1989-12-18_REVISION - M1988112 (8)

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.., • • 10. Please commit to achieving 1x10'6 cm/sec. permeability for the underlying colluvial material after it has been compacted to 95% maximum Proctor density. It will be necessary to provide verification to the Division that this permeability has been achieved prior to installation of the VLDPE. 11. Please provide a detailed discussion regarding the potential for and detection of leakage from the VLDPE liner to the underlying colluvial materials. A rationale for the placement of the monitoring wells, including the potential for those wells to intercept leakage from the tailings facility, should be included. 12. Please discuss whether the completion of angled drill holes within the colluvium beneath the tailings facility would provide beneficial information regarding leakage through the liner system. Another approach to leak detection may be to place slotted ADS piping within the compacted colluvial material with an end open to the edge of the tailings facility. In any case, a provision to assess and mitigate leakage should be addressed. 13. Please define the terms minimal and nominal as used in Section D.6.5.1 when discussing potential leakage from the disposal facility. Please discuss how verification of losses will occur. 14. Please determine and describe the percent moisture content within the tails which would constitute a saturated condition and compare this figure to the anticipated moisture within the desiccated tails. Will the amount of water entrained within the tails vary vertically? If so, describe how this would affect the long term, post reclamation stability of the facility. 15. In order for the Division to make a more adequate determination of the long term stability of the tailings disposal facility, a liquefaction potential investigation should be conducted. The stability of Phase 2 raises 1 and 2 in relation to the tails undergoing acceleration sufficient to produce this event should be specifically evaluated. 16. Section D.6.1 states that, during Phase 1, 33% of the tailings will be placed in the upper disposal area. Section D.6.5.3 states that, during Phase 1, the upper area will generate drainage at about 310 gpm, while the lower area will generate about 230 gpm. Please explain why the lesser volume of tailings generates 120 gpm more water. If these figures were used to evaluate the sizing and spatial placement of the ADS collector pipes, the design should be re-evaluated. It is essential that the drainage system be completely adequate to transmit flows from the tails to the catchment basin while minimizing the hydraulic lead within the facility. 17, The elevations contained on Figures D.6.-3 through D.6-5 should be corrected, and the figures resubmitted. -2-