Superfund Site | Marine Corps Air Station Yuma: Page 2 of 2

Status

The fourth CERCLA Five-Year Review (FYR) report for OU-1 and fifth CERCLA FYR for OU-2 was completed in January 2020.

OU-1

The OU-1 Remedial Investigation (RI) report identified six areas (Areas 1 through 6) with fuel related and/or chlorinated aliphatic hydrocarbon (CAH) 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

Active remediation is in progress in OU-1 Area 1 and Institutional Controls (ICs) in the form of restrictions on groundwater use are in place. Remedial action at OU-1 Area 1 consists of Air Sparging/Soil Vapor Extraction (AS/SVE) in the Hot Spot source area and operation of a Vertical Circulation Treatment (VCT) system in the Leading Edge Plume Area (LEPA). In addition, long-term monitoring (LTM) of groundwater is conducted on a semiannual basis.

Operation of the AS/SVE system began on Nov. 16, 1999. EPA and ADEQ concurred with permanent shutdown of the AS/SVE system in January 2019. Operation of the VCT system began on June 16, 2000. The system was shut down in December 2005 and restarted in July 2011. The VCT system was placed in temporary shutdown on Nov. 20, 2018. The VCT system currently remains shut down.

Groundwater COC concentrations have been below the EPA MCLs in all LEPA wells since 2015, while concentrations remain above the MCLs in two Hot Spot area wells. 

In 2012 1,4-dioxane was discovered in site monitoring wells above the current EPA Lifetime Health Advisory (HA) for 1,4-dioxane of 35 micrograms per liter (µg/L). 1,4-dioxane has been below the HA in all wells since May 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. 

The U.S. Department of the Navy has used Aqueous Film Forming Foam in fire training exercises, in suppressing aircraft and other vehicle fires, and in aircraft hangar fire suppression systems at many operations across the U.S. including MCAS Yuma.

Groundwater sampling for Per-and Polyfluoroalkyl Substances (PFAS) was initially conducted in OU-1 Area 1 in November 2016. 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.

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 PFOS at a level above the EPA residential Regional Screening Level.

Combined PFOA+PFOS concentrations above the HA were reported in groundwater samples from all areas included in the SI. Delineation of the extent of PFAS in groundwater at MCAS Yuma is ongoing.

OU-2

OU-2 consists of the upper 10-ft. of soil at 18 CERCLA 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 ICs to prevent unlimited use and unrestricted exposure due to remaining soil contamination.

Munitions Response Program (MRP) Sites

The MCAS Yuma MRP includes six sites. MRP 1, MRP 2 and MRP 4 are former small arms ranges; MRP 5 and MRP 6 are former firing-in buttresses. MRP-3 is a former small arms range that was remediated in 2003 through the ADEQ Voluntary Remediation Program.

MRP 1

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 Skeet Range, which consisted of 16 firing semicircles, was established in 1942, and was in use through at least 1946. Current land use includes roads and facilities supporting airfield activities.

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. 

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 (ERA) 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

A Non-Time Critical Removal Action (NTCRA) 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. 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. Post-excavation confirmation soil samples were collected to confirm that soil with COC concentrations above cleanup levels had been removed.

A Time Critical Removal Action (TCRA) was performed between September 2019 and February 2020 to excavate and remove soil impacted with PAHs and lead in response to an emerging need to place residential and recreational facilities within MRP 1a. During the TCRA approximately 30,000 tons of nonhazardous soil were excavated and disposed of at the South Yuma County landfill.

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 SRLs had been removed. 

MRP 2

MRP 2 was the location proposed for construction of a small arms range based on a 1952 preliminary Base Master Plan map. 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. 

MRP 2 was the subject of an SI in 2010. An Expanded SI was conducted in October 2014. None of the surface and subsurface soil samples had metals concentrations reported above both the MCAS Yuma background values and human health screening levels.

ADEQ concurred with the NFA designation for MRP 2 in a letter dated Sept. 3, 2015.

MRP 4

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 ICs restrict the area to industrial/commercial use.

MRP 5

MRP 5 was formerly a firing-in buttress range built prior to 1952 and removed in 1955. 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 survey and intrusive investigation were performed across 100 percent of the site to assess whether 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 3 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.

The MRP 5 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.

MRP 6

MRP 6 is located in the south-central portion of MCAS Yuma, beneath the southeastern portion of the runway area. The approximately 1-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 3 feet of clean imported soil overlain by an approximately 18-inch-thick concrete apron that is part of the aircraft hangar. 

An SI was conducted in 2010. 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 ordinance technician classified the items as munitions debris and explosively non-hazardous.

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 in the Base Master Plan.