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Giles, 1977). The principal use of water from the alluvial aquifers is for domestic and stock watering purposes; however, permeabilities are sufficient in some areas to support long-term, high-yield wells for municipal and industrial needs and irrigation, particularly in the Yampa River alluvium. Water quality is variable, depending on the underlying rock and source of alluvial material. Dissolved solids concentrations range from 82 to 2,970 mg/1 and the water may contain concentrations of arsenic, iron, manganese, nitrate, selenium, and sulfate in excess of U.S. Public Health Service Drinking Water Standards (Brogden and Giles, 1977). Ground water flow in the Upper Yampa River Basin is controlled by the geologic structure, stratigraphy and geomorphology of the general area. 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 alluvial aquifers are controlled by the geomorphology of the stream valleys and surface water flows. The Twentymile Park Structural Basin controls the regional ground water movement within regionally extensive rock strata in the general area, the Tow Creek sandstone, the Trout Creek sandstone, the Twentymile sandstone, and the strata associated with the coal seams (Figure 3). The regionally extensive rock strata are recharged in the uplands which rim Twentymile Park. The regional rock strata discharge down dip either through the Hayden Syncline northwest to the Sand Wash Basin, or to the surface streams in the Trout Creek system through outcrops and subcrops in the northern and eastern edge of the basin. The regional aquifers are under atmospheric pressure (water table conditions) near their recharge areas and under hydrostatic pressure (artesian conditions) within the basin or at discharge points. Artesian conditions are developed in aquifers which are confined by overlying and underlying strata with low permeabilities. Some wells drilled into the regional aquifers within the basin exhibit flows artesian flows. The faults in the Twentymile Park Basin may provide a conduit (pathway) for ground water communication between overlying and underlying aquifers and also between aquifers and the overlying surface streams. The faulting produces increased fracturing within and between aquifers (secondary porosity). These more porous fault zones with their higher permeabilities can control both vertical and horizontal movement of ground water between and within aquifers. In areas where hydrostatic heads are lower in the underlying aquifers than the overly surface elevation or the hydrostatic heads in overlying aquifers, the vertical movement of water through the fault system would be either upward and discharge to the surface system, or upward and recharge overlying aquifers. Mining at the Foidel Creek mine as of 1986 has not shown any significant flows from fracture zone or faults. The Foidel Creek mine permit application has submitted inflow calculations for faults that may create substantial inflows, although, to date three faults zones have not produced the originally anticipated mine inflows. -36-