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• The difference between indicated numbers is caused by <br />variable geological conditions. A ratio between brittle <br />(sandstone) and plastic (claystone) rocks in the overburden <br />has an important impact on the fracture development. More <br />plastic rock strata tend to bend without fracturing. <br />7'he increase of permeability in the zones affected by <br />subsidence can be substantial. The magnitude of increase <br />varies from about 10 to 230 times the pre-mining values <br />(Whittaker et al, 1979, Sikora, 1976, Neate and Whittaker, <br />1979). However, several studies indicated that this increase <br />is only temporary and close behind the mining face there is <br />an indication of increasing consolidation (40 meters behind <br />the Eace of longwall panel (Whittaker, 1979)). <br />The ground water quality can be altered due to <br />subsidence principally in the two following ways: <br />o caved overburden material replaces part of the coal <br />seam aquifer <br />o fracturing develops a new hydraulic connection <br />between aquifers with different water qualities <br />The first example is of interest only in cases where <br />• the coal seam acts as an aquifer. The second example applies <br />to situations where several confined aquifers ,;ontain ground <br />water of a substantially different quali*_y. As no aquifers <br />exist in the Mesaverde, the impacts of subsidence are <br />apparently negligible. <br />In addition to ground water in the Mesaverde Formation <br />it is necessary to take into account potential impacts of <br />subsidence on the aquifer in Stevens Gulch, the alluvial <br />sediments supplying water to CWI, Morrel's ranch and the <br />Muckle House. <br />Unlike the ground water in the Mesaverde Formation, <br />this aquifer could be endangered by the occurrence of <br />surface subsidence cracks. Such cracks ~+ould not necessarily <br />have to propagate to the surface to disrupt this aquifer; <br />even if they are limited to bedrock formations, they could <br />alter the ground water regime significantly. The geologic <br />and hydrologic investigations performed by CWI in Stevens <br />Gulch indicate that the lowest layers of the alluvium are <br />formed by impermeable layers such as clay and weathered <br />r~tesaverde. The impermeable layers would reduce the impact of <br />cracks that occurred beneath the alluvium. Ho~dever, it <br />cannot be ruled out that the occurrence of subsidence cracks <br />could disrupt this aquifer. H~sling of cracE:s could take <br />• <br />- 4G - <br />