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Ground Water Ground water occurs to a limited extent in all the sedimentary rocks of Upper Cretaceous age in the region. The only identified strata capable of regionally storing and transmitting water are the Tow Creek, the Trout Creek, and the Twentymile Sandstones. Locally the lenticular and interbedded sandstones of the three coal groups as well as the major coal seams will also store and transmit limited amounts of water. Wells completed in the Mancos Shale generally yield small quantities of water, less than 5 gal/min. The water is predominantly a calcium-bicarbonate type (Brogden and Giles, 1977). Total Dissolved Solids (TDS) concentration ranges from 338 to 2,590 mg/1, and the water may contain concentrations of chloride, fluoride, iron and manganese in excess of U.S. Public Health Service (1962) drinking water standards (Brogden and Giles, 1977). Although the Mancos Shale contains limited ground water that may be suitable for domestic and stock use, the extensive shales in the upper part of the formation act as relatively impermeable barriers to the subsurface movement of water. The Mancos Shale can be considered as an impermeable, confining layer underlying the Mesaverde Group, and could be classified as an aquiclude. The major bedrock aquifers in the Mesaverde Group occur within the massive, regional sandstone units of the Iles and Williams Fork Formations. Wells completed in 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 usually less than 10 gal/min (Brogden and Giles, 1977). Higher 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 in coal seams and discontinuous, lenticular sandstones 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 in character. However, water in contact with coals may be calcium sulfate in character and may contain fluoride, iron, manganese, selenium and sulfate in excess of U.S. Public Health Service drinking water standards (Brogden and Giles, 1977). TDS 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. _pg_