Environmental Protection Agency (EPA) #: AZ7570028582

Superfund National Priority List (NPL) Placement: Nov. 21, 1989

Location

The National Priorities List (NPL) site study area essentially mirrors the former Williams Air Force Base (WAFB) footprint located in Mesa, Maricopa County, Arizona, and encompasses approximately 4,043 acres. The general site boundaries are from Ray Road to the north, Ellsworth Road to the east, Pecos Road to the south, and Power Road to the west. Impacted areas vary, but with one known exception, are contained within the former WAFB footprint. An impacted groundwater plume extends less than one-quarter mile southerly beyond the former base boundary, but is still part of the NPL cleanup action.

Contaminants of Concern

Groundwater contaminants include light non-aqueous phase liquid (LNAPL) or "free product" related to aviation fuel; volatile organic compounds (VOCs) related to fuels and industrial solvents, and pesticide (dieldrin). Soil contaminants include fuel-related hydrocarbons, VOCs, Poly Chlorinated Biphenyls (PCBs), pesticide (dieldrin), metals, and per- and polyfluorinated chemicals (PFAS).

Munition debris (generally empty munition casings) have been found in a relatively small, secured area near the south central border. A few, small, empty chemical agent identification set (CAIS) vials have also been found in the munitions debris area. The few, small, empty vials appeared to contain mustard gas (liquid-state) samples (chemical warfare agent vials).

Public Health Concerns

There currently are no known exposures to contaminants of concern (COCs) in excess of applicable health-based levels. This includes groundwater, soil (surface and subsurface) and soil vapor studies conducted to date. The City of Mesa supplies drinking water to the site from an off-site source. Irrigation water is drawn from wells generally upgradient from contaminant impacted areas or from potable water sources.

Unintended exposure to contaminated soil and groundwater is a potential health hazard. Some sites include soil contaminants left in place. Land-use controls have been placed at those known sites to minimize exposure potential.

Site Hydrogeology 

Site hydrogeology includes unconsolidated and intermingled clay, silt, sand and gravel layers associated with stream channel and alluvial fan deposition from the surface to about 260 feet below the ground surface (ft bgs). This is locally designated as the Upper Unit.

From about 260 ft bgs to nearly 1,000 ft bgs are weakly consolidated silt, silty sand and gravel layers with locally occurring moderately to well-cemented siltstone associated with former playa, alluvial fan and fluvial deposits. This is locally designated as the Middle Unit.

A leaky aquitard (layer of low permeability) separates the Upper Unit and Middle Unit near WAFB.

Historically, groundwater levels in the vicinity of the former WAFB have fluctuated as groundwater pumping volumes increased and decreased. Static groundwater, circa 1900, was reported to be approximately 80 ft bgs near the former WAFB. By the 1970s, substantial groundwater drawdown was attributed to agriculture and municipal groundwater withdrawal. Since the late 1970s, however, groundwater levels have recovered (risen) 0.5 to 3 ft per year. In 2019, groundwater was encountered 129 ft bgs at Site SS017, and between 126 and 147 ft bgs at Site LF004 (shallow and deeper water zones, respectively). In 2018, groundwater was encountered between 137 and 160 ft bgs at Site ST012. Groundwater flow direction in the Upper Unit is variable, ranging from west-to-east near LF004 and from west to northeasterly near ST012. Groundwater flow direction has shifted when compared to historic data recorded during high-yield extraction well pumping years.

Action Taken

Similar to many large-scale clean up actions, the WAFB study area clean-up is divided into Operable Units (OUs), with the OUs further divided into location specific sub-sites. As of December 2020, active sub-sites include:

• Landfill No. 4 (LF004) a portion of Operable Unit 1 (OU1)
• Parcel N, a portion of OU1
• Liquid Fuel Storage Facility (ST012), a portion of Operable Unit 2 (OU2)
• Fire training area number 2 (FT002), a portion of Operable Unit 3 (OU3)
• South Desert Village (SDV), a portion of Operable Unit 4 (OU4)
• Site-wide and miscellaneous areas (many small sites are included within OU4)
• Old Pesticide/Paint Shop (SS017), a portion of Operable Unit 6 (OU6)
• Basewide, perfluorinated compounds (PFOA and PFOS [i.e., PFAS]) investigation

Of the active sub-sites, remedial activities continue for:

• A portion of Landfill No. 4 (LF004) in OU1
• Parcel N, a portion of OU1
• Liquid Fuel Storage Facility (ST012), soil and groundwater greater than 25 ft in depth (OU2)

