Superfund Site | 19th Avenue Landfill: Page 2 of 2

Site Hydrogeology

The site is situated in the southeastern portion of the west sub-basin of the Salt River Basin in central Arizona. The site is within the basin and range physiographic province. The landfill is sited on alluvial fill material that commonly occupies the structurally depressed basins of the region. No active faults are known to be present near the site.

A monitor well installation program was implemented to characterize the shallow subsurface geology in the area near the landfill. This was accomplished by drilling 12 boreholes during the summer of 1987, four of which were drilled to a depth of 300 feet or greater. Data collected from the boreholes indicate that at least five identifiable stratigraphic subunits exist within approximately 400 feet of the surface. These stratigraphic subunits belong to the upper alluvial unit with designated subunits S, A, B, C and the middle alluvial unit.
The natural groundwater flow direction beneath both cells of the landfill is to the northwest. This phenomenon is controlled primarily by the pumping of large volume irrigation wells located northwest of the landfill site. Season fluctuations can occur, however, the pumping of irrigation wells along with the natural regional flow direction controls the groundwater flow beneath the landfill.

Depth to groundwater ranges between 20 to 40 feet below ground surface (bgs) near the river, and 60 to 80 feet bgs north of the site. The current drought has resulted in lowering of the water table by 20 feet or more.

Action Taken

In 2020, the Army Corps of Engineers, on behalf of the U.S. EPA, completed the fifth Five Year Review (FYR). The FYR concluded that the site remedy is protective of public health and the environment.

Status

The COP continues to operate and maintain the treatment system and submit semi-annual groundwater monitoring reports.

As hazardous substances remain buried on-site above health-based levels, the site will continue to undergo FYRs to ensure the remedy continues to be protective of public health and the environment. The next FYR will be due in 2025.

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