The Hop·; area includes about 3,200 mi² in northeastern Arizona ~nd is mostly in the Navajo and Hopi Indian Reservations; about 400 mi of the area is south of the Navajo Indian Reservation boundary . Ground water occurs in several aquifers that are made up of one or more formations; the composite stratigraphic column indicates the relative position of the formations. The main sources of ground water are the Coconino aquifer, the Chinle Formation. the N and D aquifers, the Toreva and Bidahochi Formations, and the alluvium. The aquifers are separated by fine-grained rock units that inhibit the vertical movement of ground water.

Ground-water development has been slight, and the water is used mainly for public, domestic, and livestock supplies. In 1977 about 425 acre-ft of ground water was withdrawn in the Hopi area. The amount of ground water withdrawn annually is small compared to the potential recharge from precipitation; therefore, water levels generally are not affected by pumping from wells.

Because the ground-water system in the Hopi area has not been changed greatly by development, a large number of water-level measurements and water-quality data collected prior to 1977 are used in this report to show conditions in 1977. A few water-level measurements were made south of the Navajo Indian Reservation boundary in 1977.

The hydrologic data on which these maps are based are available, fo~ the most part, ·in computer-printout form and may be consulted at.the Arizona Water Commission, 222 North Central Avenue, Suite 850, Phoenix, and at U.S. Geological Survey offices in: Federal Building, 301 West Congress Street, Tucson; Valley Center, Suite 1880, Phoenix; and 2255 North Gemini Drive, Building 3, Flagstaff. Material from which copies can be made at private expense is available at the Tucson, Phoenix, and Flagstaff offices of the U.S . Geological Survey.