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Ofl32J5 . CHAPTER V WATER RESOURCES Bedrock aquifers Except for the La. Sal and Abajo Mountain areas where igneous rocks are exposed, the study area is nearly all underlain by a series of con- solidated sedimentary formations that will transmit water and yield it to wells and springs. The formations that are aquifers or contain mem- bers that are aquifers, listed in the order of youngest geologie age, are: Dakota sandstone; Burrow Canyon and Morrison formation; Bluff, En- trada, Navajo, and Wingate sandstones; the lower part of the Chinle and upper part of the Cutler formation; and the Rico and Hermosa formations. The formations are encountered at depths ranging from surface outcrops to more than 2,000 feet., The Navajo sandstone, being thicker, more exten- sive, and with better water conductivity than the other formations, has the greatest ground water potential, with the Entrada next in importance. Although these two formations are more than 150 feet below the surface in much of the area, they are usually 'under sufficient artesian pressure to force well water to or near the surface. The base of the Entrada is 200 to 450 feet nearer to the surface than the Navajo, making it more easily reached. . The bedrock aquifers provide a large ground water reservoir. Re- charge to the aquifers occurs in the highland created by the intrusive bodies forming the La. gal and Abajo Mountains and by the Monument Valley upwarp. It is presumed that the recharge to the aquifers was in balance with natural discharges before wells were put down in the area. Only a few wells are now operating so the natural discharge may still approxi- mate the recharge. Water rec~eries from bedrock aquifers are limited by the low trans- missivity of the aquifers. Consequently the yield of most wells is lim- ited from a few to 50 gallons per minute. Seldom do yields exceed 200 gallons per minute. A few wells located in fractured zones in the Navajo sandstone where there is high local transmissivity pump up to 2,400 gal- lons per minute. Pumping from bedrock aquifers will cause a local but continuing drop in water level if the pumping rate exceeds the capacity of the aquifers to convey water to the pump site. The quality of water from bedrock aquifers varies widely ~er the area. As a rule the salt content ranges from 200 to 8,000 parts per mil- lion. The amount of dissolved solids generally increases with the dis- tance from the recharge area and is less in the more permeable aquifers. Water chemically suitable for culinary use can be obtained from bedrock aquifers ~er the greater part of the area. The potential of bedrock aquifers as a source of ground water has not been evaluated. A comprehensive ground water investigation before or concurrent with feasibility studies of a potential surface water . 50