Of the active sub-sites, some sites only require land-use control monitoring. Land-use controls indicate a contaminant is present, but a remedy is in place to minimize exposure. A State of Arizona-administered Declaration of Environmental Use Restriction (DEUR) has been placed on:

• DP028; sewage sludge trenches located in a portion of landfill LF004 (a portion of OU1)
• LF004; the balance of Landfill No. 4 (OU1)
• ST012; shallow soils (less than 25 ft bgs) at a former liquid fuel storage facility (aviation and jet fuel) (OU2)
• FT002; the former fire training area No. 2 (OU3)
• SS016; former electroplating and chemical cleaning areas located in shop building 1085. Included in this DEUR action is a former leaking underground storage tank (LUST) with Arizona-designation LUST 0293.07 (aka Facility 1085-2) (OU4)
• SS020; a former firing range and a nearby former skeet range (OU4)
• SS021; former firing range facilities 1020 (firing-in-buttress) and 1051 (bore sighting bunker) (OU4)
• SS024; former pesticide (entomology) shop building 1010 (OU4)
• Building 1013; diesel fuel LUST (ADEQ LUST ID 0293.18) at a former wastewater facility

Pre-dating the DEUR program, the state of Arizona administered a Voluntary Environmental Mitigation Use Restriction (VEMUR) program. One site includes VEMUR-listed land use controls:

• SS019; former skeet range at South Desert Village (OU4)​

Status

Remedial activities continue at the following sub-sites:

• LF004 — Remedial activities include soil vapor extraction (SVE), In-Well Air Stripping (IWAS) and oxidant injection. SVE has been used to remove potentially harmful vapors (TCE and PCE) from the subsurface in the former above ground storage tank (AST) and Southeast Landfill areas. IWAS employs a combination of technologies to remove contaminants from groundwater and unsaturated soils. Air stripping is used within an IWAS well to aerate the groundwater and transfer contaminants from the dissolved phase into the air, where the contaminants are removed by SVE. Oxidant injection directly into groundwater using permanganate has been used in several areas of the site to supplement SVE and IWAS. At the end of 2019, both SVE and IWAS had removed the vast majority of subsurface contaminants and both systems were shut down pending confirmation soil vapor sampling. Periodic oxidant injections continue to address residual contaminants in groundwater and semi-annual groundwater monitoring will continue until all contaminants are consistently below action levels.
• ST012 — ST012 is the location of the former Liquid Fuel Storage Facility where petroleum fuel storage and distribution options were conducted from 1941 until decommissioning in 1991. SVE, Steam-enhanced Extraction (SEE), and Enhanced Bioremediation (EBR) have been employed to remove fuel contaminants from the soil and groundwater. Cleanup goals for shallow soil above a depth of 25 ft bgs were achieved following implantation of SVE treatment from 1994 to 1996. SVE continues to be operated to remove volatile contaminants from soils deeper than 25 ft bgs. SEE uses multi-phase extraction to remove LNAPL, contaminated groundwater and soil vapor from the aquifer. Steam is generated on site using boilers to provide steam to injection wells. Captured LNAPL is separated and recovered to the extent feasible to power steam generation, while contaminated groundwater is treated using onsite air stripping and granular activated carbon before being discharged to the municipal wastewater treatment plant. Soil vapor extracted during SEE is treated by thermal oxidation. SEE was implemented from September 2014 through April 2016. EBR was initiated in November 2018 to promote biological activity among bacteria that feed off of contamination present at the site. Continuing EBR activities include injection of sulfate into the groundwater and groundwater extraction to assist in circulating the sulfate throughout the contaminant plume. Periodic monitoring of groundwater will continue until attainment of all cleanup levels has been documented.
• Parcel N — The U.S. Army Corps of Engineers (USACE) conducted a Site Inspection (SI) to investigate the potential occurrence of munitions and explosives of concern (MEC) and chemical warfare material (CWM) at the 17-acre Parcel N Debris Area 1. The munitions response included digital geophysical mapping (DGM) surveys and intrusive investigations. No MEC of CWM items were identified during the investigation of geophysical anomalies, or during a surface sweep using metal detectors. Additional investigation is necessary in a portion of the site currently covered by approximately 3-5 acres of asphalt, concrete and construction debris.

Monitoring and inspections continue at other sub-sites, as required (see Action Taken section on Page 2 for a list of these sub-sites).

PFAS impact is undergoing USAF evaluation.  The next phase is a Remedial Investigation that is tentatively scheduled for 2023.

Community Information

The U.S. Air Force (USAF) periodically presents site activity updates via mailed and internet-posted flyers. Please contact USAF representatives to learn about future information updates.

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