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7.0 CROUNU WATER 7.1 Aquifers Unconsolidated Sediments and Bedrock Yield Smull lu Large Quantities of Ground Water Alluvium and talus yield more ground water than bedrock, but are less extensive. Virtually all formations in Area FI store and transmit water (fig. 7.1-I). Talus and alluvium yield small (0.5 to 20 gallons per minute) to large (100 to 500 gallons per minute) quantities of water to wells and springs, but are limited in areal extent; dis- charges fluctuate seasonally. Bedrock formations generally yield small (0.5 to 20 gallons per minute) to moderate (20 to 100 gallons per minute) quantities of ground water to wells and springs, and are wide- spread. In the northwestern and southwestern parts of the area, wells and springs discharging from sandstone and conglomerate may yield large quanti- ties of ground water. Large diameter wells in the Dakota Sandstone (table 7.1-I) also may yield large quantities o(growtd water. In very dissected terrain, outcropping bedrock aquifers generally are drained by seeps and springs (fig. 7.1-2) and only yield water that is perched above Icss permeable formations underlying the ayuifcrs. The principal bedrock ayuifcrs in the area arc the Dakota Sandstone-Purgatoire Formation, Fort Hays Limestone Member of the Niobrara I nnnation- upper part of the Carlile Shale, Raton Formation- Vermejo Formation-Trinidad Sandstone, Cuchara Formation-Poison Canyon Formation, Devils Holc Formation-Farasita Conglomerate, and volcanic rocks (table 7. {-1). Within these units, sandstone and conglomerate layers and basalt (lows transmit most of the water, and shale and coal layers generally retard flow. However, extensively fractured layers and near-surface weathered zones in shale, limestone layers, and a few thick coal seams also transmit water. Formations comprised largely of shale, in- cluding the Graneros Shale, lower part of the Carlile Shale, Smoky Hill ivtarl Member of the Niobrara Formation, Pierre Shale, and Huerfano Formation, retard the downward movement of water and confine Iluw within the aquifers. I'rccamhrian and'fcrliary imrusive and metamorphic rocks, unless extensively fractured, act as barriers [o ground-water flow. llte principal sources of gnuuul-water inlonua- tion in Area 61 are Griggs (1948), F'owcll (1952), AcLaughlin and others (1961), McLaughlin (1966), Water, Waste, and Land, Ltd. (1980), Howard 11982), and unpublished U.S. Geological Survey data.