Geology in relation to availability of water along the south rim, Grand Canyon National Park, Arizona

Abstract

A ground-water investigation of the south-rim area, Grand Canyon National Park, Ariz., was made at the request of the National Park Service to determine whether a sufficient water supply could be developed to increase adequately the supply for Grand Canyon (village) and Desert View. The Park Service estimates that an increase of about 1 cfs would be adequate for the expected expansion of the park. The oldest rock units are the Vishnu schist and the Unkar group of the Grand Canyon series of Precambrian age. The Paleozoic formations are, in ascending order, the Tapeats sandstone, Bright Angel shale, and Muav limestone of the Tonto group of Cambrian age; the Temple Butte limestone of Devonian age; the Redwall limestone of Mississippian age; and the Supal formation, Hermit shale, Coconino sandstone, Toroweap formation, and Kaibab limestone of the Aubrey group of Pennsylvanian and Permian age. The youngest rock units are the Moenkopi formation and the Shinarump member of the Chinle formation of Triassic age. The principal aquifer in the Grand Canyon village area is the Muav limestone which, in many places, yields water readily to springs. Locally, the Vishnu schist, Tapeats sandstone, Supai formation, and Coconino sandstone yield small quantities of water to springs. Outside the park, large springs issue locally from the Eedwall limestone. The major geologic structures are the monoclinal flexures associated with the East Kaibab monocline, and the Bright Angel fault. The regional dip of the strata is commonly 1° to 2° to the south and southwest. The occurrence of ground water along the south rim is related not only to the lithology of the sedimentary formations but also to the geologic structure. Ground water occurs in a series of perched water bodies in the sedimentary rock sequence. Where an aquiclude occurs, the overlying rock unit generally contains water if structural conditions are favorable, as exemplified by the small springs that issue from the Supai formation and Coconino sandstone. The main body of water in this area occurs in the Muav limestone, which is underlain by the Bright Angel shale an aquiclude. 105 106 HYDROLOGY OF THE PUBLIC DOMAIN The amount of recharge to the ground-water reservoir is believed to be only a small percentage of the average annual precipitation. Havasu Springs have a discharge of 66 cfs and are the natural discharge point for a drainage area assumed to be the same as that of Havasu Creek above the springs 2,900 square miles. In the Desert View area monoclinal flexures probably govern the direction of ground-water movement, and near Grand Canyon village the Bright Angel fault exerts an influence. The total discharge of springs between Hermit and Cottonwood Creeks is about 600 gpm. Of this, more than 500 gpm comes from two groups of springs those at Indian Garden and in Hermit Creek. Chemical analyses of water samples from springs indicate that the amount of dissolved mineral matter in the water is small. The range in dissolved solids in 14 samples from the report area was from 179 to 667 ppm, and all samples except 2 contained less than 400 ppm. Two possibilities of developing additional water for the village are capture and transport of more water from springs at Indian Garden and development of the springs in Hermit Creek. If the two sources do not provide the quantity of water needed, or if the cost of development is too high, then studies of the water resources of the north-rim area would be worth while. There is little or no possibility of developing water in the Desert View area.