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o N N <:> FlVm Geldon-- Origin of Hot Water at Glenwood Springs The origin of hot water at Glenwood Springs can be inferred from the temperature and chemistry of the water and the location of points where the water discharges from the hydrologic system. Included in this interpretation is the identification of local recharge areas for the Leadville Limestone and Dyer Dolomite. Bedrock aquifers typically are recharged in topographically high areas, where the bedrock crops out or is near the surface. Potential recharge areas for the Leadville Limestone and Dyer Dolo- mite in the vicinity of Glenwood Springs include the White River Plateau to the north, Lookout Mountain to the southeast, and the Grand Hogback to t)Ie southwest. Extensive outcrops of the Leadville Limestone and Dyer Dolomite in the White River Plateau receive a considerable influx of precipitation. However, much of this precipitation discharges as springs into rivers and creeks that drain the plateau. For example, in the East Fork of Rifle Creek upstream from Rifle Falls, springflow entering the creek over a distance of 4 mi equals 126,600 gal/ min (calculated from data in Teller and Welder, 1983, p. 11). Apparently very little of the water entering the Leadville Limestone and Dyer Dolomite in the White River Plateau circulates deeply enough to discharge at Glenwood Springs. Water temperatures at Glenwood Springs are consistent with Lookout Mountain and the Grand Hogback being the principal recharge areas for the Leadville Limestone and Dyer Dolomite in the vicinity. Temperatures of water discharging from the Leadville Limestone at Glenwood Springs consis-tently range from 111 OF to 126 oF. If the geothermal gradient at depth is as large as 1.8 of/IOO ft, which is the gradient recorded in the Wright no. 1 well between depths of 260 and 506 ft, only 4300 ft of overburden would be required to heat water from the mean annual air temperature at Glenwood Springs (48 oF) to the maximum recorded ground-water temperature of 126 oF. At Look- out Mountain, Paleozoic formations above the Leadville Limestone are about 5,200 ft thick where they have not been eroded. Tertiary lava flows of unknown thickness overlie the Paleozoic rocks locally. The combil).ed thickness of Paleozoic rocks and Tertiary lava is sufficient to heat descending ground water to temperatures observed at Glenwood Springs. In the Grand Hogback, the combined thickness of Paleozoic, Mesozoic, and Tertiary rocks probably requires some mixing of shallow and deep ground water to achieve the ground-water temperatures observed in the discharge area (assum- ing a geothermal gradient comparable to that in the Wright no. 1 well). The source of the shallow water enterin- the system could be the White River Plateau. The chemistry of water discharging from the Leadville Limestone at Glenwood Springs indicates that water in the limestone must be in contactwith evaporite deposits. Water in the Leadville Limestone at Glenwood Springs is a sodium chloride type with a large sulfate concentration and a dissolved-solids concentration of 18,000 to 22,000 mg/L. Such water is atypical of carbonate rocks where they crop out, but could occur if the water contained dissolved halite and gypsum. These rock types occur in the Eagle Valley Evaporite above the Leadville Limestone, north, east, and south of Glenwood Springs. However, geological and structural discontinuities probably allow water to percolate from the Eagle Valley Evaporite into the Leadville Limestone only to the south of Glenwood Springs. Lookout Mountain and the Grand Hogback, located in the identified area, again seem to be the principal recharge areas for the Leadville Limestone and Dyer Dolomite in the vicinity of Glenwood Springs. Recorded potentiometric heads in bedrock and alluvium support the identification of Glenwood Springs as a discharge area. In November 1984, potentiometric heads in the Leadville A-8