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• I~pound~ent Leakage <br />The MLRD has expressed concern that excessive leakage will occur from the <br />recirculating pond. Some leakage is inevitable. However, it ap ears that it <br />will be minimal. It is anticipated that the pond bottoms will b essentially <br />self-sealing. The fines and sediments will settle out of the mi a water and <br />drop to the pond floor. The existing material that was removed rom the pond <br />bottoms ~+as laboratory tested' and found to be a low plasticity c ay (CL) of a <br />type whic typically exhibits low permeability. Permeability to is were not <br />conducted on the material, since it had already been disturbed. ecause of <br />this disturbance, 1 ests will not yield a reliable approximat'on of what <br />will occur in the field. <br />• Operating Level/FYeeboard <br />The maximum operating level of the mine operations will be maint fined at least <br />three feet below the crest of the pond embankment. Three Eeet o Freeboard is <br />substantially more than is required to contain the 5-year, 6-hou storm runoff <br />From the area tributary to the pond embankment. However, the th ee foot <br />figure is held to be reasonable in view of embankment stability, wave action <br />and emergency surcharge reasons. As demonstrated in a prior rep rt, the <br />amount of runoff over the 150 acre contributory watershed above he mine area <br />is approximately 1.1 acre Eeet in a 5-year,~6-hour storm event. Given the <br />estimated 800 foot 6y 200 foot water surface area, the storm eve t occupies <br />less than one foot of depth in the ponds, leaving over two verti al feet of <br />freeboard. <br />ONSITE DRAINAGE <br />MLRD has expressed concern that not all of the mine site will ac ually drain <br />to the recirculating ponds. A slope study was performed which v riEied that <br />all drainage form the mined area in Phase I will flow into the i paundment. <br />Topography considerations make it virtually impossible to have t entire <br />permit area drain into the ponds, and it only necessary to contr 1 runoff from <br />the disturbed areas. Runoff from the disturbed area will be dir cted to the <br />ponds. When the latter stages of the mine plan are reached, new ponds <br />probably will be required and they will be sized and sited at th t time. <br />Minor sedimentation ponds will be used to collect runoff waters 'f <br />necessitated by the topography. <br />RIVER FLOODING <br />MLRD has expressed concerns that embankments in the mine site ma be <br />endangered From the 25-year Flood of the ?Fiddle Fork of the South Platte <br />River. As stated in a previous report, the river has a 33.3 squ re mile <br />watershed. This was calculated Erom the USGS 1:250,000 scale ma ing and <br />includes all areas tributary to the South Platte from its crossi g under <br />Highway 9 and just south and east of Alma. The 25-year flood wa based on a <br />regression analyses developed by the United States Geological Su vey. The 25- <br />3 <br />