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_19_ Ground Water Ground water occurs in both rock strata and alluvial bodies within the ground water study area. Rock aquifers include the Tow Creek sandstone, the Trout Creek sandstone, the Middle sandstone, the Twentymile sandstone, and the White sandstone. The Trout Creek sandstone, the Twentymile sandstone, and the White sandstone are rock aquifers currently being used for ground water supplies in the ground water study area. The Middle sandstone is not currently used as a ground water supply in the ground water study area. Lenticular and interbedded sandstones and the coal seams are also water-bearing, however, they either yield insufficient amounts of water to wells or are not continuous, and therefore cannot be considered regional aquifers. The alluvial bodies associated with the Yampa River and Williams Fork River contain limited ground water and are not considered mayor aquifers in the general area. Alluvial bodies along the Yampa River up and downstream of the ground water study area, however, are significant sources of ground water. High yield irrigation and municipal water supply wells are completed in the Yampa River alluvium in those areas. Also, the Yampa River alluvium outside the study area is widely used as a source of domestic and livestock watering purposes. Within the ground water study area alluvial ground water is not a significant source of water put to beneficial use. Ground water flow in the general area is controlled by geologic structure, stratigraphy and geomorphology. The flows of ground water in the rock aquifers are controlled by the structural folding and faulting and stratigraphy while the flows of ground water in the alluvium are controlled by the geomorphology of the stream valleys. The sandstone aquifers are under atmospheric pressure (water table conditions) near their recharge areas and under hydrostatic pressure (artesian conditions) Within the structural basins or at discharge points. Hydrostatic pressures in the sandstone aquifers increase with depth and are at a maximum at the axes of the Big and Round Bottom Synclines. Artesian conditions are developed in aquifers which are confined by overlying and underlying strata with low permeabilities. Several of the wells drilled into the rock aquifers within the study area exhibit artesian flows at the surface. The sandstone aquifers in the Big Bottom Synclinal Basin are recharged at their subcrops beneath the stream/alluvial systems of the Yampa and Williams Fork Rivers, and at their outcrops in upland areas. Flow for the Trout Creek sandstone is generally to the north-northeast while the flows in the Middle, Twentymile and White sandstones are to the north and northwest. Discharges from the Middle and Twentymile sandstones are to the north toward the center (axis) of the Big Bottom Synclinal Basin and then into the larger Sand Wash Structural Basin or to the surface/alluvial systems of the Yampa River. Infiltration from surface water and alluvial bodies may recharge the White sandstones. Likewise discharge to surface springs, seeps and streams may also occur where the White sandstones are near the surface and saturated. Declines in the piezometric water levels in Middle sandstone monitoring wells over and addacent to the Eagle No. 5 Mine indicate that discharges from this aquifer are Contributing to mine inflows. Likewise, declines in piezometric water levels in White sandstone monitoring wells adjacent to the Eagle No. 9 Mine indicate that discharges from this aquifer are contributing to mine inflows.