2023 – 2024: Long-term monitoring conducted continued monitoring concentrations remaining above the Environmental Protection Agency (EPA) maximum contaminant levels (MCLs) in the Hot Spot Area wells. 

2022: Long-Term Monitoring Plan November 2021 Report was finalized in June 2022. Groundwater monitoring concentrations remain above the EPA MCLs in the Hot Spot area wells. Removed air sparge (AS)/soil vapor extraction (SVE) treatment system above ground equipment in the Hot Spot area. 

2020 – 2021: Groundwater Contaminant of Concern (COC) concentrations have been below the MCLs in all Leading Edge Plume Area (LEPA) wells since 2015, while concentrations remained above the MCLs in two Hot Spot area wells through 2020, with one well remaining above the MCL as of 2021. Post-remediation data evaluated to determine if the Vertical Circulation Treatment (VCT) system could be permanently shut down. The Final Remedial Investigation (RI) Quality Assurance Project Plan was submitted in May 2021.

2019: EPA and ADEQ concurred with permanent shutdown of the AS/SVE system in January 2019.

2018: The VCT system was placed in temporary shutdown on Nov. 20, 2018. The VCT system currently remains shut down.

2016 – 2017: A pilot study was performed in 2016 and 2017 in OU-1 Area 1 to evaluate remedial options for reducing 1,4-dioxane in groundwater. 1,4-dioxane has been below the HA in all wells since May 2017.

2012: 1,4-dioxane was discovered in site monitoring wells above the current HA for 1,4-dioxane of 35 micrograms per liter.

2011: The VCT system restarted in July 2011. 

2005: The VCT system was shut down in December 2005.

2000: The Record of Decision (ROD) for OU-1 Area 1 is signed. The ROD remedy included active remediation, groundwater modeling, groundwater long-term monitoring, and institutional controls (ICs) to restrict the use of groundwater within the plume boundary. Operation of the VCT system began on June 16, 2000. 

1999: Operation of the AS/SVE system began in OU-1 Area 1.

2023 – 2024: Landfill Comprehensive Environmental Response Compensation and Liability Act (CERCLA) Area of Concern (CAOC) 8A investigation report completed. Annual land use controls (LUC) inspections are ongoing.

2022: The investigation results are pending for the landfill CAOC 8A. Annual LUC inspections completed.

2021: The investigation began of the closed landfill CAOC 8A which included soil, soil gas and groundwater sampling.

2020: A Final Remedy Optimization Work Plan was completed in November to evaluate the nature and extent of subsurface contamination at CAOC site 8A in order to determine if the existing LUCs can be removed or altered over portions of the site where installation of a new base entry point and badging facility is planned.

1999: OU-2 consists of the upper 10 feet of soil at 18 CERCLA CAOCs where hazardous substance disposal actions or releases may have occurred. Twelve of the CAOCs were closed with No Further Action (NFA) and three CAOCs were remediated to residential land use standards in 1999. The remaining three CAOCs (1, 8A and 10) were described in the December 1997 Final OU-2 ROD as requiring ICs to prevent unlimited use and unrestricted exposure due to remaining soil contamination.

Pre-1960: The flight line at CAOC1 was constructed.

1953: CAOC 8A is located in the southeastern portion of MCAS Yuma. This area is a former landfill (Southeast Station Landfill) that was used from 1953 to 1961 primarily for disposal of municipal wastes.

2014: A Remedial Investigation (RI) was conducted at MRP 1. Field activities included collecting 553 discrete surface and subsurface soil samples to determine the presence of munitions constituents (MC) metals and polycyclic aromatic hydrocarbons (PAHs). During the RI, no Munitions and explosives of concern (MEC) were identified. Lead shot and clay target fragments were observed on the ground surface throughout the site. 

MC metals (antimony, copper and lead) were detected in several samples at levels above both MCAS Yuma background levels and the project screening criteria. Except for five samples, all concentrations of lead were below the residential screening level of 400 milligrams per kilogram (mg/kg). PAHs were also detected at levels above the project screening criteria.

A Human Health Risk Assessment (HHRA) and Ecological Risk Assessment were conducted to determine potential exposure to contaminants in the soil. For non-residential exposure, the HHRA estimated cancer risks were within the acceptable range.

Since completion of the RI, MRP 1 has been divided into sites MRP 1a and MRP 1b. MRP 1a was further divided into the following Exposure Units based on current and anticipated future use of the site:

1. Renewable Energy Project Site and Surrounding Area,
2. Residential Area Recreational Facilities and Surrounding Area, and
3. MCAS Yuma Storage and Surrounding Area.

1946: The Skeet Range and Tower Top Range use was discontinued.

