2011-03-14_REVISION - M1977493 (62)

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j CM0001631 Page 3 F., estimated to be approximately 1.0 million cubic yards (mcy)(Climax, 1994). Oxide tailing deposition in the reservoir has resulted in impacting the water quality within the reservoir. A summary of water quality, with depth, within Eagle Park Reservoir is presented in Table 1. As shown, the water quality data indicate the water quality worsens with depth. No. 1 Tailing Dam The northeastern boundary of the ERV is bounded by the No. 1 Tailing Dam. The No. 1 Tailing l Dam is an upstream method dam, and currently has a maximum height of approximately 230 feet with a crest elevation of 11,145 feet msl. Collection pipes and horizontal drains have been added to the dam over time [1968, 1979, 1980, and 1985 (WCC, 1988)] to control seepage. Seep water emerging near the dam toe is collected, directed toward and discharged into Robinson Lake. The water quality of the seep water is shown in Table 2. As shown, the seep water is of poor quality with characteristically low pH ranging from 3.41 to 4.60, and high concentrations of metals and sulfate. Robinson Lake Robinson Lake covers an estimated 80-acre area and has a storage capacity of 3,100 acre-feet d (Wheeler, 1994). The dam was constructed in 1936 with an initial storage of 2,494 acre-feet and expanded to its current capacity in 1955. The Robinson Dam is a zoned earthen embankment with an impervious inner core, and a semipervious outer shell. Historical records indicate that cutoff trenches were excavated into weathered bedrock along the entire dam foundation. The cutoff trenches were backfilled with the same material used to construct the inner impervious core. In addition to cutoff trenches, a concrete cutoff wall was constructed into the bedrock beneath the dam to control subsurface seepage. U. u is P, Currently, Robinson Lake accepts intermittent discharges of process water from the Tenmile Tunnel and the West Tailing Delivery Line. In addition, Robinson Lake intercepts the No. 1 Tailing Dam seep water, as well as surface water runoff during precipitation events. Water quality data for Robinson Lake are unavailable. However, the discharges entering the lake are generally of poor quality. Therefore, the water quality of Robinson Lake is likely to be of poor quality. In addition to likely poor water quality, Robinson Lake has accumulated over an estimated 1.4 mcy of sludges of unknown quality. Sludges have accumulated as the lake has been used as a settling basin in the water treatment circuit at the mine. A= === = gon I .. l Environmental • •