2013-04-10_PERMIT FILE - C1981019A (11)

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RULE 2 PERMITS is generally more permeable than the overlying sandstones in the Williams Fork Formation. The transmissivity distribution for the bedrock units exhibits a trend similar to that of their hydraulic conductivities. Storativity values for the bedrock units indicated generally confined conditions at the locations of the tested wells. Geometric mean values of storativity ranged from 2.7 x 10 -5 for the G seams to 6.0 x 10 -2 for the H sandstone and 1.5 x 10-2 for the Trout Creek Sandstone. Laboratory testing (WMC, 2005) determined the permeability of the KM Layer to be less than 1x10 "10 centimeters per second (cm /sec) (2.8x10 -7 ft /day). The KM Layer consists mostly of smectite clay and appears at the same stratigraphic position as the Yampa Bed, described by Brownfield and Johnson (2008) as a regionally - persistent altered volcanic ash bed 0.5 to 5 ft thick. The low permeability and lateral continuity of the KM Layer make it an effective aquitard separating the beds within the coal sequence to be mined and the underlying rocks including the lowest part of the Williams Fork Formation and the Trout Creek Sandstone. Groundwater Flow and Water Levels - Recharge to units of the Williams Fork Formation and the Trout Creek Sandstone occurs along the outcrops on the south, east and west margins of the groundwater basin, from the saturated valley fill along the upper reaches of the stream channels, and via seepage from surface water in the stream channels in the southern areas. Recharge is thought to be greatest on the southern side of the basin in the Danforth Hills which, because of their higher elevation, tend to have lower average temperatures, more precipitation, and more snow than other potential recharge areas (WMC, 2005). Tables 2.04.7 -41 and 2.04.7 -42 summarize water -level data and Figures 2.04.7 -36 and 2.04 -37 present hydrographs for monitoring wells in the area. Water levels in valley -fill aquifer wells are typically 10 to 15 feet below ground level. The valley -fill wells exhibit seasonally higher water levels related to recharge from snowmelt runoff and infiltrating precipitation during spring and early summer. Such seasonal trends were not observed in bedrock wells. Water -level monitoring in bedrock wells and piezometers in the Collom permit expansion area has shown potentiometric elevations below the tops of the respective units (and therefore indicating perched groundwater conditions) in the southern part of the area. Water levels progressively rise above the tops of the saturated units farther downdip (WMC, 2005). The transition point from unconfined conditions where water levels are within the saturated units to confined conditions where water levels are above the tops of the saturated units occurs within the proposed Collom Lite pit. Water -level data for the bedrock wells and piezometers did not indicate seasonal trends. Most groundwater flow within the system occurs within the coal units, because of their greater hydraulic conductivities. The sandstone units are also significant in terms of groundwater flow, but they generally have lower hydraulic conductivities than the coals. The interburden units, which consist mainly of claystone and shale, have hydraulic conductivities two to three orders of magnitude lower than the coals or sandstones and do not allow significant amounts of groundwater flow. Figures 2.04.7 -34 and 2.04.7 -35 depict the potentiometric surface configurations in the F sandstone and the F/G sequence, based on water -level monitoring conducted by WMC (2005). Potentiometric surface elevations suggest flow in the F sandstone and F/G sequence in that area to be to the north and northeast. The potentiometric gradients are steeper in the south and flatten considerably to the north beyond area where the transition from unconfined to confined conditions occurs. Discharge from the bedrock units occurs primarily to the valley -fill aquifers where the steeply- dipping north limb of the Collom Syncline is dissected by streams and along the northern portions of the stream channels. Numerous small springs and seeps present in and along the sides of the stream valleys crossing the area are indications of groundwater discharge from the bedrock units, but the discharge rates are small and the springs and seeps do not comprise a significant source of discharge from the bedrock units. The valley -fill aquifers lose water to evapotraa-ispiration, primarily during the spring through early fall growing Collom — Rule 2, Page 38 Revision Date: 9/28/11 Revision No.: PR -03