GENERAL33578 - Laserfiche WebLink

Home Browse Search

E. Probable Hydrologic Consequences The Eastside Mine is located in Garfield County, approximately 3'fz miles north of Silt, Colorado. The proposed mine would have affected 167 acres of non-federal land during the life of mine. This underground mining operation would have resulted in 12 acres of surface disturbance with the surface facilities and roads. The mine is located in the Harvey Gap Drainage. The topography in the mine area is very steep, with the elevation ranging from 7846 feet to the east of the mine along the Grand Hogback down to 6350 feet at the portal area. Slopes in the area range from 10 to 50 percent. The Harvey Gap Drainage is a deeply incised north- south trending stream valley, which cuts through the resistant sandstone of the Grand Hogback. The Harvey Gap Drainage is tributary to the Colorado River. The total natural drainage azea of this stream is approximately 52110 acres (8.1 square miles). The Harvey Gap Drainage is a perennial stream with six small tributary ephemeral drainages. The nature of this drainage has been altered by construction of canals and a reservoir for irrigation. The Grass Valley Reservoir is located 600 feet upstream of the mine site on Harvey Gap Creek. This reservoir has a design storage of 5800 acre-feet. The water is impounded during high flow seasons from the headwaters of Harvey Gap and from water diverted from the headwaters of East Rifle Creek through the Grass Valley Canal. The impounded water is released from April 15 to October 15 to irrigate almost 6000 acres of land in the Silt and Rifle area. The Grass Valley Reservoir is constructed on the relatively impermeable Mancos Shale Formation. The Eastside Mine is located downstream of this reservoir, on the Williams Fork Formation of the Mesaverde Group, which overlies the Mancos Shale. The rock strata dip steeply to the south and the strata are younger in age in the down gradient d'uection as shown in Figure 1. As the drainage passes through the axis of the Grand Hogback Monocline, the topography flattens as the Tertiary- aged Wasatch Formation is encountered at the surface. This formation lies beneath the floodplain of the Colorado River and is in some cases overlain by as much as 100 feet of Quaternary alluvial deposits. The Eastside Mine was developed in coals of the Iles and Williams Fork Formations of the Mesaverde Group. The Iles Formation intertongues with and overlies the Mancos Shale in the azea. The Iles Formation is typified by marine to non-marine sandstones, siltstones, shales, and coals. The Williams Fork Formation is predominantly non-marine sandstone, siltstones, mudstones, shales, and coals. Potential aquifers in the mine area are found within the Iles Formation, Williams Fork Formation, Wasatch Formation, and the Colorado River alluvium south of the permit azea. The relatively permeable sandstone and coal units of the Iles and 11