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1 Formation in the central part of Che Big Bottom syncline, the Lewis Shale serves <br />as a confining bed that may block upward leakage of ground water from the Upper <br />Williamks Fork. Shallow wells in the Lewis Shale may yield 3 to 5 gallons per <br />minute, (Woodward-Thorfinn son, 1975). Permeability is estimated [o be about <br />10-5 gpd/ft2 for the shale. Thin sandstone strata probably have permea- <br />bilities similar to the Upper Williams Fork. <br />Alluvial Aquifer <br />The alluvial aquifer associated with the Yampa River is restricted in its occur- <br />rence to the floodplain area of the river. Alluvial wells in the Big Bottom area <br />are reported to produce up to 900 gpm (Radian, 1980). The all vial aquifer <br />undoubtzdly has high permeability and porosity, and is an excellent aquifer. <br />Estimated permeability is 500-5,000 gpd/ft2 (Radian, 1980). The thicknesses <br />Eor this aquifer are unknown. Tes[ well site GBB-3 showed 10-12 feet of allu- <br />vium. <br />The alluvial system of the drainages in the mine plan area are small until the <br />slopes of the channels drop below five per cent. Flume and Pyeatt Gulch have <br />significant alluvial systems north of the proposed permit area. The drainage <br />channels of the other gulches are narrow in the permit area and are nut very <br />extznsive. <br /> <br />2-4 62 <br />REVISED FEB 1 ~ 'R? <br />