Hydrology of Area 61, Northern Great Plains and Rocky Mountain Coal Provinces, Colorado and New Mexico

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7.0 GROUND WATER 7.1 Aquifers Unconsolidated Sediments and Bedrock Yield Small to Large Quantities of Ground Water Alluvium and talus yield more ground water than bedrock, but are less extensive. Virtually all formations in Area 61 store and transmit water (fig. 7.1 -1). 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 (2-ato 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 -1) also may yield large - quantities of ground 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 less permeable formations underlying the aquifers. The principal bedrock aquifers in the area are the Dakota Sandstone - Purgatoire Formation, Fort Hays Limestone Member of the Niobrara Formation - upper part of the Carlile Shale, Raton Formation - Vermejo Formation - Trinidad Sandstone, Cuchara 64 Formation - Poison Canyon Formation, Devils Hole Formation - Farasita Conglomerate, and volcanic rocks (table 7.1 -1). Within these units, sandstone and conglomerate layers and basalt flows 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 Marl Member of the Niobrara Formation, Pierre Shale, and Huerfano Formation, retard the downward movement of water and confine flow within the aquifers. Precambrian and Tertiary intrusive and metamorphic rocks, unless extensively fractured, act as barriers to ground -water flow. The principal sources of ground -water informa- tion in Area 61 are Griggs (1948), Powell (1952), McLaughlin and others (1961), McLaughlin (1966), Water, Waste, and Land, Ltd. (1980), Howard (1982), and unpublished U.S. Geological Survey data.