1942: The Moving Base Range, Skeet Range and Tower Top Range are established.

2021: The Final MRP Site 1a Feasibility Study was submitted in May 2021. The Proposed Plan for MRP 1a was finalized in November 2021. The selected remedy is land use controls (LUCs) for the Renewable Energy Project Site and Surrounding Area and the MCAS Yuma Storage and Surrounding Area. Under the LUCs, future land use would be restricted to industrial or commercial use. Unconditional reuse by unlimited receptors is the selected remedy for the Residential Area Recreational Facilities and Surrounding Area.

2020: A Time Critical Removal Action (TCRA) was completed in the Residential Area, Recreational Facilities Area, and Surrounding Areas exposure unit in February 2020 and the Final Removal Action Completion Report was submitted in September 2020. Results indicated the TCRA area was remediated sufficiently to attain Unrestricted Use/Unlimited Exposure.

2018: A Non-Time Critical Removal Action (NTCRA) was performed in March 2018 to excavate soil impacted with polycyclic aromatic hydrocarbons (PAHs), antimony, and lead in anticipation of construction associated with a future renewable energy project in the western half of MRP 1a. The objective of the NTCRA was to provide short-term and long-term protection of human health and the environment through the removal of soil impacted with contaminant of concern (COC) concentrations exceeding ADEQ non-residential Soil Remediation Levels (SRLs). Prior to the NTCRA, eight inactive ammunition bunkers located within and just east of the Renewable Energy Project Site were also demolished. Pre-excavation soil sampling was conducted to a maximum depth of 36 inches below ground surface (bgs). PAH concentrations above ADEQ non-residential soil remediation levels (SRLs) were confined to a maximum depth of 24 inches bgs. Subsequent excavation activities resulted in the removal and offsite disposal of approximately 21,709 tons of non-hazardous soil and 7,445 tons of Arizona Special Waste soil from an area covering approximately 25 acres. Nineteen post-excavation confirmation soil samples were collected to confirm that soil with COC concentrations above cleanup levels had been removed.

2015: A Time Critical Removal Action was performed at MRP 1b to support plans for construction of a microgrid energy facility. Based on the sample results only the top 0.5 foot of soil required excavation, resulting in the excavation of 4,432.5 tons of non-hazardous soil that was disposed of at the South Yuma County Landfill. Post-excavation soil samples confirmed that soil with contaminant of concern concentrations above residential soil remediation levels had been removed. 

MRP 1b subsequently received a No Further Action finding from EPA Region 9 in August 2015, allowing construction of the microgrid facility.

2015: ADEQ concurred with the No Further Action designation for MRP 2 in a letter dated Sept. 3, 2015.

2014: An Expanded Site Inspection (SI) was conducted in October 2014 that included the collection of 48 discrete surface and subsurface soil samples to determine whether MC were present in areas not previously investigated. Samples were collected to a maximum depth of 42 inches below ground surface. Eight subsurface samples were collected from the same locations as the SI surface sample locations. None of the surface and subsurface soil samples had metals concentrations reported above both the MCAS Yuma background values and human health screening levels.

2010: An SI was performed. Except for arsenic, metals were not detected in soil during the SI at concentrations exceeding project-screening guidelines. Arsenic was detected in surface soil at concentrations exceeding risk-based human health screening criteria (EPA residential and industrial/commercial screening levels) but was below the site-specific arsenic background concentration and the Arizona residential soil remediation level of 10 mg/kg.

2021 – 2023: In May and November each year, land use control (LUC) inspections were completed and no LUC deficiencies were noted.

2017: The MRP 4 Record of Decision (ROD) was signed on May 25, 2017, incorporating the existing institutional controls (ICs) in place at Comprehensive Environmental Response Compensation and Liability Act (CERCLA) Area of Concern (CAOC) 1, and expanding the ICs boundary to include the portions of MRP Site 4 that lie outside of the boundary of CAOC 1. The ICs restrict the area to industrial/commercial use.

2019: The MRP 5 Record of Decision (ROD) was finalized in March 2019. The selected remedy is No Action (NA). Selection of NA is based on the findings of the extensive investigations completed at the site. Hazardous substances, pollutants, or contaminants at concentrations above residential use levels have not been detected in any soil samples from MRP 5. Therefore, there is no unacceptable risk to human health or the environment under the current (industrial) use or a potential future use requiring unrestricted use and unrestricted exposure.

Because the selection of NA will not result in hazardous substances, pollutants, or contaminants remaining onsite at concentrations above those that would prevent current or future site users from unlimited use and unrestricted exposure, MRP 5 is not subject to the five-year review process.

