Superfund Site | Marine Corps Air Station Yuma
Environmental Protection Agency (EPA)#: AZ0971590062
Date Placed on National Priority List (NPL): Feb. 21, 1990
Marine Corps Air Station (MCAS) Yuma occupies approximately 4,800 acres within the city and county of Yuma, Arizona. The Site is bounded by South Avenue 3E on the east, 32nd Street on the north, East County 14th Street on the south, and the city of Yuma Main Canal on the west. Plume boundaries vary and may extend beyond the site boundary while remaining part of the Superfund site in its entirety.
Contaminants of concern
The contaminated media includes groundwater and soil. For groundwater, contaminants of concern (COCs) include chlorinated solvents (trichloroethene (TCE), dichloroethene (DCE), tetrachloroethene (PCE)) and volatile organic compounds (petroleum hydrocarbons). For soil, COCs include total residual petroleum hydrocarbons (TRPH), polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), pesticides, metals and munitions and explosives of concern (MEC). As new information becomes available, COCs may change.
This site is located in the Basin and Range lowlands province that covers most of southern Arizona. This physiographic province is characterized by elongated northwest-southeast trending fault-block mountain ranges separated by broad, deep alluvial valleys.
Hydrogeologic units defined for the site are: upper fine-grained unit, coarse gravel unit and wedge unit. Many shallow wells on the Yuma Mesa are screened in the upper fine-grained unit, and water quality is variable due to the large volume of irrigation recharge.
Groundwater in the upper fine-grained unit is generally characterized as slightly saline with total dissolved solids (TDS) of 1,000 to 3,000 milligrams per liter (mg/l). The primary regional aquifer is the coarse gravel unit that underlies the upper fine-grained unit. Groundwater in this unit is generally characterized as slightly saline with TDS of 1,000 to 3,000 mg/l.
The direction of groundwater flow beneath the site is from southeast to northwest. Water levels have declined 6 to 8-feet (ft.) since the late 1990’s
During its 70 years of operation, MCAS Yuma generated industrial wastes such as used oil, solvents, paint residues, battery acid, pesticides, herbicides, polychlorinated biphenyls, asbestos in the form of non-friable asbestos containing material (ACM), and petroleum hydrocarbons from a jet fuel leak. The ACM was scattered on top of and buried in the surface soil. It was remediated in 1999.
Established in 1991, the federal facilities agreement and assessment program identified three operable units (OUs): OU-1 contains contaminated groundwater and soil deeper than 10-ft. below ground surface (bgs). OU-2, which is comprised of 18 Comprehensive Environmental Response Compensation and Liability Act (CERCLA) Area of Concern (CAOC) sites, contains contaminated soil from ground surface to 10-ft. bgs. OU3 did not identify any specific CERCLA sites and is thus intended to be used for future sites if required.
The fourth CERCLA Five-Year Review (FYR) report for OU-1 and fifth CERCLA FYR for OU-2 was completed in January 2020. The annual physical site inspection of OU-1 and OU-2 was conducted on May 9, 2018 and document review was conducted between March and July 2018. The information presented in the 2020 FYR reflects the status of the remedies during the review period.
The U.S. Department of the Navy (DON) informed the MCAS Yuma and greater Yuma, Arizona, communities at the start of the review process in March 2018 through a public notice sent to base personnel and printed in the Yuma Sun (Yuma and regional newspaper) Friday through Sunday, March 23-25, 2018. The notice stated the purpose of the FYR and identified the types of COCs present.
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 the Arizona Department of Environmental Quality’s (ADEQ’s) Leaking Underground Storage Tank (LUST) 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, and no further action (NFA) is required in these areas.
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 November 16, 1999. The system was placed in temporary shutdown status in May 2007 and restarted in July 2013. In January 2014, the AS/SVE system performance met the Record of Decision (ROD) requirements for temporary shutdown by reaching asymptotic conditions. 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 November 20, 2018, following concurrence by EPA and ADEQ, based on the satisfaction of the ROD requirements for temporary shutdown. The VCT system currently remains shut down.
