Ground-Water Conditions in the Kaiparowits Plateau Area, Utah and Arizona, with Emphasis on the Navajo Sandstone

Blanchard, Paul

The Kaiparowits Plateau area is located in central Garfield and eastern Kane Counties of Utah and north-central Coconino Country of Arizona. The area covers about 4,850 square miles and includes and drainages of the Escalante and Paria Rivers, and several smaller drainages to the Colorado River between the Escalante and Paria River drainages. The consolidated-rock aquifer that has the most potential for ground-water development is the Navajo Sandstone of Triassic(?) and Jurassic age. The Entrada Sandstone of Jurassic age is a secondary aquifer, and the Wingate Sandstone of Triassic age presently is not developed but potentially is a significant aquifer. The top of the Navajo Sandstone ranges in depth from the surface near Boulder, on the Paria Plateau, west of the East Kaibab monocline, and in the canyons of the Escalante and the Paria Rivers, to as dee[ as 4,500 feet below land surface in the center of the Kaiparowits Plateau. The aquifer is confined naturally under the Kaiparowits Plateau and near where U.S. Highway 89 crosses the Paria River, and artificially along the shores of Lake Powell. Annual recharge to and discharge from the Navajo Sandstone is estimated to be between 8,300 and 16,900 acre-feet. The majority of recharge occurs by infiltration of precipitation into the Navajo Sandstone outcrop, especially from snowpack at higher altitudes on Boulder Mountain, and where Navajo is extensively fractured, for example, west of the East Kaibab monocline. The amount of recoverable water in storage in the Navajo Sandstone is estimated to be about 190 million acre feet. In areas where water table conditions exist in the Navajo Sandstone water is generally fresh (contains less than 1,000-3,000 miligrams per liter of dissolved solids). Dominant ions in areas of water-table conditions are calcium, magnesium, and bicarbonate. In areas of confined conditions, sodium and sulfate are the dominant cation and anion in the water. Inundation of Glen Canyon and tributary canyons by Laker Powell has caused a large increase in the altitude of the potentiometric surface near the lake, as much as 357 feet at well (A-41-8) 4dda-1, about 2.2 miles from the lake. Some increase in the altitude of the potentiometric surface gas occurred as far as 11 miles from the shoreline of the lake. Inundation has caused changes in the chemistry of ground water near Lake Powell, primarily where the ground-water level has increased to an altitude higher than that of the Navajo Sandstone-Carmel Formation contact. Major ion chemistry has been altered, the most notable change being an increase in sulfate concentration and a decrease in bicarbonate plus carbonate concentration. Analyses of where from three water-supply wells in the Glen Canyon National Recreation Area have shown arsenic concentrations in excess of maximum recommended for drinking water. The Kaiparowits Plateau area contains large quantities of energy resources. Because of the small amount of recharge to and natural discharge from the Navajo Sandstone in the study area, large withdrawals, such as those required for continued exploration for and development of those energy resources, primarily would be from storage rather than from diverted discharge.

Ground-Water Conditions in the Kaiparowits Plateau Area, Utah and Arizona, with Emphasis on the Navajo Sandstone

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