Simulation of ground-water flow and water-level declines that could be caused by proposed withdrawals, Navajo Sandstone, southwestern Utah and northwestern Arizona

Abstract

It has been proposed that water from the Navajo Sandstone be used in developing coal resources in western Kane County, Utah. The Navajo aquifer in western Kane, eastern Washington, and the southernmost parts of Iron and Garfield Counties, Utah, and in northern Mohave and Coconino Counties, Arizona, is the source of stream base flow and spring discharge in Zion National Park and Pipe Spring National Monument. The Navajo aquifer also supplies water for municipal, domestic, stock, and agricultural use in this semiarid region. Approximately 550,000 acre-feet per year of precipitation falls on the Navajo Sandstone where it crops out between the Paria River and the Hurricane Fault. According to estimates of discharge from the Navajo aquifer, from 50,280 to 68,180 acre-feet of water recharges the Navajo aquifer annually between the Paria River and the Hurricane Fault. In the areas of exposed outcrop, ground water moves from higher altitude recharge areas to deeply incised canyons where water is discharged to springs and streams. Estimates of total discharge to the North and East Forks of the Virgin River and to Kanab Creek range from 39,800 to 57,700 acre-feet per year. The estimated spring discharge for the study area is 8,140 acre-feet per year. The direction and movement of ground water farther to the north, where the Navajo Sandstone is buried beneath younger formations, is poorly defined because of the lack of wells in the area. The flow of ground water also is complicated by two large vertically offsetting faults the Sevier Fault and the Paunsaugunt Fault. In places, these faults have offsets of nearly 2,000 feet, and the hydrologic properties and influence of these faults on ground-water flow have not been determined. In lieu of further data acquisition, computer simulations were used to test various alternative concepts of the hydrologic system and its properties, particularly the possibility of east-to-west flow across the Sevier Fault. The results of three alternative steady-state models showed that flow probably does not occur across the southern part of the Sevier Fault where the aquifer is completely offset, but may occur farther north where the offset decreases. Flow across the Sevier Fault for the alternative simulations ranged from 0 to 10,000 acre-feet per year. Proposed withdrawals of about 4,000 acre-feet per year for 30 years from a well near Bald Knoll were simulated for the three alternative models. The range of storage values used was from 5 to 10 percent for specific yield and from 9.0 x 104 to 2.4 x 10'3 for storage coefficient. These simulations produced waterlevel declines from 119 to 188 feet at the Bald Knoll pumping site. No substantial water-level declines occurred in the vicinity of Pipe Spring National Monument or Zion National Park in any of the simulations. 1