The OU-1 Remedial Investigation (RI) report identified six areas (Areas 1 through 6) with fuel related and/or chlorinated aliphatic hydrocarbon groundwater contamination. Areas 4 and 5 were later identified as fuel sites, rather than CERCLA sites, and were assigned to ADEQ’s Leaking Underground Storage Tank Program. Areas 2, 3 and 6 have all achieved the EPA Maximum Contaminant Level (MCL) goals and have been closed with concurrence by EPA and ADEQ — no further action (NFA) is required in these areas.

OU-1 Area 1

The OU-1, Area 1 Record of Decision (ROD) COCs are DCE, PCE and TCE. The site is divided into three areas: the “Hot Spot” source area, the “Central Plume Area” downgradient of the source, and the “Leading Edge Plume Area (LEPA)” at the base boundary at the downgradient edge of the contaminant plume. A newer investigation into per- and polyfluoroalkyl substances (PFAS) contamination was begun in 2019.

Hot Spot source area:

• Groundwater modeling performed in 2015 concluded TCE and 1,1-DCE (DCE) concentrations above the MCL would not migrate beyond the boundary of OU-1 Area 1. Concentrations remain slightly above the MCLs in two Hot Spot area wells.
• Air Sparging/Soil Vapor Extraction in the Hot Spot source area, began in 1999, was permanently shut down in 2019 due to asymptotic concentrations and declining water levels. 

Leading Edge Plume area:

• A Vertical Circulation Treatment (VCT) system in the LEPA began operation on June 16, 2000. The system was shut down in December 2005 and restarted in July 2011. The VCT system was shutdown on Nov. 20, 2018. The VCT system was removed in 2022.
• Groundwater monitoring has continued on an annual basis since the VCT system was shut down in November 2018. COC concentrations have been below the Environmental Protection Agency (EPA) MCLs in all LEPA wells since 2015. 

A 1,4-dioxane investigation was completed in 2020. 

OU-2 consists of the upper 10 feet of soil at 18 CAOCs where hazardous substance disposal actions or releases may have occurred. Twelve of the CAOCs were closed with 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 institutional controls (ICs) to prevent unlimited use and unrestricted exposure due to remaining soil contamination. The base has placed a fence around the site and listed land use controls.

CAOC 1 (Flight Line)

CAOC 1 consists of the pre-1960 flight line (runways, aprons, and taxiways) and associated aircraft maintenance hangar facilities. CAOC 1 is located in the north-central portion of MCAS Yuma and occupies approximately 170 acres. The RI focused on the flight line areas where source areas of contamination were suspected (e.g., aircraft and vehicle wash racks, oil/water separators, fuel storage bladder locations, dry wells, miscellaneous stained soil areas, and maintenance and storage yards). Results of the RI did not reveal significant soil contamination. Chemicals evaluated in the Human Health Risk Assessment (HHRA) included PAHs, PCBs, pesticides, and metals. ICs are in place to restrict the land use of CAOC 1 to industrial/commercial use. The institutional controls are implemented through the Land Use Control Implementation Plan (LUCIP).

CAOC 8A (Southeast Station Landfill)

CAOC 8A is located in the southeastern portion of MCAS Yuma. This area is a former landfill that was used from 1953 to 1961 primarily for disposal of municipal wastes. The landfill’s disposal pits were backfilled and no longer provide an opportunity for direct human exposure to contaminated soil. The COCs are TRPH, PAHs, PCBs, and metals in surface soil. An HHRA performed for CAOC 8A indicated that direct exposure to the site surface soil does not pose an unacceptable level of risk under an industrial land use scenario. Risk associated with subsurface soil was not evaluated in the risk assessment. The OU-2 ROD remedy is ICs to prohibit continued disposal or any activities that disrupt the landfill cover. 

During 2021, an investigation of the closed landfill CAOC 8A was conducted which included soil, soil gas and groundwater sampling.

CAOC 10 (Ordnance Munitions Disposal Area, Composed of Subareas 10a and 10b)

CAOC 10 was used during World War II as a small arms shooting range for bomber gun crews. From the early 1950s to 2010, ordnance materials were stored in the magazines around the central portion of Ordnance Loop Road. The area has also been used for surface tank and drum storage. Surface spills, including liquid residues from ordnance mixing operations, were reported within this area. Suspected waste associated with this area included used oils, ordnance waste associated with nitroaromatics, fuel-related wastes, and metals. During the RI, the primary findings of the field sampling and analysis program were TRPH and PAHs in surface soil, and one anomalous lead concentration. ICs are in place to restrict the land use of CAOC 10 to industrial/commercial use. The institutional controls are implemented through the LUCIP. 

MRP 1 encompasses 370 acres in the southeast corner of MCAS Yuma and consists of three former small arms ranges identified as the former Moving Base Range, Skeet Range and Tower Trap Range. Residential facilities currently occupy approximately 25 acres in the northeastern portion of the site. 

The Moving Base Range was established in 1942. This range consisted of an oval track used to train aerial gunners in proper firing techniques by placing the gunner in a vehicle, which was driven around the track while the gunner fired at clay targets launched from trap houses. The primary ammunition used was a number 7½ shot fired from 12-gauge shotguns. The range occupied approximately 358 acres, with a safety fan that extended 900 feet from the oval track. The Moving Base Range includes CAOC 10 and is overlapped by CAOC 8A in the southeast. Residential facilities currently occupy approximately 25 acres in the northeastern portion of the site.

