1957 - 1972: The site originated as a sand and gravel company in 1957. Facility operations created large pits from the mining operations which were later backfilled predominately with municipal refuse, and various solid and liquid industrial wastes from the Phoenix area. These wastes were disposed into the unlined pits at the facility which are identified as Cell A and Cell A-1. The site was completely filled with refuse and industrial waste by late 1972.

1965 - 1979: Flood events caused the landfill to flood in 1965 and intermittently during the 1970s. These floods caused refuse and wastes to wash into the Salt River from Cell A and Cell A-1. The landfill was closed by a cease and desist order issued by the Arizona Department of Health Services (ADHS) in February 1979. COP and ADHS entered into a Consent Order in 1979 to properly close the site.

1983 - 1988: The site was placed on the EPA's NPL on September 8, 1983. ADEQ was assigned oversight authority of the project in 1988. A remedial investigation/feasibility study (RI/FS) was voluntarily conducted by COP. The RI/FS was prepared according to the requirements of CERCLA (commonly known as Superfund), and the Superfund amendments and Reauthorization Act of 1986 (SARA). The RI/FS report was submitted to ADEQ in June 1988.

1989: COP prepared a remedial action plan (RAP) under Arizona's Water Quality Assurance Revolving Fund (WQARF) rules. The RAP required the facility to design systems to control refuse wash-out, surface water quality, groundwater quality and landfill gas accumulation. The RAP was approved by a letter of determination (LOD) in September and a record of decision (ROD) was generated by the EPA and approved on September 29th.

1992: A CD between the State of Arizona and COP was signed by the United States District Court in June requiring capping of the landfill cells, removal and treatment of methane gas, monitoring of groundwater, flood control improvements and bank stabilization, and a contingency plan to treat groundwater if groundwater standards were exceeded. The Consent Decree provided legal assurance to the public that the approved remedy would be implemented as described in the LOD and the ROD.

1979 - 1996: To comply with the 1979 consent order, COP covered the site with fill material, engineered a cap for the cells, installed groundwater monitor wells, built berms around the boundary of the landfill, installed a methane gas collection system, and provided a 24-hour security guard until November 1996.

2000 - 2001: The first Five Year Review (FYR) for the site was conducted in September 2000. The FYR found that additional information needed to be obtained and identified some deficiencies. A follow-up report was completed in 2001 recommending enhancements to the methane recovery system and identified that the groundwater monitoring standards used to evaluate groundwater data had been updated by EPA. These were changed according to the updated standards. A final engineered gas collection system was approved by ADEQ in 2001 and was completed in 2002. The initial performance test was done, and Maricopa County issued the air quality permit to operate the system in fall 2002.

2002 - 2003: Enhancements to the methane gas collection system were completed in August 2002. Maricopa County issued COP an air quality permit to operate the system. In September 2003, EPA and ADEQ completed an Explanation of Significant Differences (ESD) to update the groundwater monitoring program requirements.

2005 - 2006: The second FYR was conducted in September 2005 to ensure the remedy remained protective of human health and the environment as outlined in the ROD. Minor deficiencies were noted, but COP addressed the deficiencies by May 2006. The FYR stated the remedy remained protective in the short-term, but recommended institutional controls in order for the remedy to remain protective in the long-term.

2006: The ESD to the remedy was completed by ADEQ and EPA in June identifying the appropriate institutional controls needed to ensure long-term protectiveness of the site. To fulfill one of the requirements for an institutional control that runs with the land (requirements that transfer with changes in ownership), a DEUR was negotiated between ADEQ and COP (landowner). The DEUR controls the use and access to the landfill property, and requires the facility to maintain the remedy. The DEUR was finalized in and attached to the property deed in July. A final close-out report was completed by EPA and ADEQ on August 3rd, signifying the completion of implementation of all necessary remedial actions and institutional controls.

ADEQ, EPA and COP worked together to achieve the goal of removing the site from the NPL in September. As a condition of the removal from the NPL, COP was required to maintain institutional and engineering controls as stated in the DEUR and groundwater contamination was not expected to migrate off-site. 

2007 - 2009: COP continued to operate and maintain the remedy and submit quarterly groundwater monitoring reports and data as required in accordance with the ADEQ/COP CD CIV 91-0237 and the DEUR number 2919. ADEQ and COP initiated discussions for the five-year review process.  ADEQ, EPA and COP began working on the FYR update.

2010: The third FYR report completed in September found that the remedy at the site continues to be protective of human health and the environment. Some minor deficiencies were noted, the COP plans to address these issues in 2010-2011. COP continues to operate and maintain the treatment system and submit quarterly groundwater monitoring reports and data. Currently, the COP is evaluating possible future uses for this closed landfill. 

2011: COP continued to operate and maintain the treatment system and submit quarterly groundwater monitoring reports.  Repairs were required to numerous condensate collection sumps, well metering runs, and landfill gas lateral piping as well as routine cap erosion repairs and sediment removal from the perimeter storm water drainage systems.

2012: COP continued to operate and maintain the treatment system and submit quarterly groundwater monitoring reports.  The COP requested that the DEUR be amended to allow immediate assessment of malfunctions of the landfill gas extraction system (including excavation into the landfill cover) and, to make minor repairs to the system without prior approval from ADEQ. The DEUR was amended in November.  ADEQ continues to be immediately notified of malfunctions and repairs and must approve any major changes to the system.  COP completed numerous landfill gas well metering run repairs in addition to performing routine cap erosion repairs and sediment removal from the perimeter storm water drainage systems.

2013 - 2014: COP continued to operate and maintain the treatment system and submitted quarterly groundwater monitoring reports. Some of the groundwater monitoring wells have gone dry and may need to be replaced. In mid 2014, a revised Quality Assurance Protection Plan was submitted by COP and approved by ADEQ to incorporate the updated testing requirements. COP has also updated the Operations and Maintenance Manual Plan and Well Inventory per the recommendations from the last Five Year Review.

2015: The Fourth Five-Year Review was finalized in 2015. An ESD was completed and approved. The ESD allows the COP to remove the flare station on Cell A-1 and replace it with a carbon adsorption system, due to low methane levels in the landfill.

2020: COP continued to operate and maintain the treatment system and submit semi-annual groundwater monitoring reports. The Fifth Five-Year Review was finalized in August 2020.