Local aquifers are of great importance for domestic water supplies where the three regional aquifers, the D-, N-, and C-aquifers, are too deep or have unsuitable water quality. The local aquifers include alluvial deposits, which occur in washes and stream channels throughout the basin, sedimentary and volcanic rocks of the Bidahochi Formation, and various sandstones.

The alluvium along the Little Colorado River and its tributaries is an important local source of water for domestic supplies (Figure 6). Water enters the alluvium as discharge from the D, N, and C-aquifers, as streamflow infiltration, or as direct rainfall. In thicker sections the alluvium is a steady source of water, but smaller washes can go dry because of overuse or drought conditions. Water quality varies greatly in the alluvial aquifers.

Radiochemical contamination is present in the alluvial aquifer along the Puerco River. The elevated levels of gross alpha and gross beta are caused by the movement of uranium-, radium-, and thorium-rich sediments from the 1979 Church Rock uranium mine tailing pond spill in New Mexico (Webb and others, 1988) and discharges of mine dewatering effluent which ceased in 1986 (U.S. Geological Survey, 1991b). Current movement of radionuclides is due to discharges from the sewage-treatment plant in Gallup, New Mexico (U.S. Geological Survey, 1991b). This area is considered one of the principal water-quality problem areas in the state (Arizona Department of Environmental Quality, 1990).

Basaltic volcanic rocks occur along the southeastern and southwestern edges of the Little Colorado River Plateau basin (Figure 6). The basaltic rocks form an irregular, eroded surface and vary in thickness from 0 to 3,000 feet. Groundwater is found in fractures, cinder beds, and heavily weathered zones. In the southeastern part of Navajo County, the saturated basaltic rocks, together with underlying sedimentary rocks, locally are known as the Lakeside-Pinetop aquifer. Water quality is generally very good and water is used for domestic, stock, irrigation, and Lakeside/Pinetop public supply purposes.

The Bidahochi Formation forms a local aquifer in the northeastern part of southern Apache County and near St. Johns (Figure 6). The Bidahochi Formation is composed of sedimentary and volcanic rocks. The formation generally has three members: a basal sandstone, a middle volcanic unit, and an upper sandstone member. Principal groundwater withdrawal is from the upper sandstone unit. Recharge is derived from rainfall onto the outcrop exposures.

Undifferentiated sandstones west of Show Low along the Mogollon Rim, form the locally-named White Mountain and Springerville aquifers (Figure 6). These aquifers receive recharge from rainfall and snowmelt infiltrating overlying basaltic rock units. In southern Apache County, the basaltic rocks are in hydraulic connection with the White Mountain and Springerville aquifers. Water quality generally is good and water is used for domestic, stock and Springerville and Eager public supply purposes.

Aquifers of the Little Colorado River Plateau basin contain large quantities of groundwater in storage; however, they are in a sensitive relationship with the Little Colorado River and its perennial tributaries. Lowering of hydrostatic heads by excessive groundwater withdrawals may cause some perennial reaches of the streams to dry up.