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<br />~J <br />r 1 <br />LJ <br />• Impoundment Leakage <br />?he ?ILRD has expressed concern that excessive leakage will occur <br />recirculating pond. Sone leakage is inevitable. However, it apps <br />will be minimal. It 1a anticipated that the pond bottoms will be <br />self-sealing. The fines and sediments will settle out of the mini <br />drop to the pond floor. The existing material that was removed f~ <br />bottoms was laboratory tested and found to be a low plasticity cli <br />type which typically exhibits low permeability. Permeability tell <br />conducted on the material, since it had already been disturbed. Be <br />this disturbance, lab tests will not yield a reliable approximati< <br />will occur in the field. <br />• Operating Level/Freeboard <br />The maximum operating level of the nine operations will be malnta <br />three feet below the crest of the pond embankment. Three feet of <br />substantially more than is required to contain the 5-year, 6-hour <br />From the area tributary to the pond embankment. However, the thr <br />figure is held to be reasonable in view of embankment stability, <br />and emergency surcharge reasons. As demonstrated in a prior repo <br />amount of runoff over the 150 acre contributory watershed above t <br />is approximately 1.1 acre feet in a 5-year, 6-hour storm event. <br />estimated 800 foot by 200 foot water surface area, the storm even <br />less than one foot of depth in the ponds, leaving over two vertic <br />freeboard. <br />OSSITE DRAIIiAGE <br />rom the <br />!rs that it <br />essentially <br />water and <br />~m the pond <br />y (CL) of a <br />s were not <br />:ause of <br />of what <br />ed at least <br />reeboard is <br />Corm runoff <br />foot <br />ve action <br />, the <br />nine area <br />van the <br />occupies <br />feet of <br />1fI.RD has expressed concern that not all of the mine site will actu ley drain <br />to the recirculating ponds. A slope study was performed which ver fied that <br />all drainage form the mined area in Phase I will flow into the imp undment. <br />Topography considerations make it virtually impossible to have the entire <br />permit area drain into the ponds, end it only necessary to control runoff from <br />the disturbed areas. Runoff from the disturbed Brea will be direc ed to the <br />ponds. When the latter stages of the mine plan are reached, new p nds <br />probably will be required and they will be slzed and sited at that time. <br />Ifinor sedimentation ponds will be used to collect runoff waters if <br />necessitated by the topography. <br />BIHER FLOODING <br />!f[.RD has expressed concerns that embankments in the mine site may e <br />endangered fro^ the 25-year flood of the ?fiddle Fork of the South latte <br />River. As stated in a previous report, the river has a 33.3 squar mile <br />watershed. This was calculated Ero^ the USGS 1:250,000 scale mapp ng and <br />includes all erects tributary to the South Platte from its crossing under <br />Highway 9 and just south and east of Alma. The 25-year flood was ased on a <br />regression analyses developed by the United States Geological Surv y. She 25- <br />3 <br />