1989-10-10_REVISION - M1988112 (12)

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Figure C-9. The pond liner and secondary recovery system details are contained on Figure C-8. The collection pond has been designed to contain 10.5 million gallons of water with in excess of the two-ft of freeboard. The pond will have a 500 foot long embankment with a maximum height of 22 feet. The embankment will be constructed of locally available materials, placed in lifts of no more than 12 inches at within 2X of the optimum moisture content and compacted to 95% of the standard Proctor. The pond design is based on the case where the design storm of the 100-yr recurrence interval 24-hr duration event occurs during the startup period of the facility. In this instance the drainage layer will be exposed to the water which will accumulate behind the embankments. Without a cover of tailings the seepage through the drain layer routed to the pond will be at its maximum. It has also been assumed that approximately four ft of dead storage exists in the pond and that a 48-hr loss of • pumping capacity coincides with the storm event. The volume of the seepage entering the pond during this period is calculated to be 9.4 million gallons. The approximately three inches of water falling directly on the pond will reduce the freeboard to slightly less than two feet under design storm condition. The design and height capacity relationships for the pond is shown on Figure C-9. These seepage calculations are presented in Appendix G. The pond will have a pumpback system to pump pond water to the free water pool in the disposal area for recycling back to the mill. The pumpback system will consist of two pumps and a return line with a maximum pumping capacity cof 5,000 gpm. The line will be placed within the lined collection ditch along the north side of the main embankment. The pumpback system location is shown on Figures C-3 and C-4. • D-39