Superfund Site | Former Williams Air Force Base

EPA #: AZ7570028582

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

Location

At approximately 4,043 acres in Mesa, Maricopa County, Arizona, the National Priorities List (NPL) site study area (site) essentially mirrors the former Williams Air Force Base (WAFB) footprint.  The nominal site boundary is 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 clean-up 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 perfluorinated chemicals (PFCs).  A limited, secured area contains munition debris and empty chemical agent identification set (CAIS) chemical warfare agent vials (contained mustard gas samples [liquid-state]).

Perflourooctonic Acid and Perflourooctane Sulfonate (POFA and POFS, respectively) assessment is being initiated as part of a 2017 emerging contaminant study. POFA and POFS may be generically included as PFCs by some parties and in some literature.

Public Health Impact

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.  Drinking water is supplied via City of Mesa off-site sources.  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 some sites to minimize exposure potential.

Site Hydrogeology

Site hydrogeology includes unconsolidated clay, silt, sand, and gravel 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 feet bgs to near 1,000 feet bgs are weakly consolidated silt, siltstone, silty sand, and gravel with locally-occurring moderately to well-cemented siltstone associated with former of playa, alluvial fan, and fluvial deposits. This is locally designated as the Middle Unit.  A leaky aquitard layer separates the Upper Unit and Middle Unit near WAFB.

In the vicinity of the former Williams AFB, the Upper Unit contains an unconfined regional aquifer. In areas where the aquitard does not exist, the Middle Unit aquifer is the first hydrogeologic (water bearing) layer that is encountered.

Historically, groundwater levels in the vicinity of the former Williams AFB have fluctuated as groundwater demands and pumping have increased and decreased. Static groundwater elevations circa 1900 were reported to be approximately 80 feet below ground surface near the site. By the 1970s, substantial groundwater drawdown was attributed to agriculture and municipal groundwater withdrawal.  Since the late 1970s, however, ground water levels began to recover 1 to 3 feet per year.  In 2016 groundwater was encountered by 132 feet below the ground surface at Site SS017 and 136 feet below ground surface at Site ST035.  Groundwater was encountered between 123 and 150 feet below ground surface at site LF004 (shallow and deeper water zones, respectively).  Groundwater was encountered between 140 and 160 feet at site ST012.  

In 2016 groundwater flow direction in the upper water zone ranges from west-to-east and west to northeasterly. Note that groundwater flow direction has shifted when compared to historic data recorded during high yield extraction well pumping years.