Groundwater LTM is continuing with semiannual monitoring conducted on all site wells. 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. 1,4-dioxane is considered an Emerging Contaminant and, as such, does not have an MCL. The current EPA Lifetime Health Advisory (HA) for 1,4-dioxane of 35 micrograms per liter (µg/L) has only been exceeded in two Hot Spot Area wells and 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 pilot study consisted of: (1) completion of two rounds of in-situ chemical oxidation (ISCO) injection performed in the Hot Spot area (near LTM wells 16-HS-03, 16-MW-09, and 16-MW-08); and (2) installation of a synthetic adsorbent media (AMBERSORB 560™) mobile unit in the VCT system to evaluate removal of 1,4-dioxane in extracted groundwater at the downgradient edge of the plume.
Per-and Polyfluoroalkyl Substances (PFAS)
PFAS have been identified by the EPA as Emerging Contaminants of environmental concern because of their persistence in the environment and in organisms, migration potential in groundwater, historically ubiquitous use in commercial products, and possible health effects at low levels of exposure. PFAS are man-made compounds that have been used in paper and packaging coatings, cleaning products, textiles and leather products, semiconductors, photographic products, pesticides, and Aqueous Film Forming Foam (AFFF) for extinguishing fuel fires.
Currently, the only PFAS compounds that have EPA-derived toxicity values are perfluorobutanesulfonic acid (PFBS), perfluorooctanoic acid (PFOA), and perfluorooctane sulfonate (PFOS). The EPA issued a drinking water lifetime Health Advisory (HA) for PFOA and PFOS in 2016. The DON has used AFFF 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 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.
The May 2018 data, along with results of a Preliminary Assessment (PA) completed in 2018, were used to select soil boring and groundwater sampling locations for a Site Inspection (SI) conducted in 2019. The PA identified thirteen locations where AFFF may have been released into the environment and provided an initial assessment of possible migration pathways and receptors at risk of exposure to potential PFAS contamination.
The SI field activities conducted from June to July 2019 included collecting soil and groundwater samples from temporary borings. Fifty-five soil samples were collected at 26 soil boring locations and 21 groundwater samples (including 3 duplicate samples) were collected from 18 boring locations. In addition, a single surface soil sample was collected from a retention basin outfall.
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 (RSL). Potentially complete exposure pathways to PFAS in soil include current and future MCAS Yuma workers with direct exposure to surface soil during day-to-day activities, or exposure to surface and subsurface soil during potential future construction or other ground disturbing activities. The exposure pathway is incomplete for onsite residential exposure to PFAS in soil because access to the North, Central and South Airfield Operations Areas is limited to MCAS Yuma personnel.
Combined PFOA+PFOS concentrations above the HA were reported in groundwater samples from all areas included in the SI. For all onsite areas investigated, the groundwater exposure pathways are potentially complete for human receptors. However, ICs prohibit any use of Area 1 groundwater without the written consent of MCAS Yuma, EPA, and ADEQ. The nearest downgradient production well is located more than 3,000-ft. away from the Airfield Operations Area. Delineation of the extent of PFAS in groundwater at MCAS Yuma is ongoing.
OU-2 consists of the upper 10-ft. of soil at 18 CERCLA Areas of Concern (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.
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 included PAHs, PCBs, pesticides, and metals. ICs are in place to restrict the land use of CAOC 1 to industrial/commercial use. A change in land use from industrial to residential use will require re-evaluation of the remedy. The institutional controls are implemented through the Base Master Plan.
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. CAOC 8A is entirely enclosed by chain-link fencing; access for vehicles is limited to those authorized to open the locked gates. The COCs are TRPH, PAHs, PCBs, and metals in surface soil. A human health risk assessment 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.
