover performance.

Last Modified: 2003-11-26		Important Notices

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