2007-02-14_PERMIT FILE - C1981044A (4)

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lithologies. Ground water flow in [he regional area should conform generally [o the structure as shown on the maps and cross sections presented in this permit. TOPOGRAPHIC SETTINGS The EC mine site and adjacent area lie at the foot of the prominent Williams Fork Mountains, which trend east to west immediately to the south. North of [he Williams Fork Mountains, a gradual, north sloping surface runs down through the broad Big Bottom Valley of the southward flowing Yampa River. West of the Williams Fork Mountains the Williams Fork River flows northward in a narrow valley to ajunc[ion with the Yampa River at the southern end of Big Bottom. A northwestward trending ridge situated west of Big Bottom and the Williams Fork Valley is cut by [he Yampa River, forming a narrow, fairly steep walled canyon. Just west of this ridge, the Yampa River flows through Round Bottom. REGIONAL GEOLOGICAL SETTING The EC Permit area is situated between two regional geologic features, 1) the Axial Basin Arch (anticline) and 2) Sand Wash Basin, which lie to the southwest and northeast respectively. The regional geologic setting is presented graphically on the Regional Geology and Hydrologic Monitoring Program Location Map (Map 11). The EC Mine site is located on three (3) smaller structures, these are 1) the Williams Fork anticline, 2) the Round Bottom syncline, and 3) Big Bottom syncline, which are superimposed on the northeastern limb of the axial basin arch. Major faults are known to exist just north of these anticlinal and synclinal swctures. The trend of most of the geologic structures in the area is to the northwest. Regional dips through the area are generally northward at 7 to 10 degrees. The bedrock units exposed in the permit area are predominantly cretaceous in age and include from oldest to youngest the Mancos Shale, the Iles and Williams Fork formations of the Mesa Verde group, and the Lewis Shale. The major surficial geologic unit of the vicinity is alluvium along the course of the Yampa River and its tributaries. GENERAL HYDROLOGY Ground Water Ground water occurs at the EC Mine site in bedrock aquifers and in the alluvium. The bedrock aquifers include sandstones and fractured coals in the Iles and Williams Fork formations. Less significant shale aquifers are found in the Mancos and Lewis Shales. The Yampa and Williams Fork River alluviums appear to be saturated. However, the permeabilities and water yields are so low in the vicinity of the EC Mine [hat these alluviums cannot be considered significant aquifers. Well yields in the bedrock aquifers near the mine site are generally low. The aquifers appear to have low primary permeability, but locally they can have high permeabilities. These locally high permeability zones appear to be secondary permeabilities produced by fracturing. Well yields are generally less than 25 gallons per minute (GPM). A few high yield test holes with yields up to 200 gpm have been encountered. The permeability and yield of the alluvium near the mine site are very low. Test wells in the Yampa and Williams Fork alluvium all had yields of less than three (3) gpm and most had yields of less than one (1) gpm. Higher well yields for [he Yampa River alluvium have been reported in other areas. Well yields as high as 900 gpm have been reported for wells drilled east of Craig, Colorado. The direction of flow of ground water in the bedrock aquifers appears to be in a southwesterly to northwesterly direction. The general direction of flow may be locally altered by outcrops, producing wells, or mine dewatering. Permit Revision 04-34 2.04-3 Revised 7/2/04