Hassayampa Landfill | Site History

1961 - 1980: Maricopa County began operating the Hassayampa Landfill as a municipal solid waste landfill in 1961. On February 15, 1979, The Arizona Department of Health Services (ADHS) prohibited the disposal of industrial and hazardous wastes into the City of Phoenix 19th Avenue Landfill. Hazardous and industrial wastes were then transported to the Hassayampa Landfill under a manifest tracking system from approximately February 1979 to October 1980 when disposal ceased. The wastes were disposed in a series of excavated pits on a 10-acre property adjacent to the municipal solid waste landfill. This 10-acre area later became the Hassayampa Superfund site.

1981 - 1987: In December 1981, three on-site groundwater monitor wells were constructed with ADHS funding. The first sampling in May 1982 indicated groundwater contamination by chlorinated solvents. The site was subsequently scored by the EPA and placed on the NPL on July 22, 1987.

1988: EPA and certain respondents entered into an administrative consent order on February 19, 1988 which required the respondents to conduct a remedial investigation/ feasibility study (RI/FS) under EPA direction and oversight.

1991 - 1992: The RI/FS were completed in 1991 and 1992, respectively. Hazardous substances including VOCs, and semi-volatile organic compounds (SVOCs) were detected in the soil and groundwater. EPA selected remedial actions in the Record of Decision (ROD) dated August 6, 1992

1994 - 1996: Remedial actions were implemented at the site including:

  • A groundwater pump and treat system that began operation in March 1994;
  • An engineered cap over the hazardous waste disposal areas to prevent direct contact with contaminated waste and soil left in place. This cap reduced infiltration of water, and  reduced the release of VOC vapors to the atmosphere (constructed in June 1994);
  • A SVE system with thermal oxidation treatment to remove VOCs and SVOCs from the vadose zone in areas where waste and soil contamination  were a threat to groundwater (began operation in July 1996); and
  • Access and deed restrictions.

The Consent Decree (CD), CIV94-1821PHXRCB, was signed in November 1994. The CD required all responsible parties to conduct soil and groundwater investigations, conduct a treatability study, conduct analytical modeling, cap construction, a groundwater pilot study, and remedial design and implementation of a remedy.

1998: The SVE and treatment system was shut down for repairs in September. Subsequently, under the orders of the EPA, it was not restarted because of potential concerns that dioxins were produced by the thermal oxidation process.

2000: In November, EPA’s contractor conducted the FYR of site remedies. The review found that the cap was in good condition, and the groundwater pump and treat system was achieving hydraulic containment and removed a substantial mass of VOCs. The future operation of the SVE system was deferred until additional data were obtained.

2002: In November, split effluent samples from the groundwater treatment system were collected by ADEQ and the potentially responsible parties. The samples were analyzed for 1,4-dioxane. Test results were below laboratory detection limits.

2005 - 2006: Since 1998 and prior to March 2006, the site remedy had only removed approximately 35 pounds of the contaminants of concern from the groundwater each year. Soil vapor sampling and analyses indicated increasing concentrations of VOCs. Operation of the groundwater pump and treat system was not addressing the concerns relating to the expanding soil vapor plume and off-site groundwater contamination. Two new ground water wells (MW-16UA and MW-17UA) and five soil vapor probes were installed and added to the monitoring network.  Additionally, soil vapor samples from wells extending below the basalt layer were collected and analyzed. A baropneumatic test was conducted to evaluate the effect, if any, of the basalt layer on soil vapor migration.

ADEQ and EPA determined that the existing site Conceptual Model could no longer be supported by the current site conditions. EPA and ADEQ requested the HSC to develop a new site Conceptual Model. In response to the changing conditions at the site, the HSC hired a new project manager and subsequently selected a new “Supervising Contractor” (Geosyntec Consultants) which was approved by ADEQ and EPA in accordance with the requirements of the existing CD.

ADEQ and EPA requested the HSC to provide a work plan to detail the work that will further evaluate the site conditions, update the site Conceptual Model and address existing data gaps.  The HSC submitted a Phase I work plan in July 2005. The parties met and the HSC was advised on how to finalize the work plan which involved installation of new soil vapor and groundwater monitor wells above and beneath the basalt layer and re-starting of the SVE system with an updated off-gas treatment system.

The HSC conducted site-wide groundwater and soil vapor sampling in November 2005. Implementation of the approved Phase I Work Plan was initiated after January 1, 2006.

2006: The SVE was restarted in March. SVE construction and startup testing activities included the construction of a high density polyethylene lined equipment containment pad, installation of a proprietary vapor compression/condensation treatment unit (owned and operated by GEO Inc.) and transmission piping from 11 fine-grained zone vapor wells to the treatment unit. Soil vapor monitoring probe VP-19UA was connected to the SVE system in December. Prior to the re-start of the SVE system, nine soil borings were drilled in January and February 2006 to further characterize the subsurface conditions in the vicinity of the primary source area at the site, the former solvent disposal pit (Pit 1). The data collected during that effort helped to focus the operation of the SVE system when it was re-started and further the development of the site Conceptual Model regarding fate and transport of chemicals. Once the SVE system was restarted under the Phase I Work Plan, removal of VOC mass has increased dramatically.

Two monitor wells (MW-18UA, MW-19UA) and a piezometer (MW-20UA) were installed and added to the groundwater monitoring program during the year.

In January, EPA initiated the second FYR. The second FYR report was completed by EPA in September which included a review of annual monitoring reports, a review of institutional controls, risk assessment studies, a site inspection and interviews. The report stated that monitoring data from the period of approximately the late 1990’s to early 2006 indicated upward trends in size and concentration of the vadose zone VOC vapor plume, prior to the re-start of the SVE system. VOC concentrations in several groundwater wells had also been increasing over the last few years prior to the re-start of the SVE system. The report also stated that VOC vapor concentrations beneath the basalt were found to be higher than above the basalt. The report concluded that the remedy is protective to human health and the environment in the short-term, but further data collection is needed to determine if the remedy will be protective in the long-term.