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elevation. At lower elevations, fresh water most commonly occurs in the relatively permeable sandstones and carbonate rocks, which have good hydraulic connection with the principal recharge areas in the mountains. Saline water commonly occurs in shale and siltstone strata that underlie large areas of the region. The relatively large, discharge-weighted-average concentration of dissolved solids in the s treamf low in various parts of the region results partly from inflow of saline ground water from formations like the Mane os shale and the Paradox Formation. Ground water in the alluvium generally contains more dissolved solids than streams. The principal reasons far this are the concentrating effects of evapotranspiration and the solution of minerals in the alluvium. The quality of ground water in many parts of the region is altered 6y man's activities. The seepage of highly mineralized irrigation return flow from surface sources into aquifers can deteriorate the quality of water in those aquifers. The change in San Miguel water quality at Uravan is a reflection of this. Deteriorization of ground water also results from pumpage stresses. Poorer quality saline na ter can be induced from adjacent • aquifers. Some fresh water aquifers in the region are either overlain by, underlain by, or situated between saline aquifers. Reduction of the hydrostatic pressure in the fresh water aquifer by pumping allows saline water from adjacent aquifers to move into the pumped aquifer. This potential problem exists throughout the region because of the widespread occurrence of saline ground water. One area of greatest impact resulting from the above processes is in the Paradox Valley located downstream from the Nucla Mine. The Paradox Formation, which underlies this valley is composed largely of salt, gypsum, and other salines which appear to be in hydraulic connection with the overlying unconsolidated valley deposits. In contrast, the chemical quality of ground water in some aquifers, such as in the Vi nta Basin, has improved because irrigation developments have augmented natural ground water recharge with fresher water diverted from surface sources. Significance of Cround Water. The significance of ground water on a regional basis is small when compared to the use of surface water (Price and Arnow, 1974). Cround water is primarily used for domestic consumption, stock watering, and to a much lesser extent, municipal supply; whereas surface water is the primary source for flood irrigation and the majority of municipal water supplies. Cround water and surface water use for the year 1970 in the upper Colorado River Basin are compared in Table 7-2 (Price and Ar norv, 1974). • This table shows that consumptive use of ground water is much less than surface water. 7-4 Revised 04/11/88