2016: Additional Remedial Investigation (RI) activities were conducted in February and May 2016 to investigate and remove the remaining targets of interest (TOIs) and metallic debris identified in the initial RI. Metallic debris was removed to a depth of approximately three feet through a combination of soil excavation and screening. The excavated soil was sieved through a series of screens and inspected for munitions and explosives of concern (MEC) and Material Possibly Presenting an Explosive Hazard (MPPEH). After the soil was removed, Digital Geophysical Mapping (DGM) was performed over the entire site. All additional targets were removed, and all locations cleared prior to backfilling. Soil samples were also collected from beneath the TOIs for analysis of explosives and metals. None of the targets encountered down to a depth of approximately three feet bgs were identified as MEC or MPPEH. Two expended 20 millimeter practice projectiles were detected and certified as material documented as safe (MDAS).

The results of an updated risk assessment conducted using data from the additional RI combined with previously collected data indicated the metals present in the site soils at MRP 5 do not pose an unacceptable risk to human health or the environment.

2013: An RI was conducted at MRP 5 in 2013. As part of the RI, a DGM survey and intrusive investigation were performed across 100 percent of the site to assess whether MEC items were present in subsurface soil. The geophysical investigation identified 271 anomalies as TOIs. Ten percent of these anomalies were intrusively Investigated. The intrusive investigation identified three 0.50-caliber small arm projectiles certified as MDAS.

Human health and ecological Risk Assessments were performed to evaluate potential risk and hazards associated with metals detected at the site. Results indicated MC in soil does not pose an unacceptable risk to human health or the environment.

2010: An SI was conducted at MRP 5 in 2010. The site investigation (SI) consisted of collecting surface soil samples across the site and analyzing the samples for MC metals (antimony, arsenic, cadmium, copper, lead, and zinc) and explosive constituents. No metals or explosives were detected in soil at concentrations exceeding project screening levels. A single spent 0.50-caliber cartridge, which is classified as small arms, was observed during the SI. The spent cartridge was classified as MDAS.

2021 – 2023: In May and November each year,  land use control (LUC) inspections were conducted. No deficiencies were noted.

2017: The MRP 6 Record of Decision (ROD) was signed on May 25, 2017, incorporating the selected remedy of institutional controls (ICs). ICs controlling access to subsurface soil are implemented through base planning processes by including the site IC requirements in the Base Master Plan. Requirements include review of all site use requests and Dig Permits. The remedy incorporates the actions previously performed onsite as part of the hangar and apron expansion, specifically, removal of debris consisting of wood, metal, and concrete, backfilling with three feet of clean soil, and emplacement of 12 to 18 inches of concrete. The three feet of clean soil and up to 18 inches of concrete act as Engineering Controls at the site.

2010: A site inspection (SI) was conducted. Sixteen surface soil samples were collected and analyzed for six metals (antimony, arsenic, cadmium, copper, lead, and zinc) and explosives. No metals were detected in soil at concentrations exceeding project-screening levels, and no explosive constituents were detected. Multiple expended 20mm projectiles were observed during the SI. A qualified unexploded ordnance technician classified the items as munitions debris and explosively non-hazardous. Due to an urgent mission requirement to develop MRP 6 as part of the hangar/apron expansion, no additional investigations were conducted. MCAS Yuma Explosive Ordnance Disposal Specialist conducted a sweep of the site prior to construction activities and no explosive hazards were identified.

2023: Investigation of private drinking water wells located within a mile down gradient of the on-base monitoring wells with concentrations ofPer- and polyfluoroalkyl substances (PFAS) greater than 70 parts per trillion was conducted. A public open house was conducted in April 2023 to inform the public of PFAS private well investigation. 

2019: A Site Inspection (SI) was conducted in 2019. The SI field activities conducted from June to July 2019 included collecting soil and groundwater samples from temporary borings. PFAS in soil was reported across all areas included in the SI, however, only one soil sample was reported to contain perfluorooctane sulfonic acid (PFOS) at a level above the EPA residential Regional Screening Level. Combined perfluorooctanoic acid (PFOA) and PFOS concentrations above the EPA Lifetime Health Advisory (HA) level were reported in groundwater samples from all areas included in the SI. 

2018: Additional PFAS sampling was conducted during May and November 2018. The results of sampling conducted in November 2018 showed that four of seven Hot Spot Area wells, five of eight Central Plume Area wells, and one of 13 LEPA wells had reported combined PFOA+PFOS concentrations above the HA. The combined PFOA+PFOS concentration observed at one LEPA well (A1-MW-54) represented a significant increase from previous detections at this location.

2016 – 2017: Groundwater sampling for PFAS was initially conducted in OU-1 Area 1 in November 2016.