The C-aquifer is utilized as a water supply south of the Little Colorado River and along the southern edge of the basin by Flagstaff, Heber, Overgaard, Show Low, Snowflake and Concho. North of the river the C-aquifer is too deep to be economically useful, or is unsuitable for most uses because of high concentrations of total dissolved solids. In general, the water quality of the C-aquifer degrades with increasing distance from recharge areas and at increasing depths (USBOR, 2006).
The N-aquifer occurs north of the Little Colorado River and has an areal extent of 6,250 square miles. The Navajo and Wingate Sandstones are the main water-bearing units in the N-aquifer. Groundwater flow direction varies as shown in Figure 2.1-7 and is generally south and west or north and west. The aquifer is generally unconfined but there are artesian conditions in the Black Mesa area and near Window Rock and much of the aquifer underlying the Hopi Reservation is unconfined (ADWR, 2008a). Natural recharge to the N-aquifer has recently been estimated at 2,600 to 20,246 AFA (OSM 2008). Water is discharged via springs, baseflow to streams and as underflow to drainages. N-aquifer storage estimates vary from 166 maf to 526 maf (ADWR, 1989; ADWR, 2008a).
Water levels measured in selected wells drilled in the N-aquifer vary in depth from 17 feet to 851 feet bls as shown in Figure 2.1-8. Water level changes between 1990-1991 and 2003-2004 varied in these measured wells (see Figure 2.1-7). Recent adjudication investigation on the Hopi reservation showed median well depths of 745 feet for claimed wells (ADWR, 2008a).
N-aquifer water quality is generally good and is a source of supply for the Navajo and Hopi Reservations. However, there are sites of uranium and heavy metal contamination due to past uranium mining and milling operations. Groundwater remediation activities are underway near Tuba City where a plume of groundwater contamination extends south and southeast of an uranium ore mill operation and 37 extraction wells convey water to an onsite treatment plant (DOE, 2008a)
The N-aquifer is utilized for mining operations at the Black Mesa Coal Mine operation. Until 2005, N-aquifer water was also used for the Black Mesa Coal Mine slurry pipeline that delivered coal to the Mohave Generating Station at Laughlin, Nevada. From the pre-mining period to 2003, the median water level decline was more than 23 feet in 26 wells and declines were approximately 72 feet for 12 wells in the confined part of the aquifer. (Truini, et al., 2005) To relieve impacts on the N-aquifer from pumping at Black Mesa, a proposal to use C-aquifer water withdrawn near Leupp was considered and a study undertaken that was completed in 2005 (Leake, et al., 2005). The Mohave Generating Station suspended operation in 2005, which has significantly reduced the need for N-aquifer withdrawals.
The D-aquifer overlays portions of the N- and C-aquifer in the planning area and is the smallest of the three regional aquifers. It covers about 3,125 square miles under the Navajo and Hopi reservations. The D-aquifer is composed of the Dakota, Cow Springs and Entrada sandstones. Flow direction is toward the southwest in the southern part of the aquifer and toward the northwest in the northern portion (Figure 2.1-7). Annual recharge is estimated at 5,392 acre-feet (GeoTrans and Waterstone, 1999). Recharge probably occurs along the eastern slope of Black Mesa where units of the aquifer outcrop (Lopes and Hoffman, 1997), and also locally along washes. There is some connection between the D-aquifer and the underlying N-aquifer and D-aquifer discharge also occurs via springs, baseflow to streams and as underflow along washes (ADWR, 2008a). ADWR (1989) estimated that there are 15 maf in storage in the D-aquifer.
Water level data from a well collected in 2003-2004 in the D-aquifer showed a depth to water at 271 feet bls and no water level decline since 1990-1991. Median water levels at 48 claimed wells on the Hopi reservation were 268 feet (ADWR, 2008a). Water quality is marginal to unsuitable for domestic use due to high concentrations of dissolved solids. Nevertheless, it is utilized in the north-central parts of the planning area for domestic use. |