2011-09-09_REVISION - M1990057 (3)

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Recently drilled TH -2, located on the existing TSF disturbance embankment, ended in water during the spring melt. TH -2 is drilled to a depth of 30 feet from the top of the existing embankment (12ft below the bottom of the impoundment) and ends in a layer described as "sandy clay overlaying clayey sand and gravel ". TH -2 permeability tests were not conducted. TH -2 encountered water at 25 feet, and subsequent measurements have shown water at 28 feet of depth. UMC believes the above test holes all confirm colluvium/alluvium within the TSF area, yielding unsorted sediment of varying particle sizes ranging from boulders to clay and areas of high and low permeability. Snowmelt and storm runoff water saturates the hillside and colluvium in wet periods and migrates through the colluvium/alluvium, creating the observed temporary perched water conditions. After a short period of time, the water migrates to areas lower in the colluvium/alluvium and no perched water is observed in the monitoring or tests holes. UMC has not observed artesian flows from the above wells or any evidence of movement of water in any wells resulting from adjacent hydrostatic pressures in the area. The waters are unconfined, leading to the temporary perched water condition, and there is no water gradient. The excavation of the TSF will disrupt portions of the colluvium/alluvium structure and the perched water conditions within the excavation. Excavation is anticipated to expose clay overlaying gravel lens, clayey gravels and clayey sands. The high permeability materials will be removed and segregated as material for stockpile and/or other fill operations not associated with the TSF construction. The low permeability clays will be removed and segregated for use in the clay liner. Should a gravel lens be exposed in the bottom or sides of the excavation, the lens will be over excavated to 3 feet, backfilled and compacted with clay liner material to block the water conduit. The initial excavation cut and fill operation, material stockpiling, embankment placements and other similar operations will be under the observation and supervision of either a geotechnical engineer /technician or UMC's owner engineer. It is anticipated that there will be no groundwater discharges. If groundwater is encountered during construction and cannot be infiltrated on site, UMC will apply for a Groundwater Discharge Permit from the State of Colorado Department of Public Health and Environment and notify the Division that the ground conditions are other than anticipated. The TSF liner system is designed to protect the temporary perched water conditions with a 12in clay liner, leak detection system and the 45mi1 polypropylene geomembrane liner. The TSF, as designed, is a 2 -way barrier, maintaining the tailings within the impoundment and maintaining temporary perched water conditions outside the impoundment. The leak detection system is also a 2 -way safety system, indicating the structural integrity of the 45mi1 liner and indicating the structural integrity of the clay liner. Fluid detected by the leak detection system can be clearly identified as either tailings fluids or groundwater fluids. CDRMS question 4. Page 6, Direct Shear Testing: The second paragraph indicates cyclone tailings will be placed adjacent to the embankment. This statement contradicts the statement on page 1 about the clay liner (see comment No. 1 above). The TSF cross - sections on Drawing TSF 04 do not differentiate between slime and cyclone sands as does Figures B -3 and B -6 in the slope stability analyses. Please provide DRMS with consistent narrative, drawings and figures, including a detail section of the interior face of the tailings pond showing expected thickness and order of the liner and cyclone sand layers proposed on the embankment. This will also be helpful in understanding and evaluating the proposed leak detection system.. CTL Thompson Response (September 1, 2011 letter) The cyclone tailings will be placed on top of the man-made liner on the inside face of the embankment. We differentiated between cyclone sands and slime in our initial stability analysis because we thought the M1990 -057 September 9, 2011 TR3AdequacyReview.Question 19UMCResponse.Sep9.jme.0o4