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 southern Iron and Garfield Counties, Utah, and 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 (680 million cubic meters) per year of precipitation falls on the Navajo Sandstone where it crops out between the Paria River and the Hurricane fault. On the basis of estimates of discharge from the Navajo aquifer, 50,280 to 68,180 acre-feet (62 to 84 million cubic meters) 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 (49 to 71 million cubic meters) per year. Estimated spring discharge for the study area is 8,140 acre-feet (10 million cubic meters) per year. Estimated discharge by evapotranspiration and well withdrawals for the study area is 2,400 acre-feet (3 million cubic meters) per year.

The flow of ground water through the Navajo aquifer 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 (610 meters), 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 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.

Proposed withdrawals of about 4,000 acre-feet (4.9 million cubic meters) 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 5 to 10 percent for specific yield and 9.0 × 10⁻⁴ to 2.4 × 10⁻³ for storage coefficient. Water-level declines simulated with the most realistic alternative model (flow only across the northern one-half of the Sevier fault) at the Bald Knoll pumping site were 135 feet (41 meters) with the smallest storage values and 124 feet (38 meters) with the largest storage values. No substantial water-level declines occurred in the vicinity of Pipe Spring National Monument or Zion National Park in any of the simulations.