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Closure of the Sterling Mine Heap Leach Pad

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Closure of the Sterling Mine Heap Leach Pad

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    1. Closure of the Sterling Mine Heap Leach Pad Pete Kowalewski, P.E. Vector Nevada, LLC Marc Leduc, P.Eng. Vector Colorado, LLC Chuck Stevens, Sterling Gold Mining Corporation June 2005

    2. Presentation Overview Site Overview Closure Objectives Data Availability Site Investigation Closure Design Construction Activities

    3. Site Overview

    4. Site Overview Located south of Beatty, Nevada east of the Amargosa Desert Lined facility covering approximately 20 acres Heap was rinsed following cessation of mining activities - rinsing ceased in Spring of 2003 Draindown as of April 2004 (time of closure design) was <1 gpm

    5. Site Overview Annual Precipitation = 4.6 inches Max recorded annual precipitation = 9.6 inches Annual Evaporation (Class A Pan) = 80 inches No perennial, intermittent streams, or permanently flowing springs Occasional surface flows from storm runoff Groundwater in excess of 1000 ft (bgs)

    6. Site Location

    7. Closure Objectives Provide a technically-feasible approach that meets applicable State regulations for protection of the environment Minimize closure-related costs Minimize long-term liabilities

    8. Closure Activities Regrade heap to maximum slopes of 3:1 (H:V) Construct closure soil cover Provide adequate surface water controls Provide long-term water management Convert ponds to allow passive disposal of draindown

    9. Heap Regrading

    10. Data Availability Climatic Data No site monitoring data Amargosa Desert Research Station (ADRS) 5-minute climate data (1986-1992, 1998-2003) U.S. Ecology (Hazardous waste disposal site) Daily precipitation data (1981-2002) Site Soils data No cover soil characterization completed Ore Characterization No ore hydraulic data available

    11. Site Investigation

    12. Site Investigation Cover soil investigation 16 test pits excavated Alluvial fan materials Range from silty sandy clay with gravel to sandy gravel Ore characterization Several shallow test pits excavated in heap Sandy gravel with clay

    13. Site Investigation Laboratory testing of composite borrow sample and ore Grain size analyses Moisture-density relationship (Proctor) Saturated hydraulic conductivity Soil water characteristic curve (7-pt SWCC)

    14. Closure Design Construct alluvial soil cover Minimize heap infiltration by maximizing ET effects in constructed soil cover Ensure regraded heap/constructed soil cover provide erosional stability

    15. Alluvial Soil Cover 12 inches (min) placed over regraded heap Don’t “over-compact” cover soil Ksat = 7x10-5 cm/sec @ 85% of optimum Maximize storage within cover Minimize equipment traffic over cover

    16. Cover Evaluation Predictive cover performance modeling using SoilCover® model Cover soil and ore hydraulic data from laboratory testing Climatic data from ADRS

    17. Cover Evaluation Sensitivity Analyses Saturated hydraulic conductivity of alluvial cover soil Cover thickness Precipitation (up to maximum recorded annual) Vegetation effects Evaluated case with no vegetation

    18. Cover Evaluation Results Results of Modeling 12-inch alluvial cover with poor vegetation limits heap infiltration values to deminimus values (<0.1 gpm) Removing vegetation shows gradient reversal at cover-heap interface from upward to downward No significant increase in infiltration rate

    19. ADRS Deep Infiltration Research USGS conducting research on deep infiltration at ADRS (close proximity to Sterling Mine) Installed deep vertical shafts at several locations in the Amargosa Valley Monitoring moisture contents, chloride, nitrate and sulfate in the sub-surface as indicator of deep percolation

    20. ADRS Deep Infiltration Research Study conclusion: “The observed sub-root-zone accumulations of chloride beneath undisturbed native vegetation in the Amargosa Desert support previous studies that concluded that there has been little to no deep percolation at the Nevada Test Site for thousands of years” (Stonestrom et. al., 2003)

    21. Construction Activities Perforated pipe installed in gravel at toe of heap to convey toe seepage to pond Regraded heap to maximum 3:1 (H:V) slopes 58,000 yd3 of rinsed ore moved Surface water controls (swales) constructed on heap to control runoff 12-inch (min.) alluvial soil cover placed over heap – will re-vegetate with native vegetation

    22. Post-Closure Water Management

    23. Construction Activities

    24. Construction Activities

    25. Construction Activities

    26. Construction Activities

    27. Summary 12-inch alluvial soil cover shown to effectively limit heap infiltration Conclusions appear to be supported by research at nearby ADRS Closure of the heap completed by owner with mine staff – effectively reducing closure cost

    28. Summary Involvement with project reinforced importance of investigating potential data collection and research being carried out by others in proximity to project site

    29. Thank you for your attention! Questions?

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