Isopach and structure contour mapping of thin bentonite and shale beds in an area of mapped lineaments, central South Dakota

Abstract

The N aquifer is an important source of water in the 5,400 square-mile Black Mesa area on the Navajo and Hopi Indian Reservations, Arizona. The Black Mesa monitoring program is designed to monitor long-term effects on the groundwater resources of the mesa as part of withdrawals from the aquifer by the strip-mining operation of Peabody Coal Co. Withdrawals from the N aquifer by the mine increased from 95 acre-feet in 1968 to more than 4,000 acre-feet in 1984. In 1985, withdrawals from the mine wells were temporarily reduced to about 2,500 acre-feet. Water levels in the confined area of the aquifer declined as much as 87 feet from 1965 to 1985 in some municipal and observation wells within about a 15-mile radius of the mine well field. In 1986, measurements indicated some recovery in water levels in most of these wells because of an approximate 90-percent reduction in pumpage from Peabody Coal Co. wells during the last half of 1985. 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)