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., <br /> <br />and/or placing clay soil fill on the benches and over the exposed coal (and auger boring <br />openings)..:' <br />Many of the above concerns relate directly to the ground water hydrology of the area of the <br />West Pit low wall and the adjacent bedrock, colluvium and the fill. The Division suspects <br />that the ground water hydrology of the foundational area of the fill has been impacted by <br />the excavation of the pit and the in-pit ponding of surface water runoff. In the long run, as <br />the pit deepens a portion of the ground water flow now reaching the unstable slopes <br />adjacent to the fill toe may be diverted deeper in the section. In the short term, during the <br />excavation of the lower seams, the creation of the X-Seam and D-Seam benches, and the <br />angering of the X Seam and C or D Seam, ground waters may be diverted to various seams <br />in greater quantities which could result in exacerbation of the adjacent slope stability <br />problems. Without monitoring wells (piezometers) within the fill and the underlying <br />bedrock adjacent to the low wall of the West pit most of this is gross speculation. Colowyo <br />Coal is committed to the installation of a piezometer on the First Bench of the West Pit <br />Fill. The sooner this piezometer is installed, the more information Colowyo could accrue <br />relating to the relationships of the pit and the adjacent ground water level within the fill. <br />In conclusion, the Division needs the following issues resolved prior to issuance of the <br />revision; <br />1. A few subsidence monuments should be installed in a representative area to verify <br />whether the auger webs have remained stable. Please indicate the proposed location <br />of the monuments on a map and a description of the monument construction. <br />2. Colowyo needs to address CTL/Thompson's original evaluation that a layer of <br />claystone/shale fill material should be placed on each bench where coal is exposed <br />to 'seal' the coal layer and direct free water to the south. This statement appears to <br />be in contradiction to Colowyo's statement that groundwater in the pit or from within <br />the auger holes will not pose a problem as the Colowyo pit is essentially dry. <br />The Division's decision due date for this revision is April 9, 1996. Please respond to the <br />concerns noted above prior to this date. <br />If you have any questions, please do not hesitate to call. <br />Sincerely, <br />Erica S. Crosby <br />Environmental Protection Specialist <br />c:\wp51\colowyo\960229 <br />