GENERAL35005 - Laserfiche WebLink

Home Browse Search

(Brogden and Giles, 1981). These deposits are found most extensively along the Yampa River, Trout, Middle, Foidel and Fish Creeks in the basin and consist predominately of clay, sand and lenticular, discontinuous gravel layers. The alluvium is thin or absent in areas where streams cross the resistant sandstones of the Mesaverde Group. The alluvial aquifers are wide where the streams cross less-resistent rock units of the Lewis and Mancos Shales. 2. Ground Water Although ground water occurs in all the sedimentary rocks, the only identified strata capable of regionally storing and transmitting water are the Tow Creek, the Trout Creek and the Twentymile sandstones, along with the lenticular and interbedded sandstones of the three coal groups. 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 gpm (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 within the discontinuous, lenticular sandstones associated with coal seams and the coal seams themselves within the tesaverde Group. Existing operations mine the Wolf Creek, Wadge and Lennox seams, and consequently interrupt the hydrologic regimes associated with these coals. While ground water within the Iles and Williams Fork Formation is predominantly calcium and sodium-bicarbonate types, 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, 1977). TDS levels range from 76-185 mg/1 and limited yields from water-bearing units associated with the coals preclude significant use of this water. The geologic formations within the study area are described with respect to their general water bearing characteristics, their water quality parameters and their use in the following paragraphs. The formations will be described from oldest to youngest. Wells tapping the Mancos Shale generally yield small quantities of water, less than 5 gpm. The water is predominantly a calcium-bicarbonate type (Brogden and Giles, 1977). Dissolved solids 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 t9ancos Shale contains -13- ~ ..