| Summary: | 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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