2010-12-17_REVISION - C1981019 (153)

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Groundwater 5.4 Summary of groundwater flow conditions 75 The main water - bearing units underlying the proposed Collom Mine are the coal seams. The sandstone units also transmit groundwater, but have lower hydraulic conductivities than the coal seams. The mudstone interbeds act as confining units. Recharge occurs during the spring and early summer, principally to the southern outcrops located in the Danforth Hills. Lesser- amounts of recharge may occur from surface infiltration and from the saturated steam channel valley fill. In the southern portion of the site, water levels are perched in the units down to the F/G sequence. North of this area, the units become fully saturated and the head starts to rise above the top of water bearing units. At the north edge of the proposed pit, there is 200 to 400 ft of excess pressure head in the F/G sequence and the H sandstone. Excess pressure heads are expected to continue to increase down dip. Water discharges out of the units laterally into the stream drainages or vertically as inter -unit leakage. Structural deformation at the axis of the syncline may create pathways for groundwater to migrate into the upper units and then into the valley fill. Discharge of groundwater into valley fill in the lower part of the stream channels is not apparent due to the thickness of the valley fill. Most groundwater flow within the system is within the coal units, which range in hydraulic conductivity from approximately 0.38 to 0.0004 ft/day. The highest hydraulic conductivities were observed in the Fab, Ga, and Gb seams, with average values of C approximately 0.19 ft /day. The hydraulic conductivities in the interburden are two or three orders of magnitude lower. The modest storativity of the coals ( -5 x 10 -4 ) indicates that only small volumes of groundwater need to be removed to lower the heads. The bottom of the groundwater system relevant to the Collom Mine appears to be defined by the KM, a 4 -ft thick clay layer with very low vertical permeability. The KM is likely an aquiclude, effectively separating the units above it from the K and lower units of the Williams Fork Formation and Trout Creek Sandstone. Valley fill groundwater Valley fill in the stream channels is derived from the Williams Fork Formation. The valley fill is confined to the incised stream channels, which tend to widen downstream. The thickness of the valley fill also tends to increase downstream. In general, the valley fill is composed of heterogeneous, poorly - sorted clayey silts and sands. Slug tests of the valley fill wells show a wide range of hydraulic conductivities, from 196 to 0.0014 ft/day (Table 5.6 and Figure 5.9). The geometric mean of the available data is 3.3 ft/day. This variability in hydraulic conductivity is normal in valley fill groundwater systems, where the lithology can shift from a clayey sand to a sandy gravel in a very short distance. • 2572 -R2 Colowyo Coal Company Water Management Consultants