2011-06-20_REVISION - M1990057 - Laserfiche WebLink

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TAILINGS DISPOSAL Operating History A 4.6 acre tailings disposal facility (TSF) was approved and phase 1 of the TSF was constructed in 1990. This first phase facility is located immediately south of the grinding /flotation mill and is approximately one acre in size. The TSF currently holds approximately 1,000 tons of tailings material. This TSF was phase one (1) of several phases for the development of the 4.6 acre disposal area. A total of 330,000 tons of tails were planned for the TSF. A tailings impoundment dam was constructed from available onsite soils borrowed from the tailings disposal area as outlined in the recommendation of CTL /Thompson, Inc., Consulting Geotechnical and Materials Engineers, Permeability Study Tailing Pond Area, April 10, 1990, presented as Appendix A. The tailings disposal area is underlain by thick lenses of highly impervious silty clay with enclosed, thin, dense sand beds with occasional pebbles. The 1990 CTL Thompson study of the native materials in the TSF indicated 2 -9 ft of stiff, sandy clay underlain by dense, clayey gravel and clayey, gravelly sand and permeability in the 2 to 7 ft/year in the non - permeable clays and permeability in the 5000 ft/yr, in localized permeable 1.5 ft thick gravel lenses at 8 ft of depth on the north edge of the facility. The tailings dam embankment was constructed of compacted high clay content alluvial material as recommended in Section 34, "Tailings Disposal" of SME Mineral Processing Handbook, N.L. Weiss, Editor, 1985. The pond bottom was constructed of local clays, with no synthetic liner. CTL/Thompson, Inc. conducted embankment compaction tests and presented the results in a July 27, 1990 report, Percent Compaction Measurements Existing Tailing Pond Embankment, presented as Appendix B. The report concluded the average compaction percent was 90.1 for the 10 embankment test locations. Two of the test locations were not well compacted, averaging 75.1 %, with the other 8 test locations averaging 93.9 %. A subsequent inspection by CMLRD, indicated 92.5% of Proctor Density as acceptable for the tailings disposal area (Memo presented as Appendix C). The plan for the 4.6 acre tailings disposal area was to construct an embankment to utilize about only one acre of pond area at one time with drainage of tails water going to a suitable collection point where a collection pump will reclaim the water for re -use in the milling process. These affected areas of about one acre were planned to be somewhat pie- shaped, and when the near limit of capacity is reached, another pie- shaped section would be cleared, and a proper embankment of compacted material constructed. The TSF facility, based on investigations by the initial mill owner and CTL Thompson, Inc., concur that no adverse impact on surface or ground water would occur with construction of the tailings disposal area as proposed in the permit. This area is geologically defined as old lake beds. It is well described in USGS Professional Paper 148 by Emmons, Irving & Laughlin, 1927. Communication with the Colorado State Engineers, Ground Water Section, in 1990, indicates the first known aquifer in this area to be 80 to 100 ft below surface. This data is based on well logs of the nearest water wells to the mill site. CTL Thompson, Inc. determined permeability rated over the majority of the proposed tailings disposal site to be as low as 2 to 7 ft per year, and initial construction did not include a synthetic liner. M1990 -057 June 6, 2011