Superfund Site | Hassayampa Landfill

EPA #: AZD980735666

Superfund National Priority List (NPL) Placement: July 22, 1987

Location

The Hassayampa Landfill (site) is located about 10 miles west of Buckeye, Arizona, and approximately six miles east of the Palo Verde Nuclear Generating Station. The site consists of about 10 acres formerly used for hazardous waste disposal which lies adjacent to the 47-acre former sanitary landfill. The plume boundary varies and may extend beyond the site boundary, but remains part of the Superfund site in its entirety.

Contaminants of Concern

The contaminants of concern for groundwater include various volatile organic compounds (VOCs) such as 1,1-dichloroethene (DCE); trichlorotrifluoroethane (Freon 113); 1,1,1-trichloroethane (TCA); 1,1-dichloroethane (DCA); trichloroethene (TCE); tetrachloroethene (PCE); trichlorofluoromethane (Freon 11); 1,2-dichloroethene (DCE); 1,2-dichloropropane; and toluene. Soils beneath the waste pits contain VOCs, heavy metals, pesticides, and lime wastes. Contaminants of concern at the site may change as new data becomes available.

Public Health Impact

Risk assessment results indicate that potential health risks may exist for individuals who might ingest the contaminated groundwater or come into direct contact with hazardous wastes present. The landfill is capped; therefore, there is no potential for adverse health effects due to inhalation of VOCs in the air or direct contact with the hazardous wastes present below the ground surface. Contamination in the groundwater is contained within the site boundaries. The groundwater contamination is restricted to the shallow aquifer which is not used as a potable water source.

Site Hydrogeology

The site is located on the broad southward-sloping alluvial plain of the Hassayampa River Basin. The basin is bounded on the east by the White Tank Mountains, on the south by the Buckeye Hills, and on the west by the Palo Verde Hills. The altitude of the land surface at the site is approximately 910 to 915 feet above mean sea level.
Regional hydrogeologic units in the area of the site include in order of increasing depth: recent alluvial deposits, basin-fill deposits, and the bedrock complex. Groundwater levels in the vicinity of the site generally lie below the base of the recent alluvial deposits. However, where saturated, the recent alluvial deposits may yield moderate quantities of groundwater to wells. The thickness of the basin-fill deposits appears to exceed 1,200 feet in the vicinity of the landfill.

The basin-fill deposits comprise the principal source of groundwater to wells in the area of the site, and are generally referred to as the regional aquifer. Within a three mile radius of the site, 349 groundwater wells have been identified, 172 of which potentially service individual residences. These wells yield groundwater from the regional basin-fill deposits aquifer. The reported depths range from five feet below land surface to 250 below land surface. The nearest downgradient domestic well is about 2,500 feet south of the site. The basin-fill deposits have been classified in order of increasing depth into the upper, middle, and lower alluvium units. The upper alluvial unit (UAU) beneath the site was subdivided in order of increasing depth into the upper alluvium deposits, basaltic lava flow unit, subunit A, and subunit B. The upper alluvium subunit consists of a coarse-grained part and a fine-grained part. The average depth to the base of the coarse-grained part is about 34 feet, while the average depth to the base of the fine-grained part is about 58 feet. The basaltic lava-flow consists of vesicular, basaltic rock and is part of the Arlington Mesa basalt flows. This subunit appears to thin and dip towards the north.

The presence of contaminated groundwater in subunit A indicates that the basaltic lava flow unit is not an impermeable unit. The part of the UAU from the base of the basaltic lava-flow subunit to the top of the middle alluvial unit is the uppermost water bearing part of the regional aquifer.
The direction of groundwater flow in subunits A and B is generally to the south, although local variations in the flow direction may occur. The average depth to the water table beneath the Site is 73 feet below ground surface.

Action Taken

In 2015, the HSC conducted a pilot study to evaluate the efficiency and cost-effectiveness of changing from the current cryogenic treatment system to carbon treatment for the VOC-contaminated soil vapor. The result of this pilot study indicated that changing the system to carbon was feasible and was switched in early 2016.

In 2016, the U.S. EPA completed an Explanation of Significant Differences (ESD) #2 that clarified the remedial action objectives for the Site, which were not expressly identified in the 1992 ROD and updated the soil vapor performance standards (SVPSs).

In early 2017, the HSC and the U.S. EPA, along with ADEQ, revised and updated the Performance Monitoring and Verification Plan, which included the updated SVPSs.

Status

Sampling and monitoring events continue at the Site and are reported in Semi-Annual and Annual Reports. The next FYR is due in 2021.