The Skeet Range, which consisted of 16 firing semicircles, was established in 1942, and was in use through at least 1946. This range was used to train gunners and as a recreational facility for military personnel. The Skeet Range occupied approximately 74 acres, the majority of which is encompassed by the boundary of the Moving Base Range. Current land use includes roads and facilities supporting airfield activities. CAOCs 8A and 10, which have remedies in place, overlap a portion of the Skeet Range.

The Tower Trap Range was established in 1942 and was used through approximately 1946. During the RI, no MEC were identified. Lead shot and clay target fragments were observed on the ground surface throughout the site. This range consisted of five firing positions with trap houses and was used for gunner training and recreation. The trap range occupied approximately 51 acres and the safety fan extended 900 feet from the firing points. The Tower Trap Range is encompassed by the boundary of the Moving Base Range, except the southernmost and easternmost portions. The eastern portion overlaps CAOC 9, which previously received an NFA determination, and the remainder of the site overlaps CAOC 8A. 

Munition constituent (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.

An HHRA and Ecological Risk Assessment were conducted in 2015 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.

MRP 1a

A Non-Time Critical Removal Action was performed in March 2018 to excavate soil impacted with PAHs, antimony, and lead in anticipation of construction associated with a future renewable energy project in the western half of MRP 1a. 

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.

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.

MRP 1b

In April 2015, a TCRA 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 COC concentrations above residential soil remediation levels (SRLs) had been removed. 

MRP 1b subsequently received an NFA finding from EPA Region 9 in August 2015, allowing construction of the microgrid facility.

MRP 2 was the location proposed for construction of a small arms range based on a 1952 preliminary Base Master Plan map. However, the range did not appear on subsequent aerial photos or maps, nor was any evidence located in historical records that a small arms range was built or used at the site. If the range was built, its use would have been restricted to pistol and rifle ammunition based upon the presence of nearby base facilities. MRP 2 is currently part of the Yuma County Airport. Most of the site is currently paved and is used as a taxiway and parking apron for small planes. Hangars and buildings have been constructed at the site to provide administrative and operations facilities for the airport. Small unpaved areas comprise approximately 0.6 acres of the 11-acre site.

MRP 2 was the subject of an SI in 2010. 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 SRL of 10 mg/kg.

An Expanded SI was conducted in October 2014, which included the collection of 48 discrete surface and subsurface soil samples to determine whether MC were present in areas not previously investigated. None of the surface and subsurface soil samples had metals concentrations reported above both the MCAS Yuma background values and human health screening levels.

MRP Site 4 is a former small arms range located in the north-central portion of MCAS Yuma. Approximately 198 acres of the 240-acre site is located beneath the current runways, aprons, and associated airfield facilities, which effectively act as a cover system that minimizes potential exposure to MC. The site is also located almost entirely within OU-2 CAOC 1. Surface and shallow subsurface soil at MRP 4 are considered an affected media because metals potentially resulting from MC (e.g., arsenic, cadmium, copper, lead, and zinc), as well as PAHs, PCBs, and pesticides were detected during the OU-2 RI at CAOC 1. Environmental restrictions, in the form of ICs, have been successfully implemented since the late 1990s for OU-2 CAOC 1.

The MRP 4 ROD was signed on May 25, 2017, incorporating the existing ICs in place at 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.

MRP 5 was formerly a firing-in buttress range built prior to 1952 and removed in 1955. The former firing-in buttress was used to zero-in fixed aircraft guns. Ammunition used included 0.50-caliber and 20-millimeter (mm) projectiles, which were the typical munitions for the types of military aircraft operated at MCAS Yuma at the time the range was used. MRP 5 is located south of and adjacent to a combat aircraft loading area and covers less than 1 acre. 

An SI was conducted at MRP 5 in 2010. No metals or explosives were detected in soil at concentrations exceeding project screening levels.

An RI was conducted at MRP 5 in 2013. As part of the RI, a Digital Geophysical Mapping (DGM) survey and intrusive investigation were performed across 100 percent of the site to assess whether Munitions and explosives of concern (MEC) items were present in subsurface soil. Results indicated MC in soil does not pose an unacceptable risk to public health or the environment.

Additional RI activities were conducted in February and May 2016 to investigate and remove remaining targets of interest 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 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 public health or the environment.

MRP 6 is a former firing-in buttress, which consisted of an earthen mound used as a backstop or target area. The site potentially contains MEC and/or MC-contaminated soil in the subsurface. MRP 6 is located in the south-central portion of MCAS Yuma, beneath the southeastern portion of the runway area. The approximately one-acre site is located entirely within the restricted area of the airfield east of and adjacent to a combat aircraft loading area. MRP Site 6 is covered by three feet of clean imported soil overlain by an approximately 18-inch-thick concrete apron that is part of the aircraft hangar. 

The potential contaminants at MRP 6 are MEC and MC from ammunition when the site was used as a range to zero-in fixed aircraft guns. An SI was conducted in 2010. 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. The Feasibility Study is dated December 2013 and the Proposed Plan is dated March 2014.

The MRP 6 ROD was signed on May 25, 2017, incorporating the selected remedy of ICs. ICs controlling access to subsurface soil are implemented through base planning processes by including the site IC. 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.