2008-03-31_PERMIT FILE - C1980007A (34)

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West Elk Mire Site Hydrogeoloey The most significant hydrogeologic units present within the West Elk Mine property include (from bottom to top) the Mancos Shale, Mesaverde Formation (including the Rollins Sandstone, Lower Coal Member, Upper Coal Member, and the Barren Member), and the colluvium and alluvium along the drainage side-slopes and valley bottoms, respectively (Map 9). A more detailed discussion of the stratigraphic units within the pen-nit area can be found in Section 2.04.6, Geology Description. The general significance of these stratigraphic units from a hydrogeological perspective is discussed below. The extent and orientation of these units is best depicted in a representative stratigraphic section through the permit area as compiled from drill hole information (Figure 4 in Section 2.04.6). This stratigraphic section has been prepared using the top of the Lower Marine Sandstone unit as a datum reference and the "best fit" method. This technique "matches," to the nearest degree possible, the rock types and sequences of lithologies portrayed on the graphic logs of adjacent drill holes. The Mancos Shale, which underlies the Mesaverde Formation within the West Elk Mine property, is a marine shale formation of considerable thickness (approximately 2,000 to 3,000 feet thick). Due to its extremely low permeability, this shale unit acts as an aquitard, inhibiting the downward migration of groundwater from the basal sandstone member (Rollins Sandstone) of the Mesaverde Formation. The Mesaverde Formation is typically 2,500 feet thick at the mine and consists of sandstone, shale, clay, and interbedded coal. As mentioned in Section 2.04.6 - Geology Description, this formation is the coal-bearing formation in the region and is divided into five main members; the Rollins Sandstone Member, the Lower Coal Bearing Member, the Upper Coal Bearing Member, the Barren Member, and the Ohio Creek Member (Dunrud 1989). Within these members, locally continuous permeable sands may contain groundwater, which generally is recharged in the active zone from meteoric waters. However, the discontinuous nature of the more permeable zones within this formation clearly indicate that the Mesaverde Formation at and in the vicinity of West Elk Mine does not contain any formations which could be classified as aquifers. Only the Rollins Sandstone, and to a lesser degree, the Upper and Lower Marine Sandstones between the Lower and Upper Coal Members, provide the most continuous sandstone units within the Mesaverde Formation. The colluviurn within the West Elk Mine property is generally comprised of locally derived materials carried down slope by rain wash, rock fall, landslides or other gravitational means. Colluvium is typically thin, but may be up to 30 feet thick ii landslide debris deposits. Numerous springs are known to emanate from these deposits, indicative of the active groundwater regime discussed previously in this section. Alluvial deposits within the coal lease area are generally confined to relatively continuous, but narrow strips in the lower reaches of the larger drainages and are usually less than 25 ft. thick. Where these deposits contain sufficient saturated thickness, the more permeable nature of these materials can yield sufficient water to sustain domestic and livestock supplies. Locally, only wells in the alluvium of the North Fork supply such yields. 2.04-58 Revised June 2005 PRIG, March 2006; Rev. Apri12006 PRIO, Sep. 2007PR12