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 extent of the soil and groundwater contaminant plumes vary and may extend beyond the site boundary, but remains part of the Superfund site in its entirety.

Contaminants of Concern

The contaminants of concern in 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 and enclosed by a perimeter fence; 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 in an alluvial-filled basin of the Hassayampa River and has been influenced by the nearby river as well as the Quaternary Arlington Mesa basalt flows. A regional aquifer, consisting of basin-fill deposits, underlies the Site and comprises the principal source of groundwater to wells in the area. The regional groundwater flow direction is generally to the south-southwest. Wells within three miles of the Site provide drinking water for approximately 350 people and irrigation water for an estimated 2,800 acres of farmland. The nearest downgradient domestic well is about 1,000 yards south of the Site.

Based on the predominant soil types identified in field studies, the shallow subsurface at the Site includes two general units in the vadose zone (unsaturated soils): an upper coarse grained zone (CGZ) and a lower fine-grained zone (FGZ). The CGZ generally extends from ground surface to approximately 30 feet below ground surface (bgs). The FGZ extends from approximately 30 to approximately 60 feet bgs and consists predominantly of silts and fine sands. Beneath the FGZ there is a basaltic lava flow unit that is typically approximately 20 feet thick but tapers out or is not present to the north of the Site. This basalt flow unit originated from Arlington Mesa to the south. An unsaturated zone, approximately 10-feet thick, is also currently present beneath the basalt unit.

Two groundwater units, Unit A and Unit B, have been defined at depth. The current water table occurs in silts and fine sands of Unit A at a depth of approximately 80 feet bgs. Unit B is composed of somewhat coarser materials than Unit A and is typically encountered at about 100 feet bgs and has a thickness of over 100 feet. Unit B is a productive aquifer in the region. Piezometric levels in Unit B are 7 to 8 feet lower than those in Unit A, indicating a downward gradient.

Action Taken

A Soil Vapor Extraction (SVE) system was installed and operated at the Site beginning in 1994, with intermittent operations between 1997 and 1999, when the SVE system was stopped. The SVE system was re-started in 2006 and has been operating more or less continuously since that time. Since 2006, the SVE system has removed over 200,000 pounds of volatile organic compounds (VOCs) from subsurface soil. The annual rate of VOC removal is declining, as is typical of SVE systems.

In 2015, the Hassayampa Steering Committee (HSC) conducted a pilot study to evaluate the efficiency and cost-effectiveness of changing from the cryogenic treatment system to a carbon treatment for the VOC-contaminated soil vapor. The result of this pilot study indicated that changing the treatment system to carbon was feasible and the switch occurred 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 record of decision (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.

Through 2018, the HSC continued groundwater monitoring, and operations and maintenance of the soil vapor extraction carbon treatment system and the groundwater recovery and treatment system. Results of groundwater monitoring performed in April 2018 indicated that VOC contaminants in groundwater, exceeding site specific performance standards, were limited to within the site boundary. Soil vapor VOC concentrations continued to decrease and the HSC performed a SVE system shutdown and rebound test. That test indicated that it was appropriate to continue operating the SVE system.

Hassayampa Landfill’s SVE system was shut down for a third rebound test for nearly one year spanning from July 2019 to June 2020. The system was restarted in June 2020 as the concentration results for VOCs met if not exceeded the rebound performance standard criteria during the nine month rebound monitoring event.

Also a groundwater extraction and treatment system (GETS) operates at the site.  The pumping rate from the two connected extraction wells is low due to soil characteristics. Although, the GETS removes a much lower quantity of VOCs compared with the SVE system, it modifies the groundwater flow direction and helps contain contaminated groundwater on the site by drawing water toward the extraction wells.

Status

The SVE system is currently shut down for a fourth rebound test, however, the GETS operation is ongoing. Sampling and monitoring events continue at the site and are reported semi-annually in January and July. The site continues to undergo Five Year Reviews (FYR) to ensure the remedy remains protective of public health and the environment. The fifth FYR report was released in September 2021.

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