Revised On: Feb. 20, 2024 - 1:03 p.m.

2019 – 2020: ADEQ operated a soil vapor extraction (SVE) system and conducted regular groundwater monitoring at the site. The Proposed Remedial Action Plan (PRAP) was completed June 2019 and the Record of Decision (ROD) was completed June 2020.

2018: ADEQ began drafting a site PRAP, installed three groundwater monitoring wells, operated a SVE system, and conducted regular groundwater monitoring at the site. The Feasibility Study (FS) Report was completed in April 2018.

2017: ADEQ began drafting an FS Report and conducted regular groundwater monitoring at the site.

2016: ADEQ conducted regular groundwater monitoring at the site and installed two additional groundwater monitoring wells to better define the lateral and vertical extent of contamination. Two soil borings were installed at the source area to assess volatile organic compound (VOC) concentrations in soil and soil vapor. Additionally, ADEQ conducted groundwater modeling to examine the effect of natural attenuation at the site.

2015: ADEQ released the Final Remedial Investigation (RI) Report and the FS Work Plan in July. ADEQ evaluated additional remediation alternatives and technologies available for the site. Quarterly groundwater monitoring continued at the site.

2014: ADEQ continued to monitor wells near the extraction well on a quarterly basis to monitor the effectiveness of the pump-and-treat system. In addition, the entire monitor well network at the site was sampled during the year to monitor the extent of the plume.

The groundwater pump-and-treat system was shut down in September to evaluate the need for continued operation. 

In June, the soil vapor extraction/air sparge (SVE/AS) system was shut down to evaluate the need for continued operation.

2013: ADEQ continued to operate the SVE/AS and groundwater pump-and-treat systems. 

ADEQ continued to conduct groundwater monitoring of wells near the extraction well on a quarterly basis to monitor the effectiveness of the pump-and-treat system. In addition, the entire monitor well network at the site was sampled during the year to monitor the extent of the plume.

ADEQ installed five additional shallow monitor wells near the center and in the southwest area of the groundwater plume. ADEQ also installed an additional deep monitor well, MW-119D, near Salt River Project (SRP) well 29E-1.5S located at the SRP canal and Cooper Road

2012: In June, ADEQ drilled and sampled three borings in the immediate vicinity of the main source area, the former drywell.

The sample results indicated significant amounts of Tetrachloroethene (PCE) were still present in the soils near the drywell. ADEQ installed two additional SVE wells in these borings, SVE-106 and SVE -107.

ADEQ continued to operate and maintain the SVE/AS  and groundwater pump-and-treat systems.

ADEQ continued to conduct groundwater monitoring of wells near the extraction well on a quarterly basis to monitor the effectiveness of the pump-and-treat system.  In addition, the entire monitor well network was sampled twice to monitor the extent of the plume. 

2011: ADEQ continued to operate and maintain the SVE/AS and groundwater pump-and-treat systems.

The evaluation of the capture zone of extraction well EW-101 indicated a capture zone of approximately 300 feet wide, which encompassed the source area at the location of the drywell.

2010: In July, ADEQ completed construction of the piping from the groundwater pump-and-treat system to the Town of Gilbert sanitary sewer system and to the SRP canal. Continuous automated operations began in August.

In November, ADEQ installed an additional SVE well where previous sampling indicated a likely surface spill of PCE had occurred.

Groundwater monitoring of wells near the extraction well was conducted to determine the effectiveness of the pump-and-treat system. In addition, the entire monitor well network at the site was sampled twice this year to monitor the extent of the plume. 

2009: Initial start-up of the AS system occurred in May.

The Town of Gilbert approved ADEQ's plans to discharge treated groundwater to the water treatment plant located immediately east of the site. SRP and ADEQ finalized the agreement for construction in the SRP right of way and to discharge to the canal south of the site.

2008: In February, ADEQ installed three additional monitor wells off-site to continue to determine the extent of the plume. Concentrations of PCE in these wells indicated the groundwater contamination plume extends north of Guadalupe Road.

Initial startup of the SVE system occurred in December.

