The southern part of the Black Mesa area includes about 2,500 mi² in northeastern Arizona and is entirely in the Navajo and Hopi Indian Reservations. The main source of water is the ground water in the several aquifers that are made up of one or more formations. The aquifers are stacked one on the other and generally are not hydraulically connected; the composite stratigraphic column indicates the relative position of the formations. The main waterbearing units are the N and D aquifers, the Toreva and Wepo Formations, and the alluvium. The geologic structure and topographic relief preclude a uniform depth to water in the area; therefore, recommended drilling depths should be determined on an individual site basis. Ground-water development has been mainly for public, domestic, and livestock supplies, In 1975 about 500 acre-ft of ground water was withdrawn from the aquifers in the southern part of the black Mesa area; mo st of the water was from the N aquifer. In 1951-76-the period for which data were used to compile these maps-withdrawals from the N aquifer resulted in water-level declines in parts of the area . Measurable changes in water levels have not occurred in the other aquifers in the 25-year period.

The hydrologic data on which these maps are based are available, for the most part, in computer-printout form for consultation at the Arizona Water Commission, 222 North Central Avenue, Suite 800, 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.