Hydrogeologic framework and characterization of the Truxton Aquifer on the Hualapai Reservation, Mohave County, Arizona

The U.S. Geological Survey, in cooperation with the Bureau of Reclamation, developed this study to determine an estimate of groundwater in storage in the Truxton aquifer on the Hualapai Reservation in northwestern Arizona. For this study, the Truxton aquifer is defined as the unconfined, saturated groundwater in the unconsolidated to semiconsolidated older and younger basin-fill deposits of the Truxton basin overlying bedrock. The physical characteristics of the Truxton aquifer have not been well characterized in the past. In particular, the depth to impermeable granite bedrock and thickness of the basin are known in only a few locations where water wells have penetrated into the granite. Increasing water demands on the Truxton aquifer by both tribal and nontribal water users have led to concern about the long-term sustainability of this water resource. The Hualapai Tribe currently projects an increase of their water needs from about 300 acre-feet (acre-ft) per year to about 780 acre-ft per year by 2050 to support the community of Peach Springs, Arizona, and the southern part of the reservation. This study aimed to quantitatively develop better knowledge of aquifer characteristics, including aquifer storage and capacity, using (1) surface resistivity data collected along transects and (2) analysis of existing geologic, borehole, precipitation, water use, and water-level data.

The surface resistivity surveys indicated that the depth to granite along the survey lines varied from less than 100 feet (ft) to more than 1,300 ft below land surface on the Hualapai Reservation. The top of the granite bedrock is consistent with the erosional character of the Truxton basin and exhibits deep paleochannels filled with basin-fill deposits consistent with the results of surface resistivity surveys and borehole logs from wells. The estimated average saturated thickness of the Truxton aquifer on the Hualapai Reservation is about 330 ft (with an estimated range of 260 to 390 ft), based on both resistivity results and the depth to water in wells. The saturated thickness might be greater in parts of the Truxton aquifer where paleochannels are incised into the granite underlying the basin-fill sediments. The estimated groundwater storage of the Truxton aquifer on the Hualapai Reservation ranges from 420,000 to 940,000 acre-ft and does not include groundwater storage in the aquifer outside the Hualapai Reservation boundary. In addition, the calculation of total storage in the Truxton aquifer does not determine nor indicate the availability and sustainability of that groundwater as a long-term resource. These results compared well with studies done on alluvial-basin aquifers in areas adjacent to this study. The part of the Truxton aquifer on the Hualapai Reservation represents about 20 percent of the entire aquifer.