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the Colorado and Utah parts of the upper Colorado Region have been estimated by <br />Robinson (1958) to be more than 2 million acre -feet per year. Ground water occurs <br />under both water table and artesian conditions. Water table conditions commonly exist in <br />shallow alluvial aquifers along the larger streams, in principal recharge areas, and in the <br />relatively flat -lying rocks that predominate certain sections of the region. Artesian <br />conditions occur locally throughout the region but are prevalent in the bedrock aquifers <br />of the major structural basin. <br />Regional Ground Water Quality. In the upper Colorado Region, fresh water is <br />generally available from shallow aquifers in most consolidated units in areas above 7,000 <br />feet in elevation. At lower elevations, fresh water most commonly occurs in the <br />relatively permeable sandstones and carbonate rocks, which have good hydraulic <br />connection with the principal recharge areas in the mountains. Saline water commonly <br />occurs in shale and siltstone strata that underlie large areas of the region. <br />The relatively large, discharge- weighted- average concentration of dissolved solids in the <br />streamflow in various parts of the region results partly from inflow of saline ground water <br />from formations like the Mancos shale and the Paradox Formation. Ground water in the <br />alluvium generally contains more dissolved solids than streams. The principal reasons <br />for this are the concentrating effects of evapotranspiration and the solution of minerals in <br />the alluvium. <br />The quality of ground water in many parts of the region is altered by man's activities. <br />The seepage of highly mineralized irrigation return flow from surface sources into <br />aquifers can deteriorate the quality of water in those aquifers. The change in San Miguel <br />water quality at Uravan is a reflection of this. Deterioration of ground water also results <br />from pumpage stresses. Poorer quality saline water can be induced from adjacent <br />aquifers. Some fresh water aquifers in the region are either overlain by, underlain by, or <br />situated between saline aquifers. Reduction of the hydrostatic pressure in the fresh water <br />aquifer by pumping allows saline water from adjacent aquifers to move into the pumped <br />aquifer. This potential problem exists throughout the region because of the widespread <br />occurrence of saline ground water. One area of greatest impact resulting from the above <br />processes is in the Paradox Valley located downstream from the NHN Mine area. The <br />Paradox Formation, which underlies this valley, is composed largely of salt, gypsum, and <br />other salines which appear to be in hydraulic connection with the overlying <br />unconsolidated valley deposits. In contrast, the chemical quality of ground water in some <br />aquifers, such as in the Uinta Basin, has improved because irrigation developments have <br />augmented natural ground water recharge with fresher water diverted from surface <br />sources. <br />Significance of Ground Water. The significance of ground water on a regional basis is <br />small when compared to the use of surface water (Price and Arnow, 1974). Ground <br />water is primarily used for domestic consumption, stock watering, and to a much lesser <br />extent, municipal supply; whereas, surface water is the primary source for flood irrigation <br />and the majority of municipal water supplies. Ground water and surface water use for the <br />year 1970 in the upper Colorado River Basin are compared in Table 2.04.5 -2 (Price and <br />Section 2.04.5 Page 8 April 2011 <br />