Bedrock aquifers in the lower Dirty Devil River Basin area, Utah, with special emphasis on the Navajo sandstone

Abstract

The lower Dirty Devil River basin area in southeastern Utah has an area of about 4,300 square miles (11,1140 square kilometers) and ranges in altitude from about 3,700 to more than 11,000 feet (1,130 to 3,350 meters) above mean sea level. Precipitation, the main source of water in the area, ranges from slightly less than 6 inches (152 millimeters) per year in the lowlands to more than 30 inches per year (762 millimeters) in the Henry Mountains and along the western boundary. Rocks that crop out in or underlie the area range from the Precambrian to the Holocene in age. The thickness of the composite section of sedimentary rocks ranges from about 7,300 feet (2,200 meters) to about 23,000 feet (7,000 meters). The Entrada, Navajo, Wingate, and Coconino Sandstones and rocks of Mississippian age are considered major aquifers because of their large areal extent or thickness or their known locally large yields to wells. The chemical quality of the water in these aquifers ranges from fresh to briny. The permeability of the aquifers in the area is affected by folding, faulting, and igneous intrusion. These geologic processes have locally enhanced ground-water circulation by fracturing or impeded circulation by offsetting permeable beds or sealing some zones with rocks of lower permeability. The total hydrologic system in the lower Dirty Devil River basin has estimated long-term average annual inflow and outflow of about 1.6 million acre-feet (1,970 cubic hectometers), of which about 1.55 million acre-feet (1,910 cubic hectometers) is derived from precipitation. An estimated 96 percent of the water available to the area is consumed by evapotranspiration. The estimated gross annual average ground-water recharge is 34,000 acre-feet (42 cubic hectometers), of which 5,000 acre-feet (6.2 cubic hectometers) recharges the Navajo Sandstone. Recoverable fresh to moderately saline water stored in the Navajo, Wingate, and Coconino Sandstones is estimated to be 210 million acre-feet (259,000 cubic hectometers), of which 89 million acre-feet (110,000 cubic hectometers) is stored in the Navajo alone. Long-term large withdrawals from the Navajo Sandstone are feasible. Withdrawal of 12,000 gallons per minute (757 liters per second) over a period of about 36 years probably would diminish the amount of water in storage by less than 1 percent. The withdrawals, would eventually diminish the average annual discharge of the Dirty Devil River by possibly as much as 13 cubic feet per second (0.37 cubic meter per second). A change of this magnitude on the flow of the Colorado River would be too small to measure.