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. N N o CO Mountains; elsewnere the presence of ephemeral streams indicates very high evaporative losses. In addition. to high evaporation, ground ~ater is discharged by stream seepage, phreatophyte cons~"'ption, and by an estimated well ~ithdrawal of 2,100 AF/yr (Price and Arnow, 1974; U.S. Bureau of Recla~tion, 1976). Most of this withdrawal is for irrigation water and is derived fram alluvial aquifers in the Farmington area, including the 10l,er Animas and La Pl"ta valleys. The geology of bedrock aquifers in the Upper San Juan River Basin is complex. There are aver 15 major. water-bearing geologic units in the basin and ~ny minor aquifers. Of the major aquifers, only four have the potential ~f yielding 100 gpm or more to wells-- the :;acimiento and San Jose formations and the Gallup and Cliff House sandstones (u.S. Bureau of Land Mgm~~ 1977). The Naci~ento and S~~ J0se formations cover a large part of the basin, extending from roughly the eastern half of San Juan County and the western half of Rio Arriba County. When joined ~ith t~o other units, the Fruitland and Menefee formations, nuch of the surface rock area of the San Juan Basin is geologically described. . Bedrock aquifers near the Animas River generally yield less than 25 gpm, most commonly 1 to 10 gpm. A fe~ wells yield as ~uch as 50 gpm. Tne more common aquifers are the Dakota and Cliff House sand- stones and ~he MenQf~e and ~orrison formations. Flowing artesian wells are common (Brogde~ and Giles, 1976a). Ca~cium and sodium bicarbonates are typical ions found in these aquifets. The Menefee. Formation provides degraded water because of water move~ent through coal beds. Dakota Sandstone contains an average dissolv"d solid con~ tent ~f only 273 to 440 ~g/liter (Brogden and Giles, 1976b). In the region of Ute ~lountain Indian Reservation, bedrock aqui- fers provide only 1 to 15 gpm to wells. The Dakota Fo~tion provides much of the stock '",ater for the region. Host ~ells in this formation are artesian. ~uch of the Ute Nountain Indian Reservation is.under- lain by the Hesaverde Group of sandstones. However, this unit is not an important source of .ater because it has been drained by the deeply incised Mancos River and its tributaries (Irwin, 1966). Some wells tapping the Dakota Formation are sulfurous because of contact with interstitial coal seams; these wells ar" not domestically useable. Alkalinity can also be quite high; some samples have a pH level as high as 11.7 but most ~ere in the range of 7.5 to 8.5. Dis- solved solids ranged fr= 465 to 6,000 mg/liter in vadous areas .of the Dakota Sandstone (Irwin, 1966). Most ground ~ater samples in southwestern Colorado are high in calcium and magnesium and, there- fore, ar" very hard. In general, hardness and TDS increase with aquifer depth. .. 0-21