2011-03-14_REVISION - M1977493 (62)

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CM0001639 Pa e 11 g supporting calculations for impoundment sizing are presented in Appendix A. The impoundment dam will be constructed in similar fashion as the other dams in the ERV. A 1 zoned earthen embankment will be constructed with an impervious soil core and pervious soil shell. A drainage layer will be placed on the downstream face of the core to provide relief of excessive seepage pressures within the embankment. A cutoff wall will be excavated along the entire length of the embankment to decrease underfloor of poor quality water through the bedrock. Optionally, a slurry wall or grout curtain can be placed to further decrease subsurface ground water flow. a. . A collection drain, similar in construction to the Robinson Lake Drain (Section 3.1.2), will be installed along the toe of the impoundment dam. The purpose of the collection drain will be to collect seep water from the impoundment dam and pump it back into the impoundment. As part of the impoundment construction, the county road through the ERV may need to be realigned. The realignment, shown on Figure 3, will be completed as part of impoundment construction. Passive Treatment of Seep Water Optionally, the No. 1 Tailing Dam seep water could be treated passively by routing the seepage through an oxic or anoxic limestone drain located downstream of the dam and directly C discharging to Robinson Lake. The drain location is illustrated on Figure 4. Case studies highlighting the use of limestone drains (Kleinmann, 1991, Ziemkiewicz, et al, 1994, Bureau of Mines, 1994) have shown drains to be effective in treating acid mine drainage (AMD) to acceptable water quality levels. Oxic drains, or open limestone channels (OLC), are beds of crushed limestone that are exposed to oxygen. Earlier studies on OLCs concluded that these types of drains have limited effectiveness in treating AMD because of armoring of limestone with iron hydroxide. However, recent studies by Ziemkiewicz (1994) have shown that, if sufficient contact areas are provided, OLCs can be effective for long-term treatment of AMD. Anoxic limestone drains (ALD) are beds of crushed limestone from which oxygen is excluded, creating reducing conditions in the drain. The reducing conditions prevent formation of iron hydroxides' and armoring of the limestone. ALDs have been shown to be effective for treating AMD. However, these types of drains are more delicate systems and, even without the introduction of oxygen, can precipitate aluminum hydroxides that can plug the drain. The inefficiencies associated with either OLC or ALD drains can be greatly reduced by oversizing the drains. The application of a limestone drain to treat the No. 1 Tailing Dam seeps will require further engineering design based on current seep water quality and discharge rate u= ? - - • =_=r N OT N Environmental