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• The ground water quality can be altered due to subsidence principally in the two <br />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 the coal seam acts as an aquifer. <br />The second example applies to situations where several confined aquifers contain ground <br />water of a substantially different quality. As no aquifers exist in the Mesaverde, the <br />impacts of subsidence are apparently negligible. <br />In addition to ground water in the Mesaverde Formation, it is necessary to take into <br />account potential impacts of subsidence on the near surface colluvial aquifers. <br />Unlike the ground water in the Mesaverde Formation, this aquifer could be <br />• endangered by the occurrence of surface subsidence cracks. Such cracks would not <br />necessarily have to propagate to the surface to disrupt this aquifer; even if they are limited <br />to bedrock formations, they could alter the ground water regime. The geologic and <br />hydrologic investigations performed in Stevens Gulch near the Bowie #1 Mine indicate that <br />the lowest layers of the alluvium there are formed by impermeable layers such as clay and <br />weathered Mesaverde. The impermeable layers would reduce the impact of cracks that <br />occurred beneath the alluvium. However, it cannot be ruled out that the occurrence of <br />subsidence cracks could disrupt this near surface ground water. Healing of cracks could <br />take many years and there is little probability that any man-made mitigation is feasible <br />because of the considerable thickness of the alluvial deposits. <br />In summary, the impacts of potential subsidence on ground water in the Mesaverde <br />Formation are negligible. Potential fracturing of the formation and the local increase of <br />permeability above the extracted coal seam would only affect potential perched water <br />bearing zones. As none of them has been used in the past and is not likely to be used in <br />the future, the impact is negligible. The subsidence impact on near surface colluvium <br />appear to be negligible. <br />• <br />- 23 - 10!00 <br />PR-o v <br />t <br />