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-21- Limited ground water also occurs within the discontinuous, lenticular sandstones associated with coal seams and within the coal seams themselves within the Mesaverde Group. The permeabilities of these water bearing units are quite low, generally an order of magnitude less than the fractured sandstone aquifers. Ground water within the Iles and Williams Fork formation is predominantly calcium and sodium-bicarbonate types. However, water in contact with coals may be calcium sulfate type and contain fluoride, iron, manganese, selenium and sulfate in excess of U.S. Public Health Service drinking water standards (Brogden and Giles, 1917). Dissolved solids concentration of water within the Iles and Williams Fork Formations ranges from 334 to 1,460 mg/1 (Brogden and Giles, 1977), with the water contained in the coals and thin discontinuous sandstones generally being of poorer quality than that from the massive regional sandstone aquifers. Because of the poor quality and limited yield of water-bearing units associated with the coals, use of this water is not significant in the area. Limited ground water yields have been obtained from the Lewis Shale but use of water from this unit is considered insignificant. Where the Lewis Shale is present in the area, it acts as an impermeable confining layer creating artesian conditions within the underlying aquifers of the Mesaverde Group. The highest reported well yields of ground water aquifers in the basin are produced by the unconsolidated alluvial deposits of the Yampa River and its tributaries. Well yields range from 5 gal/min to as much as 900 gal/min (Brogden and 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. The Hayden Syncline controls the ground water movement within regionally extensive rock strata in the general area, including the Tow Creek sandstone, the Trout Creek sandstone, the Twentymile sandstone, the coal seams, and the strata associated with the coal seams (Figure 4). The regionally extensive rock strata are recharged in the uplands which edge the syncline. The regional rock strata discharge downdip through the Hayden Syncline northwest to the Sand Wash 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. Wells drilled into the regional aquifers downdip and within the syncline exhibit artesian flows.