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SubTerra, Inc. Peabody Twentymile Coal LLC 09/27/2014 Foidel Creek Mine Permit Revision for Longwall Mining in the Wolf Creek Seam - Subsidence Evaluation 6. Evaluation of Potential Subsidence Impacts to Water Wells 6.1. Local Hydrology The Foidel Creek mining area is part of a larger enclosed groundwater basin containing five significant groundwater aquifers, the Fish Creek Sandstone, Twentymile Sandstone, the Wadge Seam, the strata immediately above and the underlying Trout Creek Sandstone, and the Wolf Creek Coal Seam. These aquifers are confined by shale layers above and below. However, joints and faults may serve as conduits between lithologic units. The Twentymile Sandstone outcrops above the Foidel Creek mine forming the cliffs to the south of the mining area. Aquifers of interest in the mining area therefore include the Wadge Seam, the underlying Trout Creek Sandstone, and the Wolf Creek Seam which is being extracted. 6.2. Coal Mine Roof Collapse and Near Seam Effects on H drolo As noted in Section 2, as a longwall face retreats, the immediate roof strata separate at bedding planes and pre - existing sub - vertical discontinuities and cave behind the face supports. Caving propagates until it is arrested through bulking and /or intersection with more competent overlying beds. As the longwall face retreats further beyond the section, the overlying beds fail in bending and deform towards the centroid of the excavation ultimately resulting in subsidence at the ground surface. Changes in hydrology will occur in and immediately above the extraction area proportional to the degree of fragmentation with the largest increases in transmissivity associated with the zone of caving. 6.3. Prior Experience There is a wealth of information in the literature regarding impacts to water wells from underground room and pillar and longwall coal mining: • The onset of permeability change occurs well in advance of observed surface subsidence, usually when the panel is a distance equal to seam depth from the well. • Wells completed in the zone of caving typically dry up as the longwall approaches i.e., prior to the coal being mined. • Experience at the Foidel Creek mine suggests that wells completed in and just above the coal seam and within 500 -feet of the mining area will be affected during panel development. • Little response to mining for shallow wells. Higher permeability change in the tensile zone (near the headgate and tailgate) than in the compressive zone (near the panel center)5. • Shallow wells may experience depressed water levels but generally recover in 2 to 3 months. • Undermined wells typically experience an increase in specific capacity. 5 Booth, .J., 1986. Strata Movement Concepts nd the Hydrological Impact of Underground Coal Mining, Ground Water, V. 24, No. 4, July -Aug, 1986., pp 507 -515. 16 PN: 2013 -26