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<br />o <br />N <br />f-" <br />en <br /> <br />layers, with discharges that rarely exceed 50 gallons per minute. <br />Hydraulic-conductivity values for the Devonian and Mississippian carbon-ate rocks <br />hydro stratigraphic unit range from less than 0.001 to more than 100 feet per day; transmissivity <br />values range from less than 0.1 to 47,000 square feet per day. The storage coefficient determined <br />from a flowing-well test at Glenwood Springs is 0.0005. Hydraulic-conductivity values for the Penn- <br />sylvanian and Permian sandstone hydrostratigraphic unit range from less than 0.0001 to 20 feet per <br />day. Hydraulic-conductivity values for local aquifers typically range from less than 0.0001 to 2 feet <br />per day. Hydraulic-conductivity values for confining layers range from less than 0.0001 to 0.25 foot <br />per day. <br />The Devonian and Mississippian carbonate rocks hydrostratigraphic unit in the Glenwood <br />Springs area consists of the Dyer Dolomite and the Leadville Limestone. The temperature and chem- <br />istry of water discharging from the Leadville Limestone in 18 hot springs and seepage areas at <br />Glenwood Springs indicate that most of the water originated in highlands to the south. Moisture from <br />precipitation and snowmelt in the Lookout Mountain and Grand Hogback areas descends to the <br />Leadville Limestone and Dyer Dolomite through several thousand feet of upper Paleozoic, Mesozoic, <br />and Tertiary sedimentary rocks; locally, the moisture descends through Tertiary basalt. The water is <br />heated as it descends through the overburden and becomes saline by dissolution of halite and gypsum <br />in the Eagle Valley Evaporite. The hot, saline water moves through fractures and solution channels in <br />the Leadville Limestone and Dyer Dolomite toward the Glenwood Springs area, where it mixes with <br />cool, freshwater from the White River Plateau, and discharges as hot springs or seeps into stream <br />alluvium. Artesian flow from wells completed in the Leadville Limestone at Glenwood Springs affects <br />discharges from nearby hot springs and water levels in the alluvium. <br />Northwestern Colorado encompasses parts of four physiographic provinces-the Colorado <br />Plateaus, Southern Rocky Mountains, Wyoming Basin, and Middle Rocky Mountains. These four <br />provinces are segmented into 13 uplifts and 4 basins. <br />Paleozoic rocks in the study area comprise 27 geologic units with an aggregate thickness <br />that generally increases from northwest to southeast. The Paleozoic rocks range from less than 2,000 <br />ft to more than 3,000 ft thick in the Uinta Mountains and Sand Wash basin and are as much as 18,000 <br />ft thick in the Elk Mountains. The Paleozoic rocks are absent in the center of the Park Range, Gore <br />Range, Rabbit Ears Range, Middle Park basin, Front Range, Sawatch Range, and Uncompahgre <br />Plateau because of erosion or nondeposition. Paleozoic rocks are underlain by Precambrian sedimen- <br />tary, igneous and metamorphic rocks (Tweto, 1980) and generally are overlain by Triassic formations <br />consisting mostly of shale. Triassic formations are absent only in the Elk Mountains, southeastern <br />Piceance basin, and Front Range; in these areas, the Entrada Sandstone of Jurassic age overlies the <br />Paleozoic rocks. <br />Based on lithologic and hydrologic properties, the Paleozoic rocks in northwestern Colorado <br />are grouped into 11 hydrostratigraphic units. As used in this report, the term "hydrostratigraphic unit" <br />includes regional aquifers, local aquifers, and confining layers. Regional aquifers are hydrostrati- <br />graphic units typically composed of sandstone, limestone, or dolomite that generally yield usable <br />supplies of water throughout most of a region. Local aquifers are hydrostratigraphic units typically <br />composed of sandstone, limestone, and dolomite with shale layers, sedimentary textures, or secondary <br />structures that in some locations may inhibit extracting usable supplies of water from the unit. Confin- <br />ing layers are hydrostratigraphic units typically composed of fine-grained clastic, biogenic, or chemi- <br />cal sedimentary rocks or crystalline rocks that generally do not yield usable supplies of water <br />throughout most of a region. <br />Regional Aquifers -- Regional aquifers in the study area include the Devonian and Missis- <br />sippian carbonate rocks hydrostratigraphic unit and the Pennsylvanian and Permian sandstone <br /> <br />A-4 <br />