NIBW Superfund Site: Geology and Groundwater Hydrology

The NIBW Superfund Site is located along the Indian Bend Wash in south Scottsdale. In this area, the underlying geology consists of a fairly thick sequence of sediments that extends to depths of over 1,000 feet below the ground surface.

The NIBW Site is part of an area referred to as the Paradise Valley basin, which is a broad, sediment-filled trough surrounded by the McDowell Mountains to the northeast and the Phoenix Mountains, Camelback Mountain, and the Papago Buttes on the west and southwest. Granite and other consolidated rocks forming the mountains along the basin margins generally underlie the basin sediments. The sediments filling the basin are alluvial deposits consisting of layers of gravel, sand, silt, and clay. Alluvial deposits, or alluvium, are derived from the weathering of exposed rocks in adjacent or sometimes quite distant mountains and transported and deposited into the basin by streams over a period of hundreds of thousands or even millions of years.

Since the early 1980s, More than 150 groundwater monitor wells have been installed and dozens of site specific studies have been completed by the Environmental Protection Agency and companies involved in the remediation effort. This has given us a good understanding of the complex geology and hydrology of the NIBW area.

Groundwater is present in three distinct layers of sediment known as the upper alluvium unit (UAU), the middle alluvium unit (MAU), and the lower alluvium unit (LAU).

NIBW upper alluvium unit (UAU)

UAU sediments consist mostly of coarse-grained sediments (sand and gravel) with lesser amounts of finer-grained silt and clay that occur primarily in the uppermost soils. The coarse-grained sediments are much like the sand, gravel and boulders present in the riverbed of the Salt River. The UAU extends from the ground surface to a depth ranging from about 90 to 190 feet across the NIBW area, with an average thickness of about 140 feet.

Groundwater is typically found at the base of the UAU in most of the NIBW Site in what is termed an unconfined aquifer. An unconfined aquifer is like a sandbox filled with porous sediments. The groundwater surface, or water table, rises when recharged and falls when water is pumped out or moves vertically into underlying layers. The UAU in the northern NIBW Site has little or no groundwater whereas there is a saturated thickness of several tens of feet in areas to the south. In those areas where groundwater is found in the UAU, the direction of movement is generally from east to west.

NIBW middle alluvium unit (MAU)

The MAU represents the finer-grained portion of the basin deposits and consists chiefly of weakly cemented silt and clay layers with sand and gravel interbeds. The MAU is typically 500 to 600 feet thick throughout most of the NIBW Site but thins to the west along the basin margin. For example, the MAU in some locations west of Scottsdale Road is 60 feet thick or less and disappears all together as the basin sediments encroach upon the exposed bedrock of the Papago Buttes just southwest of the NIBW Site.

The MAU is generally saturated with groundwater, but the layered silt and clay beds confine the aquifer to predominantly horizontal or lateral flow and limit vertical groundwater movement. Pumping of production wells largely controls groundwater movement in the MAU. A cone of depression, or sink, for MAU groundwater is generally centered around extraction wells that pump from the MAU in the central part of the site. Extraction wells completed solely within this unit are capable of producing over 1,000 gallons per minute of groundwater. The pattern of groundwater movement is also influenced by the thinning of the MAU sediments towards the southwestern part of the Site. In this region, impediments to vertical flow are not as prevalent, and MAU groundwater can migrate into the underlying LAU aquifer.

NIBW lower alluvium unit (LAU)

The LAU is generally a coarse-grained unit, however LAU sediments range in size (from clay to boulders), and in how strongly they are cemented. Consolidation within the LAU increases with depth at the site. Although total thickness of the unit has not been defined at many locations in the NIBW area, thickness of LAU sediments is interpreted to be very large (at least several thousand feet) near the center of the Paradise Valley basin and to decrease substantially toward the basin margins. Across the center of the NIBW Site, the top of the LAU is found approximately 600 to 700 feet below the ground surface.

The LAU is a confined and fully saturated aquifer in the NIBW area. Extraction wells completed within the LAU are capable of producing several thousand gallons per minute or more of groundwater. Because the aquifer is so productive, many groundwater wells operated by the area water providers derive much of their supply from the LAU. The heavy pumping of the LAU controls groundwater flow within the unit and also induces a downward gradient, or driving force, for movement of groundwater from the overlying UAU and MAU into the LAU. Since the MAU generally restricts vertical movement of groundwater in the NIBW area, much of the recharge of groundwater to the LAU occurs along the basin margin, particularly to the southwest of the Site, where the MAU is thin or absent. As such, groundwater moves northeast in the LAU from the southwest margin recharge area, north through the central part of the site, and converges toward the cone of depression created by extraction wells in the vicinity of the Arizona American Water Company well field.