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Jim Stark <br />Page 2 <br />September 12, 2006 <br />The Trout Creek Sandstone is an aquifer of regional extent and underlies most of the <br />mining impact azeas. The potential impact from mining activities to the Trout Creek <br />Sandstone is limited. The Trout Creek Sandstone in under confined conditions. The <br />Trout Creek Sandstone aquifer is separated from the lowest coal seam to be mined by <br />approximately 590 feet in south Taylor. Units below the lowest coal seam to be mined <br />consist of sandstones, shales, and coals. The shale units have low permeability and may <br />be a barrier to subsurface migration of leachate. As indicated in the updated PHC for PR- <br />02, about 165 feet above the Trout Creek Sandstone is a two-foot thick smectite clay <br />layer (Km) that is laterally extensive through the area. This layer has been found to be of <br />low permeability and is an additional impediment to downward groundwater flow. <br />According to the revised PHC drilling by Colowyo between 1995 and 2005 has found <br />that no significant water is encountered in the South Taylor pit until drilling proceeds <br />below the ICm bed. For these reasons described above, it is unlikely that the Colowyo <br />operation will impact the Trout Creek Sandstone. <br />It is the belief of the DRMS that coal mining operations at Colowyo do not have the <br />potential to negatively impact bedrock groundwater, and that no point of compliance <br />need be established for bedrock groundwater. <br />Backfill and Excess Spoil Groundwater <br />The Colowyo coal mine is a multi-seam, open-pit, operation. The pits are excavated to <br />approximately 330 feet deep, and are then backfilled to approximate original contour. <br />The original stratification of geologic units is not reconstructed during this backfill <br />process. Placed backfill materials are not as dense as in situ geologic formations <br />(materials are blast cast, back-cast by dragline, end-dumped from trucks). Being less <br />dense, they have more voids, are more permeable, and have higher transmissivity values. <br />In addition, the 300-foot backfill section becomes a more homogenous section. As <br />precipitation penetrates these areas it becomes groundwater. <br />Such groundwater would develop as a result of the mining method, and is not considered <br />a pre-existing groundwater aquifer/saturated zone thought would be impacted by the <br />mining. These zones of developed groundwater are not deemed aquifers and the water is <br />known to be of degraded quality. Thus the belief of the DRMS is that points of <br />groundwater compliance aze not required in areas of mine pit backfill. <br />During the open-pit mining process, the replacement volume of mined materials expands <br />by approximately 20%, requiring the development of excess spoil fills for the excess <br />material. These fills approximate the pit depths in thickness, and are similar to <br />chazacteristics of backfilled areas. However, they include underdrain systems designed to <br />minimize groundwater reservoirs. In the event that groundwater reservoirs do develop, it <br />is the belief of the DRMS, as with backfilled azeas, groundwater points of compliance are <br />not required in excess spoil fills. Spoil groundwater is of known degraded quality and is <br />