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. 

2007: The groundwater recovery system removed and treated approximately 35 pounds of VOCs in 2007. Routine monitoring was on-going to collect supplemental data for use in the continued refinement of the site Conceptual Model. The HSC continued to submit quarterly soil vapor and groundwater monitoring reports in a timely manner. EPA, ADEQ and the HSC met on a quarterly basis to discuss issues related to the site.

The HSC submitted a Phase I report to EPA and ADEQ in June 2007. The Phase I work completed in 2006 and early 2007 was designed to collect supplemental data for addressing remaining data gaps and resume control of the soil vapor plume. Specific Phase I work included soil boring investigations, groundwater well installations, aquifer tests and the re-start of the SVE system. The Phase I report also included interpretations of the current state of the site, in particular the soil vapor and groundwater plumes. The Phase I report was approved in October.

2008: ADEQ, EPA and the HSC prepared for several activities to start Phase II of the site remediation including:

Letter Work Plan 1 – Groundwater Recovery System Optimization

• Upgrade the PLC and data logger systems
• Update capture zone analysis
• Convert extraction wells, EW-01UA and EW-02UA, to monitoring
• Evaluate the need for a new groundwater extraction well

Letter Work Plan 2 – Update Monitoring and Reporting Programs

• Review and optimize soil vapor monitoring program
• Update quality assurance project plan and site assessment plan as needed

Letter Work Plan 3 – Unit B Groundwater Analyses

• Complete a two-dimensional modeling to simulate and analyze the Unit A and Unit B seasonal water level fluctuations
• Develop the possible ranges of Unit A vertical hydraulic conductivities that best fit the simulations

Letter Work Plan 4 - Optimization of the Restarted SVE System

• Refine the conditions for and timing of the transition from the GEO system to air-phase carbon
• Design of carbon system
• Develop soil vapor performance standards
• Develop SVE closure criteria and testing protocols

In June, EPA and ADEQ approved an updated monitoring schedule for soil vapor monitoring. The revision included updating monitoring locations, sample collection procedures, laboratory testing methods and quality assurance and quality control protocols.

At the request of EPA and ADEQ, the HSC installed a new Unit A monitor well at the site during July. The new well, designated MW-21UA, was located to provide water quality and water level data in an area that was viewed as a data gap between groundwater monitor wells MW-3UA and MW-16UA.

2009: The HSC continued to evaluate the performance of the SVE system. ADEQ and EPA worked with the HSC to develop revised clean-up objectives that will establish the future shut down of the SVE system. HSC began the process of conducting a pilot test to determine the effects of shutting down two of the soil vapor extraction wells. In December, an Explanation of Significant Difference was signed revisiting the soil vapor performance standards.

2010: The pilot test is still underway for the soil vapor extraction system. EPA and ADEQ requested an updated operation and maintenance manual (O&M). EPA and ADEQ continue to review the comments to the updated Quality Assurance and Performance Plan (QAPP).

2011: The third FYR was completed in September. The current remedy was determined to be protective of human health and the environment in the short term. The FYR determined that several potential issues might affect protectiveness in the long-term. The issues include:

• More aggressive source area treatment in order to accelerate removal of VOCs and help protect the Unit B aquifer
• The current groundwater monitoring well network could benefit from more wells
• Nitrate injection into the Unit B aquifer
• Updates to the Operation & Maintenance Manual, Sampling and Analysis Plan, and the Performance Monitoring and Verification Plans.

​Two SVE wells were installed near the source area to help enhance the SVE system and have shown positive results.

2012: Work continued on the items addressed in the FYR. The results of the pilot test determined that two soil vapor wells could remain off without affecting the remedy. 

2013: In addition to ongoing groundwater and soil gas monitoring at the site, the Performance Monitoring and Verification Plan has been updated to describe procedures in order to accurately evaluate compliance with pre-determined groundwater and soil-vapor performance standards.  Three onsite Unit B deep-wells were video logged in June to assess well construction and integrity after VOCs were detected in well MW-01UB.  The results of the videos logging showed no observable damage to the wells.

2014: The Performance Monitoring and Verification Plan is in progress of finalizing updates in describing procedures in order to accurately evaluate compliance with pre-determined groundwater and soil-vapor performance standards.

2015: The SVE and groundwater recovery systems operated through the year and removed approximately 14,989 pounds of VOCs. A pilot test for a new (vapor-phase) granular activated carbon (GVAC) SVE system was implemented in August and ran into December. Results of the pilot test indicated that the GVAC SVE system generally provided higher extraction vacuums at monitoring locations. Monitor well MW-21UA was abandoned and replaced with well MW-21UAR. Well MW-21UA was partial screened in Unit B and downward vertical groundwater gradient from Unit A to Unit B was the main concern and reason for abandonment.

2016: The SVE and groundwater recovery systems operated through the year and removed approximately 3,280 pounds of VOCs. Based on the positive results of the GVAC SVE pilot test, the full scale GVAC SVE system was installed and operational in August.

2017: The GVAC SVE system operated into September when it was shut down for rebound testing, performed in October and December. The groundwater recovery system operated through the year. The remedial systems removed approximately 817 pounds of VOCs.

2018: Based on the December 2017 rebound test results, the GVAC SVE system was restarted in January. The detection of 1,2-dichloropropene (1,2-DCP) in soil vapor was the one compound that exceed the soil vapor performance standards. Due to low influent VOC concentrations, the GVAC SVE system was shut down in July for a second rebound test. Results of this second rebound test are pending.