Ground Water in the Navajo National Monument Area

Christensen, Paul

The Peabody Coal Company operates strip coal mines in the Navajo Indian Reservation. The operation withdraws considerable quantities of water from the N multiple-aquifer for industrial use. Navajo National Monument which is located 19 miles northwest of the Peabody Coal field also utilizes the same aquifer for water. The rocks in the area are a sequence of Triassic, Jurassic, and Cretaceous rocks composed of mudstones, siltstones, and sandstones. Coal resources are located in the Cretaceous rocks, particularly in the Wepo formation which caps Black Mesa. The N multiple-aquifer consists of the Triassic-Jurassic Navajo sandstone, the Triassic Kayenta formation, and the Triassic Lukachukai member of the Wingate sandstone. The principal lithology of these units is sandstone. The Navajo sandstor2is the chief aquifer in this system. The Carmel formation overlies the Navajo and confines the N multiple-aquifer. The Triassic Chinle formation underlies the Wingate sandstone and acts as the lower confining unit to the aquifer. Structurally, the Organ Rock Monocline plunges the N multiple-aquifer beneath Black Mesa. At the Peabody well field depths to the Navajo sandstone approach 2500 feet. Artesian conditions exist. The Navajo sandstone at Betatakin no. 2 well is exposed at the surface and unconfined conditions occur. Betatakin no. 2 well is located neat the very steep-walled Tsegi Canyon, which exposes the contact between the Navajo and Kayenta and largely drains the Navajo, thus limiting the saturated thickness of the Navajo in such areas. In the. Black Mesa area, the transmissivity values of the Navajo range from 75 to 350 square feet per day; the hydraulic conductivity from 0.40 to 0.70 feet per day. The artesian part of the aqUifer has storage coefficients from 10-3 to 10-4 , and the unconfined part of the aquifer from 0.10 to 0.15. The recharge area for the artesian portion of the aquifer has an area of about 100 square miles, compared to the surface area of 3,000 square miles of the aquifer beneath Black Mesa. Rate of groundwater movement is about 2 to 4 feet per year. Betatakin no. 2 well has a total depth of 753 feet. The static water level sits at 535 feet BSL. An aquifer test showed the water table dropped 57 feet after 19.3 hours of pumping at a discharge of 14.2 gpm. The coefficient of transmissibility is 127 gpd/ft. Specific capacity is 0.27 gpm/ft. No stabilization occurred. A recovery of 90 per cent occurred 80 minutes after pum.ping ceased. Domestic wells near Betatakin no. 2 well also utilize water from the Navajo sandstone. These wells. probably will not affect water levels at Betatakin no. 2 well at current discharge rates. An annual withdrawal of 4,200 acre feet of water by the Peabody Coal Company probably far exceeds the recharge rate. The mining of the groundwater could have an effect upon the Betatakin no. 2 well. U.S.G.S. monitored wells around the northern periphery of Black Mesa.have shown no noticeable effects on the groundwater table there to date. Currently, the U.S.G.S. is constructing a model to determine the long-term effects of continued withdrawals at the Peabody water well field. It is planned for release spring 1980.