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-23- The major bedrock aquifers in the Mesaverde Group occur within the massive, regional sandstone units of the Iles and Williams Fork Formations. Wells tapping the Tow Creek sandstone and Trout Creek sandstone of the Iles Formation and the Twentymile sandstone of the Williams Fork Formation are used primarily for domestic and stock purposes. Well yields from these sandstone aquifers are generally less than 10 gal/min (Brogden and Giles, 1977). Nigher yields generally occur at greater depths where water in the sandstone is under artesian pressure, and where the sandstones have been fractured extensively. Hydraulic conductivity values for fractured sandstones of the Iles and Williams Fork Formations range from 3.7 to 26 feet/day (Brogden and Giles, 1977). Limited ground water also occurs within the discontinuous, lenticular sandstones associated with coal seams and 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 (8rogden and Giles, 1977). Dissolved solids concentration of water within the Iles and Williams Fork Fornations 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 Twentymile Park Basin. Limited ground water yields have been obtained from the Lewis Shale but use of water from this unit is considered insignificant in the Twentymile Park Basin. 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 can be obtained from the unconsolidated alluvial deposits of the Yampa River and its tributaries. Well yields range from 5 gal/min to as much as 9U0 gal/min (Brdgden 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, particularily in the Yampa River alluvium. Water quality is variable, depending on the underlying rock and source of alluvial material. Dissolved solids concentration ranges 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).