Geohydrology and effects of water use in the Black Mesa area, Navajo and Hopi Indian Reservations, Arizona

The N aquifer is the main source of water in the 5,400-square-mile Black Mesa area in the Navajo and Hopi Indian Reservations in northeastern Arizona. The N aquifer consists of the Navajo Sandstone and parts of the underlying Kayenta Formation and Wingate Sandstone of Jurassic and Triassic age. Maximum saturated thickness of the aquifer is about 1,050 feet in the northwestern part of the area, and the aquifer thins to extinction to the southeast. Water is under confined conditions in the central 3,300 square miles of the area. To the east, north, and west of Black Mesa, the aquifer is exposed at the surface, and water is unconfined. The aquifer was in equilibrium before about 1965. Recharge of about 13,000 acre-feet per year was balanced primarily by discharge near Moenkopi Wash and Laguna Creek and by evapotranspiration. At least 180 million acre-feet of water was in storage. The estimated average hydraulic conductivity of the aquifer is 0.65 foot per day. The confined storage coefficient is estimated to be about 0.0004 where the aquifer is thickest, and the estimated unconfined storage coefficient ranges from 0.10 to 0.15.

Ground-water withdrawals that averaged 5,300 acre-feet per year from 1976 to 1979 have caused water levels to decline in wells in the confined part of the aquifer. Withdrawals include an average of 3,700 acre-feet per year to supply a coal-slurry pipeline from a coal mine on Black Mesa. Six observation wells equipped with water-level recorders have been used to monitor aquifer response. The water level in one well 32 miles south of the mine declined 17 feet from 1972 through 1979 and 3.5 feet during 1979.

A mathematical model of the N aquifer was developed and calibrated for equilibrium and nonequilibrium conditions. The model was used in part to improve estimates of aquifer characteristics and the water budget, and it successfully reproduced the observed response of the aquifer through 1979. The model results indicate that about 95 percent of the 44,000 acre-feet of water pumped from 1965 to 1979 was withdrawn from storage, but the reduction amounted to less than 0.03 percent of total storage. Water-level declines through 1979 were estimated to be more than 100 feet in an area of 200 square miles. Four projections of future water-level changes were made using the model. The most probable projection indicates that water-level declines would exceed 100 feet in an area of 440 square miles by 2001. Most of the decline would be recovered within a few years if withdrawals at the mine ceased. By 1990, however, municipal-supply pumpage is expected to exceed pumpage at the mine, and this pumpage would continue to have significant impacts on water levels in the Black Mesa area.