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70 Groundwater <br />1 <br />5.3.4 Groundwater levels in bedrock <br />Groundwater levels for the hydrogeological units described above have been measured <br />in various piezometers and wells installed in the Collom area. Groundwater elevations <br />are shown on Figures 5.11 through 5.14 for the F sandstone, F/G sequence, <br />H sandstone, and I sandstone, respectively. <br />The groundwater levels in the Collom Mine-are also illustrated in the cross sections <br />shown in Figures 5.2 through 5.5. Measured piezometric surfaces for each of the four <br />units that are being monitored (i.e., F sandstone, F/G sequence, H sandstone and <br />I sandstone) are indicated on the cross sections. <br />South area <br />In general, potentiometric levels are in or just above each sequence on the south <br />(upgradient) side of the Collom study area, and progressively rise above the sequence <br />downdip. <br />The southern portion of the mine is characterized by steep groundwater gradients and <br />perched water levels that are unconfined (water levels within the respective water <br />bearing units). Water level data for the southern portion of the proposed Collom Mine is <br />based on data from the MWC /PC -04 -29 well nest (FS, FG and H), PC -04 -281 and <br />C PC -04 -381. At MWC /PC- 04 -29, a comparison of the groundwater elevations and the <br />water bearing units that generate the observed heads indicates that water levels in the <br />F sandstone and H sandstone are within their respective units. The water levels in the <br />F/G wells are approximately 25 ft above the top of the F/G sequence. <br />The I sandstone piezometer (PC -04 -281) has been dry since it was installed, indicating <br />that the unit is not saturated at this location. At PC -04 -381, the water level occurs 19 ft <br />above the top of the I sequence. Water levels in this piezometer have been slowly rising <br />since the well was installed, which may indicate that the water levels are still <br />equilibrating. The change has only amounted to 5 ft. <br />In summary, in the south portion of the mine area, groundwater exists in perched zones <br />that are not under significant pressure. Conditions are similar to the existing Colowyo <br />Mine and dewatering measures are probably not required. <br />Central area <br />Water levels in the central portion of the mine area in both the F sandstone and <br />F/G sequence range between about 7,360 and 7,100 ft in elevation. Water levels in the <br />F sandstone show varying amounts of head above the screened unit, with the highest <br />values on the west side of the study area. At PC- 04 -36FS the water level resides in the <br />F sandstone. At PC- 04 -04FS the water table occurs approximately 92 ft above the <br />sands between the F5/6 coal and the Fab coal. In PC- 04 -05FS, the water table occurs <br />at an elevation of 7,023 ft, 70 ft above the top of the F sandstone. <br />• <br />2572 -R2 <br />Colowyo Coal Company <br />Water Management Consultants <br />