Predicting long-term seepage from waste-rock and tailing facilities
has become an integral part of both permitting and closure in most
states. S.E.T. separates seepage prediction into three components,
as listed below, to provide the most accurate and defensible prediction
possible.
This three-component approach was applied by S.E.T. to predict
when and at what rate seepage may occur from waste rock piles at
a metal mine in southwest Arizona. A geotechnical investigation
was conducted to measure in-situ hydraulic conductivity using a
tension infiltrometer and moisture content using sonic drilling
techniques.
The field data was used as input to the hydrologic model SOILCOVER
to predict net infiltration rates. Percolation through the piles
was simulated based on state-of-the-art conceptual models of unsaturated
flow in waste rock and supported by field observations.
S.E.T. predicted seepage rates from a tailings facility at a
metal mine in southwestern U.S. using a similar approach as outline
for the waste rock facility above.
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Analysis and Design of Soil Covers
Closure of mine waste and heap leach facilities often require
analyses of alternative cover designs and selection of the most
effective economical alternatives.
In evaluating the long-term hydrologic performance of cover systems,
S.E.T. uses both empirical models (i.e., the U.S. EPA Health Model)
and physically-based models (i.e., SOILCOVER, UNSATH AND OPUS)
to predict net infiltration. Physically-based models provide a
more accurate quantification of evaporation than empirical models,
and at sites where evaporation is a significant component of the
water balance, this approach results in the most accurate and defensible
prediction of net infiltration to the underlying waste.
S.E.T. developed alternative cover designs for mine sites by
employing empirical infiltration models as a preliminary screening
level tool and physical-based models to optimize cover design.
Results of infiltration modeling for representative cross sections
were compared to performance criteria to assess the acceptability
of a particular cover design. This optimization methodology resulted
in savings of millions of dollars in construction cost.
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Solute Transport Modeling
The unsaturated zone can act as an efficient attenuator of solutes
due to the processes of diffusion, dispersion, and moisture storage.
These relevant processes have typically been ignored in the past,
despite their significant effect on solute transport times and
concentrations. S.E.T. conducts unsaturated zone field and modeling
studies to predict the effect of the unsaturated zone on solute
transport.
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