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RULE 2 PERMITS <br />is generally more permeable than the overlying sandstones in the Williams Fork Formation. The <br />transmissivity distribution for the bedrock units exhibits a trend similar to that of their hydraulic <br />conductivities. Storativity values for the bedrock units indicated generally confined conditions at the <br />locations of the tested wells. Geometric mean values of storativity ranged from 2.7 x 10-5 for the G seams <br />to 6.0 x 10"2 for the H sandstone and 1.5 x 10-2 for the Trout Creek Sandstone. <br />Laboratory testing (WMC, 2005) determined the permeability of the KM layer to be less than 1x10-10 <br />centimeters per second (cm/sec) (2.8x10-7 ft/day). The KM layer consists mostly of smectite clay and <br />appears at the same stratigraphic position as the Yampa Bed, described by Brownfield and Johnson <br />(2008) as a regionally -persistent altered volcanic ash bed 0.5 to 5 ft thick. The low permeability and <br />lateral continuity of the KM layer make it an effective aquitard separating the beds within the coal <br />sequence to be mined and the underlying rocks including the lowest part of the Williams Fork Formation <br />and the Trout Creek Sandstone. <br />Groundwater Flow and Water Levels - Recharge to units of the Williams Fork Formation and the Trout <br />Creek Sandstone occurs along the outcrops on the south, east and west margins of the groundwater basin, <br />from the saturated valley fill along the upper reaches of the stream channels, and via seepage from surface <br />water in the stream channels in the southern areas. Recharge is thought to be greatest on the southern side <br />of the basin in the Danforth Hills which, because of their higher elevation, tend to have lower average <br />temperatures, more precipitation, and more snow than other potential recharge areas (WMC, 2005). <br />Table's 2.04.7-41 and 2.04.7-42 summarize water -level data and Figures 2.04.7-36 and 2.04-37 present <br />hydrographs for monitoring wells in the pit area. Water levels in valley -fill aquifer wells are typically 10 <br />to 15 feet below ground level. The valley -fill wells exhibit seasonally higher water levels related to <br />recharge from snowmelt runoff and infiltrating precipitation during spring and early summer. Such <br />seasonal trends were not observed in bedrock wells. Water -level monitoring in bedrock wells and <br />piezometers in the Collom permit expansion area has shown potentiometric elevations below the tops of <br />the respective units (and therefore indicating perched groundwater conditions) in the southern part of the <br />area. Water levels progressively rise above the tops of the saturated units farther downdip (WMC, 2005). <br />The transition point from unconfined conditions where water levels are within the saturated units to <br />confined conditions where water levels are above the tops of the saturated units occurs within the Collom <br />Pit. Water -level data for the bedrock wells and piezometers did not indicate seasonal trends. <br />Most groundwater flow within the system occurs within the coal units, because of their greater hydraulic <br />conductivities. The sandstone units are also significant in terms of groundwater flow, but they generally <br />have lower hydraulic conductivities than the coals. The interburden units, which consist mainly of <br />claystone and shale, have hydraulic conductivities two to three orders of magnitude lower than the coals <br />or sandstones and do not allow significant amounts of groundwater flow. <br />Figures 2.04.7-34 and 2.04.7-35 depict the potentiometric surface configurations in the F sandstone and <br />the F/G sequence, based on water -level monitoring conducted by WMC (2005). Potentiometric surface <br />elevations suggest flow in the F sandstone and F/G sequence in that area to be to the north and northeast. <br />The potentiometric gradients are steeper in the south and flatten considerably to the north beyond area <br />where the transition from unconfined to confined conditions occurs. <br />Water level data collected in 2015 from the Colowyo monitoring wells in the Trout Creek Sandstone were <br />used to inlcude the poteniometric surface on Map 10B. At well W-95-02, the top of the sandstone is at <br />about 6,160 ft amsl and the water level is above the land surface elevation of approximately 6,787 ft amsl, <br />with a level of approximately of 6,819 ft. amsl. W-95-15 has the top of the sandstone at approximately <br />5,533 ft. amsl with the water level of 6682 ft. amsl. UL -95-45 has an elevation of 6,627 ft amsl, with the <br />top of the Trout Creek Sandstone at 6,027 ft amsl and a water level at 6,532 ft. ams]. <br />Collom — Rule 2, Page 25 Revision Date: 4/7/17 <br />Revision No.: RN -07 <br />