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Waste rock cover system field trials at the Myra Falls Operations : a summary of three years of performance monitoring O'Kane, Mike; Januszewski, Steve; Dirom, Gavin C.
Four field test plots were constructed in 1997 on a sloped waste rock surface at the Myra Falls Operations of Boliden Westmin (Canada) Limited to evaluate hydraulic performance of alternate cover system designs. The field test plot cover systems were designed to control oxygen ingress and water infiltration to the underlying waste rock while providing a medium for a grass and legume vegetation cover. The test plots consisted of a bare waste rock surface (control plot), and three additional plots consisting of native compacted till placed directly on the waste rock surface, overlain by non-compacted till. Two of the test plots included a compacted till layer ameliorated with flyash and bentonite, respectively, to improve moisture retention and reduce permeability. The third test plot consisted of a compacted layer of native till overlain by non-compacted till. A performance monitoring system was installed in each test plot to evaluate net percolation and oxygen ingress to the underlying waste rock. In addition, each test plot has profiles of moisture retention, water content, and temperature sensors installed at up-slope and down slope locations. The field data collected to date demonstrates that infiltration to the underlying waste rock has been reduced to as little as 1% of precipitation for the compacted till-bentonite ameliorated cover system, and less than 10% for the compacted till-flyash ameliorated cover system as well as the compacted native till cover system. The presence of the overlying non-compacted till and associated vegetation significantly impacts infiltration to the waste rock during the summer and fall. Precipitation occurring as rainfall is stored in the non-compacted growth medium overlying the compacted layer, and subsequently "released" back to the atmosphere as evapotranspiration. The presence of the compacted layer limits water infiltration to the underlying waste rock during wet fall, winter, and spring conditions. The native till and flyash ameliorated cover systems have not functioned as oxygen ingress barriers during the period monitored. The poorer performance of the native till cover system was expected, but this was not the case for the flyash ameliorated system. The reduction of moisture conditions in the compacted tillflyash layer and the compacted native till layer was a result of atmospheric demand for moisture during the summer. A reduction in the moisture conditions within the compacted till-bentonite layer was also observed, although to a much less extent. Increasing the thickness of the overlying non-compacted layer should significantly reduce the demand for moisture from the compacted layers, and thus prevent desaturation and eventually an increase in oxygen ingress. The optimum cover system design will be determined after all field data is used to calibrate a soilatmosphere model, and this model's output used as input for a site specific hydrogeologic model. Then the relative technical and economic advantages of each cover system option can be evaluated to select the optimum waste rock cover system for the Myra Falls site.
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