ADEQ worked on finalizing the construction design for the conveyance of treated groundwater to the SRP lateral south of the site and the Town of Gilbert wastewater treatment plant to the east of the site.

2007: In May, ADEQ installed three groundwater monitor wells off-site to continue to determine the extent of the plume. In August, ADEQ conducted an SVE pilot test to determine unsaturated zone properties.

2006: ADEQ completed an Early Response Action (ERA) evaluation at the site. During this ERA evaluation, ADEQ investigated the depth of groundwater contamination at the site and installed an extraction well (EW-101) located northwest of the drywell. ADEQ determined the concentrations of PCE in the soil and soil vapor at the site and installed several soil vapor monitor wells, SVE wells, and groundwater air sparge wells. ADEQ also collected and analyzed additional shallow soil samples to begin to define the extent of surface soil contamination indicated by previous samples collected.

ADEQ installed Well MW-105 located 600 feet to the northeast and Well MW-106 located 1,200 feet west of the site. Analytical results from MW-105 and MW-106 indicated the extent of the groundwater contamination plume had not been defined. 

2005: ADEQ conducted quarterly groundwater monitoring at the site.

2004: ADEQ conducted a preliminary investigation and placed the site on the WQARF Registry in June.  The site's Eligibility & Evaluation Score was 33 out of a possible 120.

2003: ADEQ installed two additional monitor wells.

2002: ADEQ sampled MW-102, west of the drywell, and MW-103, south of the drywell. The concentrations of PCE in groundwater samples from the two monitor wells were well above the AWQS.

2001: Analytical results from groundwater samples collected from a Town of Gilbert monitor well (G-9) indicated PCE and Trichloroethene (TCE) at concentrations above the AWQS of 5.0 micrograms per liter (µg/L).

1996: Groundwater monitoring at the site continued and PCE was detected at concentrations as high as 6,600 µg/L in monitor well MW-101, located north of the drywell. 

1994: Cyanide-contaminated soils in the area of the drywell were removed to a depth of three feet. Three groundwater monitor wells were installed at the site. Arsenic, chromium and lead were detected above residential soil remediation levels (SRLs). No minimum groundwater protection levels (GPLs) were exceeded in the soil samples. Groundwater samples detected PCE above the AWQS of 5.0 µg/L. Groundwater samples were also analyzed for cyanide, benzene, ethyl benzene, toluene, xylene and total petroleum hydrocarbons (TPH) but were not detected. Arsenic and barium were detected in the groundwater samples, but below the AWQS.

1990: Analytical results of soils samples obtained at a depth of 12 inches indicated arsenic was present above the SRL. PCE and copper were also detected, but below residential SRLs. A soil vapor survey was also conducted indicating high concentrations of PCE were present in the soil vapor on the east side of this parcel at a depth of four to five feet below ground surface (bgs). Total petroleum hydrocarbons (TPH) were also detected in the deep borings at this property, which may have been related to the diesel-fired boiler.  ADEQ has no current remediation standard for TPH in soil.

1989: Operations on the eastern portion of the site were investigated by the ADEQ Hazardous Waste Inspections Unit. Several shallow soil samples were collected from the property. Samples collected from the sump of the drywell detected cyanide above the residential SRL for hydrogen cyanide of 11 mg/kg. Arsenic, lead and copper were also present above the respective residential SRLs.

1983 – 1984: Unichem (aka United Chemical Corporation) discontinued operations at the site prior to 1983. In July 1983, Unichem sold the property to Aztec Resources, which operated a gold extraction plant at the site. The process reportedly used cyanide baths to extract gold from scrap materials and mine tailings. In September 1984, Unichem reacquired the property.

1977: Unichem purchased the site and constructed facilities for the production of copper sulfate from scrap metal. A drywell was constructed to a depth of 79 feet on site, near the center of the concrete pavement that served as a foundation for the processing plant. The copper sulfate production process used aqueous ammonia, lix (a petroleum-based compound) blended with kerosene, and sulfuric acid to extract copper from the scrap metal. A diesel-fired boiler with heat exchangers was used to heat the process stream before the crystallization of the copper sulfate. PCE was reportedly used to manufacture a coolant, possibly used in the crystallization process.