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impacts on Aquifers <br />Ground Water Occurrence <br />Ground water occurs in both rock strata and alluvial bodies within the <br />ground water study area. Rock aquifers include the Trout Creek Sandstone, <br />the Middle Sandstone, the Twentymile Sandstone, and they White Sandstone. <br />The Trout Creek Sandstone, the Twentymile Sandstone, and the White <br />Sandstone are rock aquifers currently being used for ground water supplies <br />in the ground water study area. The Middle Sandstone is not currently used <br />as a ground water supply in the ground water study area.. Lenticular and <br />interbedded sandstones and the coal seams are also water bearing; however, <br />they either yield insufficient amounts of water to wells or are not <br />continuous, and therefore cannot be considered regional aquifers. The <br />alluvial bodies associated with the Yampa River and Williams Fork River <br />contain limited ground water and are not considered major aquifers in the <br />general area. Alluvial bodies along the Yampa .River up- and downstream of <br />the ground water study area, however, are significant sources of ground <br />water. High yield irrigation and municipal water supply wells are <br />completed in the Yampa River alluvium in those areas. .41so, the Yampa <br />River. alluvium outside the study area is widely used as a source of <br />domestic and livestock watering. Within the ground water study area, <br />alluvial ground water is not a significant source of water put to <br />beneficial use. <br />Ground water flow in the general area is controlled by geologic structure, <br />stratigraphy and geomorphology. The flows of ground water in the rock <br />aquifers are controlled by the structural folding and faulting and <br />stratigraphy while the flows of ground water in the alluvium are controlled <br />by the geomorphology of the stream valleys. <br />The sandstone aquifers are under atmospheric pressure (crater table <br />conditions) near their recharge areas and under hydroste+tic pressure <br />(artesian conditions) within the structural basins or at: discharge points. <br />Hydrostatic pressures in the sandstone aquifers increase: with depth and are <br />at a maximum at the axes of the Big and Round Bottom Syr.+clines. Artesian <br />conditions are developed in aquifers which are confined by overlying and <br />underlying strata with low permeabilities. Several of the wells drilled <br />into the rock aquifers within the study area exhibit artesian flows at the <br />surface. <br />The sandstone aquifers in the Big Bottom Synclinal Basin are recharged at <br />their subcrops beneath the stream/alluvial systems of the Yampa and <br />Williams Fork Rivers, and at their outcrops in upland areas. Flow for the <br />Trout Creek Sandstone is generally to the north-northeast while the flows <br />in the Middle, Twentymile, and White Sandstones are to the north and <br />northwest. Discharges from the Middle and Twentymile Sandstones are to the <br />north toward the center (axis) of the Big Bottom Synclinal Basin and then <br />into the larger sand wash structural basin or to the surface/alluvial <br />systems of the Yampa River. Infiltration from surface water and alluvial <br />bodies may recharge the White Sandstones. Likewise, disc=harge to surface <br />springs, seeps, and streams may also occur where the White Sandstones are <br />near the surface and saturated. Declines in the piezometric water levels <br />in Middle Sandstone monitoring wells over and adjacent to the Sagle No. 5 <br />Mine indicate that discharges from this aquifer are contributing to mine <br />inflows. Declines have also been observed in piezometric water levels in <br />White Sandstone monitoring wells adjacent to the Beagle No. 9 Mine. <br />In the Round Bottom Synclinal Basin, ground water flow di.rectione in rock <br />aquifers have not been determined. The Division postulates, based on the <br />geologic structure, that ground water flows west and north downdip to the <br />axis of the Round Bottom Syncline in the vicinity of the 2 West Area of the <br />16 <br />