2023 – 2024: Groundwater sampling was conducted in March and October annually.

2021 – 2022: ADEQ monitored and sampled groundwater in March and October annually.

2020: ADEQ monitored and sampled groundwater in February/March and October. The Proposed Remedial Action Plan (PRAP) was completed by ADEQ in July.

2019: ADEQ monitored and sampled groundwater in April/May and October. Additional soil gas sampling was conducted at the West Osborn Complex parcels in September and October. ADEQ installed a new monitor well in the deep aquifer in November.

2018: ADEQ monitored and sampled groundwater in January. Contaminant concentrations in groundwater were similar to previous sampling events. ADEQ installed new monitor wells in the shallow and deep aquifers in November and December.

2017: In November, a five-day soil vapor extraction (SVE) pilot test was performed evaluate if this remedy is effective for addressing volatile organic compound (VOC) impacts within the vadose zone.

2016: ADEQ monitored and sampled groundwater in April and conducted soil gas investigation to identify potential additional source of groundwater contamination. Contaminant concentrations in groundwater were similar to previous sampling events.

2015: ADEQ monitored and sampled groundwater in April and October. Contaminant concentrations were similar to previous sampling events.

2014: ADEQ modified the Proposed Remedial Action Plans (PRAPs) to address comments received and based on recent groundwater data. ADEQ monitored and sampled monitor wells in the shallow and deep aquifers in March; conducted vertical profiling in three deep wells in May; and conducted limited rehabilitation activities at City of Phoenix (COP) wells COP-70 and 71 in June. ADEQ monitored and sampled monitor wells in December.

2013: ADEQ completed PRAPs for the Shallow Groundwater System (SGWS) and the Lower Sand and Gravel Subunit (LSGS). ADEQ monitored and sampled monitor wells in the deep aquifer in April and in the shallow and deep aquifers in October.

2012: ADEQ approved Feasibility Study (FS) reports submitted by United. ADEQ is proceeding with the completion of the proposed remedial action plans (PRAPs).

2008 – 2011: United continued to monitor groundwater quality at the site in both the shallow and deep contaminated aquifers below the former West Osborn Complex (WOC) facility. Monitor wells in both the shallow and deep aquifer were sampled twice each year.

2007: ADEQ in conjunction with United agreed that the deep aquifer and the shallow aquifer needed to be further characterized and remediated separately. The FS Work Plan prepared by United was revisited due to evidence that the shallow plume migrated further south than anticipated. An addendum to the FS Work Plan for the placement of new monitor wells was approved by ADEQ in August. Drilling activities were initiated in September. Results from these wells were incorporated into the FS for the shallow plume.

2006: United conducted the FS to evaluate specific remedial measures and strategies required to meet the Remedial Objectives (ROs) to remediate groundwater. In June, as part of the FS, United installed additional monitor wells to further define the extent of shallow groundwater contamination emanating from the WOC property. These new wells showed concentrations of trichloroethylene (TCE) above the Arizona Aquifer Water Quality Standard (AWQS) of 5.0 micrograms per liter (µg/l).

2005: In April, ADEQ issued the proposed RO report Comments were received from the public and ADEQ issued the final RO report in May. The Remedial Investigation (RI) then became final. In July, a notice was issued to the public indicating the availability of the final RI report and final RO report.

United submitted the FS Work Plan for review and it was approved in June. United installed five additional wells between May and June as part of the ongoing FS. A groundwater report was submitted to ADEQ in August documenting the installation and sampling of the new wells.

2004: United completed the soil cleanup at the property and permanently shut down the SVE system in March. United abandoned a WOC irrigation well in July, that well is believed to have been the conduit to the deeper contamination found at the site.

In August, ADEQ issued the draft RI report. A West Central Phoenix (WCP) Community Advisory Board (CAB) meeting was conducted in November, to discuss the RI report, as well to obtain input on remedial objectives (ROs) for the site.

1999 – 2002: United operated a SVE system from August 1999 through October 2002 to remediate contaminated soil beneath the WOC property.

1998: The site was placed on the WQARF Registry with a score of 47 out of a possible 120.

1996: United and ADEQ entered into a consent decree in Federal Court to conduct the remedial investigation (RI) and FS at the site, and pay oversight costs. ADEQ also received $250,000 on past and future costs.

1987 – 1997: In 1987, the WCP area was designated a WQARF priority list site. In 1997, ADEQ established the WQARF Registry, which replaced the priority list.

1982 – 1989: VOCs were first detected in groundwater in the WCP area in July 1982. The City of Phoenix (COP) detected TCE in four municipal public supply wells, COP wells #70, #71, #151, and #152. The Arizona Department of Health Services, Salt River Project (SRP), and the COP confirmed the presence of VOCs in the groundwater with sampling in 1983, 1985, and 1986. Groundwater from COP wells #70 and #71 contained the highest concentrations of TCE and, therefore, were immediately shut down. Wells #151 and #152 were monitored for VOC concentrations from 1982 until 1989. As a result of sampling conducted during February of 1989, COP elected to take both wells #151 and #152 off-line on March 7, 1989.

1950 – 1970: The WOC facility was originally one large property (about 15 acres) located near 35th Avenue and Osborn Road. From the late 1950s to the mid 1970s, companies at the property manufactured electronic components. These businesses used TCE and other chemicals in the production and cleaning process. ADEQ learned that large quantities of TCE and other wastes were disposed of in septic tanks and seepage pits at the WOC property from the late 1950s until the late 1960s. TCE may have also been dumped onto the ground surface.