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flP,?,f\,? .... ,'0. . GROUND WATER Bedrock Aquifers Bedrock aquifers in the study area generally are sandstone and fractured bedrock such as sandstone, shale, siltstone, and coal. Fine-grained rocks require significant fractures to transmit and store ground water. The extent of fractures in many of the rocks is unknown and aquifer-characteristic data are limited. The Wingate Sandstone (Upper Triassic) is the oldest rock unit developed for a water supply. Bedrock units that yield the most water to wells are the Mesaverde Formation and the Mancos Shale (Upper Cretaceous), Burro Canyon Formation and the Dakota Sandstone (Lower Cretaceous), The geologic units shown on plate 2 and summarized in table 2 are grouped into hydrogeologic units and shown by a geologic symbol, Hydrogeologic units represent aquifer systems (such as the Burro Canyon Formation and the Dakota Sandstone), confining beds (such as the Mancos Shale), or groups of rocks of similar water-bearing characteristics (such as igneous and metamorphic rocks). No wells were reported to have been completed in the Wasatch Formation (Tertiary). Sandstone and conglomerate zones within this formation have reasonable potential for yielding water for domestic well production; however, the formation's occurrence at higher altitudes and inaccessible locations has prevented significant development of these water resources. Fractured igneous and metamorphic rocks of Precambrian and Tertiary age locally may be aquifers in the eastern part of the study area. Precambrian metamorphic rocks may be found below consolidated sedimentary rocks at most locations in the basin. If a well is to be developed in the igneous and meta- morphic rocks, the yield will depend on the number of fractures intersected below the water level in the well. Yields generally decrease with greater depth because most fractures are in the upper part of these rocks. Wells drilled in the broken Tertiary basalt have reported yields of 6 to 25 gal/min. The Wingate and the Entrada Sandstones have produced 11 to 15 gal/min in this basin, but are areally limited. The Entrada Sandstone, Morrison Forma- tion, and Dakota Sandstone may be potential aquifers, but are more limited in areal extent. Based on well-production data, the Burro Canyon Formation and the Dakota Sandstone represent the best opportunity for development in the western part of the lower Gunnison River basin with reported well yields ranging from 5 to 14 gal/min. Ten wells were reported to have been completed in the Mancos Shale. One produced as much as 20 gal/min, but another yielded only 1 gal/min. This for- mation generally has little potential for yielding sufficient quantities of water for domestic or agricultural uses. 9