Laserfiche WebLink
i) As stated in the mine plan narrative, the lower pit area, excavated in the white <br />mazble bedrock, has shown a high degree of permeability due to fracture and <br />jointing associated with mountain building earth forces. The current pit <br />excavation contains a potential storage volume on the order of 10 acre feet. <br />This volume appeazs to be about half of a 10 year, 24 hour event estimate for <br />the storm analysis results sent to the writer for this site, estimated to be in the <br />range of 16-18 acre feet. Colorado Mazble proposes to maintain a minimum <br />storage volume of ibis magnitude as mining proceeds northward. In the <br />unlikely event that a 100 yeaz, 24 hour event might fill the pit azea to the <br />lower staging area elevation, water would flow across the bedrock contact <br />southwazd along a path paralleling the white mazble bedrock outcrop along <br />the lower staging azea, F.S. Road 228, and down gradient to Taylor Gulch <br />drainage. Colorado Mazble proposes to maintain a potential flow path on <br />bedrock material where possible, from a point along the eastern side of FS <br />Road 228 and lower staging area contact to Taylor Gulch. <br />ii) Colorado Mazble maintains that historic record and operating experience offer <br />the best technical assessment regazding permeability rates for a structure of <br />this size and type. The mine operator proposes to maintain a photographic <br />log of lower pit conditions taken during or immediately after major storm <br />events or high snow melt conditions to support its contention that the lower pit <br />bedrock displays a high rate of percolation and will meet OSE 72 hour <br />requirement. During the past twenty yeazs of this structure being in place and <br />operating as described, no water has been noted standing in the bottom of the <br />lower pit, even after lazge storm events and high snow melt periods. <br />iii) Colorado Mazble contends that expanding the minimum tower pit storage <br />capacity to no less than 35 acre feet and maintaining that minimum storage <br />capacity during the life of mine operation and as a final closure structure left <br />in place after mining will add a high degree of certainty that no water will <br />spill from the lower pit during a 100 yeaz/24 hour storm event. <br />iv) As mentioned above, Colorado Mazble, Inc., plans to keep a minimum 35 acre <br />feet storage capacity in place in the lower pit through the life of mine <br />operation and left as a final site control after mining for storm related surface <br />water dischazge control. During the first several yeazs of mine operations <br />from present, the lower pit wi111ook little different than at present, save for <br />extension of the pit to the north as mining progresses. As more storage <br />capacity is gained over time due to mazble removal, the bottom level mine <br />benches will be backfilled and graded to slopes of no steeper than 3:1, capped <br />with topsoil no less than 12" in depth, and seeded per approved native grass <br />mixture. Backfilling of the lower mining benches will be performed to help <br />buttress the lower reaches of the pit excavation faces on both footwall and <br />hangingwall sides of the ore body. Backfilling operations in the lower pit will <br />commence only when the minimum 35 acre feet of storage within the <br />remaining pit area can be maintained. Final flow path from southern end of <br />