The N aquifer is an important source of water in the 5,400 sq-mi Black Mesa area on the Navajo and Hopi Indian Reservations. The Black Mesa monitoring program is designed to monitor long-term effects on the groundwater resources of the mesa as a result of withdrawals from the aquifer by the strip-mining operation of Peabody Coal Company. Withdrawals from the N aquifer by the mine increased from 95 acre-ft in 1968 to more than 4,480 acre-ft in 1986. Water levels in the confined area of the aquifer declined as much as 90 ft from 1965 to 1987 in some municipal and observation wells within about a 15-mi radius of the mine well field. Part of the drawdown in municipal wells is due to local pumpage. Water levels have not declined in wells tapping the unconfined area of the aquifer. Chemical analyses indicate no significant changes in the quality of water from wells that tap the N aquifer or from springs that discharge from several stratigraphic units, including the N aquifer, since pumping began at the mine. (USGS)