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. A change in land use from industrial to residential use will require re-evaluation of the remedy. The institutional controls are implemented through the Base Master Plan.
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 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.
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-ft. from the oval track. By 1952, ammunition storage buildings had been constructed on the site. The Moving Base Range includes CAOC 10 and is overlapped by CAOC 8A on 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. 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-ft. 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.
An RI was conducted at MRP 1 in 2014. Field activities included collecting 553 discrete surface and subsurface soil samples to determine the presence of Munitions Constituents (MC) metals and PAHs. 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 of 10-4 to 10-6 and the estimated non-cancer Hazard Index (HI) was less than 1. For hypothetical future residential exposure, the estimated cancer risks were at the upper end of the acceptable cancer risk range of 10-4 to 10-6; the predominant contributor to the risk estimates was the PAH benzo(a)pyrene. The estimated non-cancer HI was greater than 1. However, the predominant contributor to the non-cancer hazard was antimony, which was detected above residential RSLs in only three samples. Due to the lack of habitat at the site, exposure to ecological receptors is considered insignificant.
Since completion of the RI, MRP 1 has been divided into sites MRP 1a and MRP 1b.
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. 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 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 (in.) bgs. PAH concentrations above ADEQ non-residential SRLs were confined to a maximum depth of 24-in. 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.
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. Following an initial excavation to 6-in. bgs, soil samples were collected to determine the locations of any “hot spots” requiring additional deeper excavation. This process continued until all soil containing COC concentrations above ADEQ residential SRLs was removed. During the TCRA approximately 30,000 tons of nonhazardous soil were excavated and disposed of at the South Yuma County landfill.
In April 2015, a TCRA was performed at MRP 1b to support plans for construction of a microgrid energy facility. The TCRA was conducted over the area where the 1-acre microgrid facility was planned and an adjacent environmental clearance area to be used for equipment storage during construction of the facility.
The Remedial Action Objective (RAO) for MRP 1b was to remove, by excavation and off-site disposal, surface and subsurface soils containing COCs at concentrations exceeding ADEQ residential SRLs. Pre-excavation soil sampling was conducted at 13 locations to determine the depth of the excavation. Based on the sample results only the top 0.5-ft. 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 1b subsequently received an NFA finding from EPA Region 9, 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. Field sampling completed during the SI included collection and analysis of eight soil samples, which were analyzed for six metals (antimony, arsenic, cadmium, copper, lead, and zinc) associated with small arms munitions.
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 RSLs) 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. Samples were collected to a maximum depth of 42-in. bgs. 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.
The potential for risks to human health and the environment associated with exposure to MC in soil was assessed under current and anticipated future site conditions. All COC detections were below background or risk-based screening values protective of industrial and residential exposure. Therefore, the risk assessment determined exposure to soil at MRP 2 would not result in unacceptable human health risk. ADEQ concurred with the NFA designation for MRP 2 in a letter dated September 3, 2015. .
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 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. The 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 material documented as safe (MDAS).
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 MEC items were present in subsurface soil. The geophysical investigation identified 271 anomalies as targets of interest (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.
Additional RI activities were conducted in February and May 2016 to investigate and remove the remaining TOIs and metallic debris identified in the initial RI. Metallic debris was removed to a depth of approximately 3-ft. through a combination of soil excavation and screening. The excavated soil was sieved through a series of screens and inspected for MEC and Material Possibly Presenting an Explosive Hazard (MPPEH). After the soil was removed, 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 3-ft. bgs were identified as MEC or MPPEH. Two expended 20-mm practice projectiles were detected and certified as 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.
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. 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.
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 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-ft. 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 UXO 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 technician conducted a sweep of the site prior to construction activities and no explosive hazards were identified.
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. 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 3-ft. of clean soil, and emplacement of 12 to 18-in. of concrete. The 3-ft. of clean soil and up to 18-in. of concrete act as Engineering Controls at the site.