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Groundwater 69 <br />C <br />• H sandstone —This unit is the first significant sandstone below the <br />Gc seam. The sequence is confined, which leads to increases in the <br />head above the sequence from south to north across the Collom Mine. In <br />the north portion of the Collom Mine, the potentiometric head is 300 to <br />500 ft above the H sandstone. It will probably need to be depressurized <br />to reduce excessive pore pressures that could cause floor heave. <br />/ sandstone — This is a sandstone unit above the I coal seam and lies <br />about 50 to 75 ft below the bottom of the pit. Similar to the H sandstone, <br />some depressurization may be required in this unit. <br />Lower sequence — This includes the units between the I sandstone and <br />the top of the Trout Creek Sandstone. A key unit within this sequence is <br />the impermeable KM layer. Units below the I sandstone and above the <br />KM layer are expected to have a minimal effect on dewatering. Units <br />below the KM are hydraulically isolated from the overlying units. <br />Trout Creek Sandstone — This is the uppermost bed in the Iles Formation. <br />It is hydraulically isolated from the Collom Mine and is not anticipated to <br />have any effect on dewatering. <br />5.3.2 Site -scale structures <br />( The effects of site -scale structures on groundwater flow were evaluated using the <br />hydrogeologic and geologic data. As discussed in Section 2, a well- defined joint system <br />pervades the sandstone units in the existing mine and the proposed mine. Drawdown in <br />the F/G sequence may indicate that the joint pattern creates preferential flow paths. The <br />drawdown from the pilot dewatering test shown in Figure 5.18 is asymmetric, with the <br />greatest drawdowns oriented along the N70 °W lineation and the least amount of <br />drawdown oriented along the N30 0 E lineation. Site -scale structures are not expected to <br />have any significant effect on dewatering /depressurization of the Collom Mine. <br />5.3.3 Hydraulic characteristics <br />Groundwater flow occurs principally in the coal seams, due to their relatively high <br />hydraulic conductivity. Based on the aquifer tests (Tables 5.6 and 5.7 and Figure 5.9) as <br />well as the drawdown curves produced from the pilot dewatering test (Appendix 5.H), the <br />most permeable units are the Fab, Ga, and Gb coal seams. The coal seams have <br />hydraulic conductivities in the range of 0.2 to 0.04 ft/day. Aquifer tests conducted in <br />wells that screen the entire F/G sequence have lower hydraulic conductivities (generally <br />less than 0.1 fUday) due to the presence of the siltstone and sandstone interburden. <br />Storativity values range from approximately 4 x 10 -4 to 4 x 10 -6 . The F/G sequence and <br />the Gc seam had the highest values (10 -4 to 10 -5 ), while the Ga coal had the lowest <br />(10 -6 ) . These values are in the middle to lower range of values typical encountered in <br />confined, water - bearing units (Freeze and Cherry, 1979). <br />2572 -R2 <br />Colowyo Coal Company <br />Water Management Consultants <br />