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Superfund Site | Former Williams Air Force Base: Page 2 of 3

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)​