The N-aquifer has an areal extent of 6,250 square miles and also occurs north
of the Little Colorado River (Figure 8). The Navajo Sandstone and, where it is
present, the Wingate Sandstone are the main water-bearing units in the
N-aquifer. The aquifer generally is under water-table conditions (unconfined)
although an artesian (confined) zone exists in the Black Mesa area. Water-level
declines have occurred in this confined area as a result of groundwater
withdrawals for the Black Mesa Coal Mine slurry pipeline operated by Peabody
Coal. Water levels in the confined area are much more sensitive to pumping than
those in the unconfined area. Metered groundwater pumpage for the coal slurry
pipeline ranged from a low of 43 acre-feet in 1969 to a high of 4,740 acre-feet
in 1982; in 1989, 3,450 acre-feet were withdrawn (U.S. Geological Survey, 1992).
Precipitation falling on the exposed aquifer units is the main source of recharge for the N-aquifer. Groundwater in the N-aquifer moves southward and southeastward under Black Mesa. The flow divides under the mesa, moving westward and eastward (Eychaner, 1981). Water in the N-aquifer is of good quality and suitable for most uses. The N-aquifer is a source of groundwater supply for the Navajo and Hopi Reservations, as well as the Black Mesa area.