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West Elk Mrne • Conceptual Description of Subsidence Impacts to Springs The key to evaluating whether or not any given spring will be adversely affected by the E Seam mining in the pemut azea is to define its elevation relative to both the mine seam and the ground surface. Essentially, the higher the elevation of a spring (and spring source), the smaller the probability of adverse impact as a consequence of fractures that extend upward from the mine. However, for a spring to avoid impacts from subsidence, in addition to being above the mine fracture zone, the source of the spring needs to be far enough below the ground surface to avoid surface fractures that may be the result of subsidence. This phenomenon is described by Liane Kadnuck (U.S. Bureau of Mines) in her paper, Response of Surface Springs to Longwall Coal Mining Wasatch Plateau, Utah, where she states, "The limited spring response observed at this site may be attributed to several factors; (1) The thickness and composition of overburden present which contained a massive competent sandstone, which causes bridging in the overburden, limiting subsidence, (2) Presence of formations containing swelling clays, and (3) The elevation of the spring above the elevation of estimated heights of fracturing and caving in the overburden." This basic concept is elaborated as follows. Within the South of Divide mining area, the overburden thickness for the E Seam ranges from 400 to 1,200 feet. Springs (and their sources) can be affected by subsidence in two different ways. First, fractures can extend upward from the mine seam and intersect the spring or spring source. As explained in • Sections 2.05.6 (6)(e)(i)(C&D), Subsidence Zone Description, the height of the caved/fractured zone extending upwazd from the mine seam is conservatively estimated at 280 feet. Secondly, there is a small probability of surface cracks developing in association with the mining (see Section 2.05.6 (3)(b)(iii & Viii) Streams for estimate of surface crack probability) and the typical maximum depth of such cracks in the South of Divide mining area is conservatively estimated at 25 to 35 feet. These two distances indicate that, for a spring to avoid any potential impacts, the source of the spring must be at least 280 feet above the uppermost seam that is being mined and more than 25 to 35 feet below the ground surface. From a practical standpoint, because every spring (by defmition) "daylights" at the ground surface, there is an extremely small risk that any given spring will encounter a surface crack. However, it is essential to recognize the following: • Site specific evidence demonstrates that surface cracks are most likely to occur over chain pillazs, barriers and mine boundaries. Consequently, if a spring and its source are not located in these azeas, they will be unaffected by surface cracks. This is true for most of the springs in the South of Divide mining azea. • The spring source could easily be unaffected by a surface crack, even if the spring itself is affected (at the point where it surfaces). In this case, the spring would re-emerge at another location. ~I 2.05-166 Revised November 2004 PR/0