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<br />carbonate aquifer has a low porosity, typically less than 2% by volume; because of this, hydraulic <br />conductivity reflects the ability of water to move through fractures in the rock rather than through <br />intergranular porosity, as in sandstones. The very high hydraulic conductivity in the <br /> <br />o Glenwood Springs area can be explained by intense fracturing rocks in the keystone block (Fig. 1) of <br />au the anticline beneath the area. <br />0, <br />Cl' <br /> <br />To have flow of ground water, both hydraulic conductivity (a rock property) and a hydraulic <br />head (an environmantal parameter) are needed. Fig. 2 is a map showing the hydraulic head (with <br />respect to sea level) on the carbonate aquifer over the northwestern quarter of Colorado, based on <br />well and spring data. Regionally, the greatest hydraulic head, over 9000 feet (referred to Sea Level), <br />is found in the White River Plateau to the north of Glenwood Springs. At G1enwood Springs, the <br />head is about 5000 ft, again referred to Sea Level, a drop of nearly 4000 ft. If no flow were taking <br />place through the aquifer, the artesian head at G1enwood Springs would be nearly 4000 ft (1760 <br />pounds per square inch). <br /> <br />The observed head at Glenwood Springs with no flow is only about 30 feet above River level. <br />The lessening of the head is explained by flow resistance as water moves from areas of high head to <br />areas of low head. Fig. 4 shows values of flow (cubic feet per square foot per day) computed from <br />the data of Figures 2 and 3. Lateral movement of ground water in the carbonate aquifer increases by <br />a factor of 10 to 100 in the immediate vicinity of G1enwood Springs. <br /> <br />In summary, regionally, warm ground water in the carbonate aquifer is pressured to move into <br />rocks beneath Glenwood Springs. There, because the top seal of the aquifer has been breached by <br />keystone faulting of an anticline involving the carbonate aquifer, great volumes of water escape to <br />form the Glenwood thermal springs and the Dotsero warm springs, a few miles up river. This escape <br />of water from the aquifer causes a drawdown of aquifer pressure along the Colorado River. The <br />hydraulic sink formed by the springs causes water to flow into the springs from three quadrants, <br />those to the north, east, and south, as shown by the flow arrows in Fig. 3. <br /> <br />3. Local Hydrogeology: At G1enwood Springs, 14 or so thermal springs discharge about 300 <br />million cubic feet of water per year containing about 440,000 tons of salt. The number of springs has <br />been stated differently by various reporters. Some water escapes in swampy places along the River, <br />and there is at least one spring that flows into the bed of the River. The most prominant springs are <br />shown on Fig. 5. <br /> <br />Spring water flows immediately into the River. The Colorado River Valley is filled to a depth <br />of about 100 feet of alluvium which forms a reservoir for additional thermal water supplied from the <br />underlying bedrock as well as normal water provided by the River. <br /> <br />The alluvium forms an open aquifer. Draw down tests in well Redstone well #21-9 (Figure 5) <br />caused reduction in flow from alluvial wells. and springs at distances up to 1100 feet away, as well as <br />in the well Wright No.1, which penetrates bedrock 4400 feet away. The alluvial aquifer is character- <br />. ized by a high degree of communication, recharged by warm saline water from the beached carbonate <br />aquifer beneath and from cool, clear river water from above. <br /> <br />Geophysical evidence can be used to provide a rough estimate the volume of the open aquifer. <br /> <br />2 <br />