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ARCOSA <br /> exposed along Doudy Draw and the interchannel area towards Coal Creek. The outcrop is <br /> approximately 3,500 feet west of the quarry. The Hygiene dips 450-550 to the east,which would cause it <br /> to occur at a depth greater than 3,000 feet under the site. The 3,000 foot interval is composed of <br /> extremely low permeability shales and claystones of the Pierre Shale which makes any percolation from <br /> the quarry impossible. The recharge areas are up gradient from the quarry so the possibility of affecting <br /> the Hygiene aquifer is nil. <br /> The last confined aquifer in the area is the sandstones of the Dakota Group which form the first <br /> resistant hogback along the mountain front, approximately 1.5 miles west of the site. Because the dip of <br /> the Dakota averages 450, these beds would be very deep( >6,000 feet) below the site and contain <br /> highly mineralized, non-potable water. It is concluded no confined aquifers will be affected by the quarry <br /> operation. <br /> The only other geologic units in the quarry area which contain ground water and are permeable enough <br /> to function as aquifers, are the pediment and alluvial gravels. These gravel deposits are of three ages, <br /> with the Rocky Flats Alluvium being the oldest. <br /> The Rocky Flats is a yellowish-brown bouldery gravel, which contains clay, silt, and sand layers.This <br /> unit was formed by alluvial fan deposition on an eroded surface of Pierre Shale at the quarry site. The <br /> thickness of the alluvium is highly variable, ranging from 0 to 35 feet in response to channel trends <br /> which follow the low areas of the erosional surface. Several paleo channels are discernable from <br /> borehole data. These trend primarily northeast. <br /> The intermediate age gravel deposit is a terrace deposit which is generally less than 10 feet thick. It <br /> was deposited by Coal Creek at a higher previous level. Like the Rocky Flats Alluvium, this gravel <br /> unconformably overlies the Pierre Shale. This unit is highly weathered. Like the Rocy Flats, it is <br /> cemented by caliche. <br /> The youngest gravel deposit is a terrace and flood plain deposit formed by Coal Creek in recent time. <br /> This unit probably averages 10 to 15 feet thick. <br /> Water infiltrating the gravels from various sources saturates the lower portions of the gravels. The water <br /> then flows generally northeast. The subsurface flow is controlled primarily by the topography of the <br /> erosional bedrock surface,with the greatest volume of water following the thickest alluvium in the old <br /> channels. The hydraulic conductivity of the Rocky Flats Alluvium is estimated at 35 ft/day(Hurr, 1976). <br /> It is likely the two 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 /> The quarry operation will remove these gravels to expose the Pierre Shale for mining. Approximately <br /> 95% of the Pierre is composed of extremely low permeability claystone and shale. The other 5%or less <br /> of the formation composed of thin sandstone and siltstone beds that are generally less than 4 inches <br /> thick at the quarry site.Water in the gravel units is perched over the Pierre Shale contact and emerges <br /> as seeps when this contact is exposed in excavation. The flow rate is dependent upon whether a buried <br /> channel or interchannel area is exposed. The quarry operation will have little effect upon the ground <br /> water.The only anticipated change is a slight lowering of the local ground water table. The localized <br />