2010-01-25_PERMIT FILE - C1981044A (4)

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Sandstone Aquifers. The term "aquifers" as defined by 30CFR817.126 and by Rule 4.20.4 are renewable resource lands that serve as a "significant source of water supply to any public water system". However, as discussed in Section 2.04.7, Hydrology Information, the sandstone aquifers which underlie the EC permit area do not serve as a "significant source" or even a minor source of water supply to a public system and as such cannot be classified as a renewable resource land. The four {4) major sandstone aquifers within the permit area are 1) Trout Creek Sandstone, 2) Twentymile Sandstone, 3) Middle Sandstone, and 4) White Sandstone. The depth of the sandstone aquifers varies throughout the mine area. The average thickness for the major sandstone aquifers is as follows: 1) Trout Creek Sandstone - 75 feet, 2) Twentymile Sandstone - 75 feet, 3) Middle Sandstone - 90 feet, and 4) White Sandstone - 300 feet. The Trout Creek Sandstone lies below all mineable coal seams within the permit azea. The Twentymile and Middle Sandstones lie above the proposed mining horizon. However, they both iie sufficiently above the "E" Coal Seam (No. 6 Mine) and the "F" Coal Seam (No. 5 Mine) so as to be situated in the zone least impacted by mining. It should be noted that a portion of the planned five- (5) year (1998 - 2003) mining in the No. 6 Mine lies outside the outcrop of the Twentymile Sandstone. Refer to the Subsidence Map (Map 30). A detailed discussion of the hydrology of the sandstone units is presented in Section 2.04.7, Hydrology Information. Description of Worst Possible Consequences of Subsidence if it Occurred on Structures or Renewable Resource Land The results of the Subsidence Survev presented later in this section indicated that no material subsidence damage or diminution of reasonable foreseeable use, as defined in Rule 2.05.6(6)(e)(ii)(A), will occur to the structures and renewable resource lands that have been identified as a worst possible consequence of subsidence either planned or unplanned. This conclusion is based on the subsidence and hydrologic monitoring experience at EC, including both longwall and full extraction room-and-pillar mining. Pre and Post-Minine Land Use There are presently a variety of land uses within the planned mining area. In addition to the mining, which EC is conducting in the area, the surface is utilized for residential purposes, grazing of livestock and cultivation of wheat and hay. The area also serves as a transportation corridor containing a state highway, an oil pipeline and a railroad spur line. Planned mining and subsidence is not expected to have a significant impact on any of these uses. In all of these cases, the post-mining land use is expected to remain the same as the pre-mining land use. Worst Case Subsidence Description Whenever a large quantity of material is mined underground, the surface as well as the overburden above the coal seam will move in response. This ground movement will consist of both vertical and horizontal components. The current and planned longwall mining conducted at EC is designed to maximize extraction of the coal resource. All underground support is removed from the extraction area. The overburden strata caves into the void and the surface subsides in proportion to the ability of the overburden strata to "bulk-up" and compensate for the extracted coal thickness. Therefore, the maximum subsidence that can occur will occur. Planned subsidence is anticipated to result in the development of surface subsidence troughs over single and multiple longwall panels. Minor surface cracking may occur where overburden and surface soil cover are thin. However, this surface cracking will be transitory in nature and should close up or be filled in by surface wash within 12 to 24 months. The worst possible consequences of subsidence are as follows: • For the maximum extraction height of 12 feet in the No. 6 Mine longwall panels, the maximum longwall Permit Revision 04-34 2.05-50 Revised 7/2/04