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<br />Tssm6t <br />The only other geologic units in the quarry area which contain ground water and are <br />permeable enough to function as aquifers, are the pediment and alluvial gravels. These <br />' gravel deposits are of three ages, with the Rocky Flats Alluvium being the oldest. <br />The Rocky Flats is a yellowish-brown bouldery gravel, which contains clay, silt, and <br />' sand layers. This unit was formed by alluvial fan deposition on an eroded surface of <br />Pierre Shale at the quarry site. The thickness of the alluvium is highly variable, <br />' ranging from 0 to 35 feet in response to channel trends which follow the low areas of <br />the erosional surface. Several paleo channels are discernable from borehole data. These <br />' trend primarily northeast. <br />t The intermediate age gravel deposit is a terrace deposit which is generally less than 10 <br />feet thick:. It was deposited by Coal Creek at a higher previous level. Like the Rocly <br />Flats Alluvium, this gravel unconformably overlies the Pierre Shale. This unit is highly <br />' weathered. Like the Rocky Flats, it is cemented by caliche. <br />' The youngest gravel deposit is a terrace and flood plain deposit formed by Coal Creek <br />in Recent time. This unit probably averages 10 to 15 feet thick. <br />Water infiltrating the gravels from various sources saturates the lower portions of the <br />gravels. The water then flows generally northeast. The subsurface flow is controlled <br />' primarily by the topography of the erosional bedrock surface, with the greatest volume <br />of water following the thickest alluvium in the old channels. The hydraulic conductivity <br />' of the Rocky Flats Alluvium is estimated at 35 ft/day (Hurr, 1976). It is likely the two <br />other gravel units have similar if not slightly higher conductivities due to a lesser <br />' degree of cementation. These units are interpreted to be hydraulically interconnected. <br />i , The quarry operation will remove these gravels to expose the Pierre Shale for mining. <br />Approximately 95% of the Pierre is composed of extremely low permeability claystone <br />and shale. The other S% or less of the formation composed of thin sandstone and <br />' siltstone beds that are generally less than 4 inches thick at the quarry site. ~'Jater in <br />the gravel units is perched over the Pierre Shale contact and emerges as seeps when <br />' this contact is exposed in excavation. The flow rate is dependent upon whether a <br />' -26- <br />1 <br />