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Examples of Modeling: Groundwater Impact Evaluation
Re-initiation of copper mining in thehistoric Robinson District near Ely Nevada
PTI Environmental Services. 1994. Hydrogeochemistry of the Robinson Mining District, White Pine County, Nevada. Prepared for Magma Nevada Mining Company, Tucson AZ.
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Wells and springs in the Robinson District
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Model objectives
Predict impact of mine dewatering and operations on water quantity and quality impacts to springs and creeks near mining district, particularly Murry Springs, water supply for Ely, Nevada
Predict infilling rates for pit lakes for pit lake water quality predictions
Evaluate tailings disposal scenarios in Liberty Pit
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Groundwater modeling steps
Compile regional and local hydrogeologic data Develop conceptual flow model Configure model grid Estimate input parameters from existing data Determine reasonable boundary conditions Estimate areal recharge Develop and calibrate steady-state model Calibrate transient model Make predictions: impacts from dewatering and potential
for contaminant transport to springs, pit lake infilling, tailings disposal scenarios
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Model domain and MODFLOW grid
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Groundwater model: Regional topography and model domain
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Regional model layers – Boundary conditions
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Example of data evaluated: Discharge at Murry Springs v. average precipitation
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Estimate recharge from precipitation data using method of Maxey and Eakin (1949)
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Geologic Cross section through Mining District
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Regional geology
Local geology from Mine block model
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Regional geology
Mine site geology
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Calibrated hydraulic conductivities for geologic units
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Model calibrated to reproduce current measured groundwater levels and discharge from springs
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MODPATH (Particle tracking) indicate no flow paths from mine facilities (lakes, waste rock or tailings impoundment) to Murry Springs
Contaminant transport modeling not needed to evaluate potential for water quality impacts
Particle tracking
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Model uncertainty
Sensitivity analysis was conducted using range of values for K and recharge
Model was found to be most sensitive to K for a few regionally extensive and higher K geologic units
Simulations conducted by varying input parameters within reasonable (literature values) ranges did not alter model results sufficiently to change conclusion that there will be no impact to springs
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Pit lakes projected to create cones of depression as the result of evaporative losses, in perpetuity
Murry Springs outside of cone of depression/capture zone – flows not affected by dewatering or long-term presence of lakes
Mine not predicted to impact spring water quality
Conclusions
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Robinson groundwater model evaluation
XModel well-documented
XModel revaluated as dewatering/mining progressed
XSensitivity analysis conducted
XModel calibrated to existing data (steady state and transient)
XParameter estimator used
XModel boundaries, domain, input parameters clearly described and reasonable
XPublic domain code
XConceptual site model clearly presented
NoYesEvaluation criteria