Williams Air Force Base operated as a military flight training school in Mesa, Arizona from 1941 until it closed in 1993. The 4,043-acre property was placed on the National Priorities List in 1989 after decades of operations left behind a wide range of contamination. The Air Force divided cleanup into six operable units covering 13 originally identified sub-sites plus additional areas found later. Over 3,800 acres have since been transferred for reuse as Phoenix-Mesa Gateway Airport and an Arizona State University campus.
Contaminants include benzene, trichloroethene, tetrachloroethene, toluene, naphthalene, lead, arsenic, beryllium, chloroform, dieldrin, and 1,4-dichlorobenzene, along with total petroleum hydrocarbons. These were found in soil, groundwater, and soil gas. More recently, per- and poly-fluoroalkyl substances (PFAS), chemicals used in fire-fighting foams, were detected in groundwater near the LF004 Landfill, the FT002 Fire Training Area, the ST12 Fuels Spill Site, and an area near the fire station.
Early cleanup actions included capping a drainage ditch in 1988, removing 20 underground storage tanks between 1990 and 1991, and excavating a pesticide drum burial site in 1991. At the ST12 Liquid Fuels Storage Area, steam enhanced extraction removed over 2.5 million pounds of hydrocarbons from 2014 to 2016, though the Air Force estimates 3 to 6 million pounds remain in the ground. An enhanced bioremediation pilot study is now underway there. The LF004 Landfill has groundwater treatment running for solvent contamination, and the FT002 Fire Training Area received additional soil vapor extraction after an earlier bioventing approach proved ineffective. Surface soil cleanup is mostly complete, with contaminated areas capped or covered with clean soil. Arizona State University housing in the South Desert Village area is restricted to adults over 18 because of lead-contaminated soils beneath a 6-foot clean soil cap.
The EPA has determined that human exposure is under control at the site. However, groundwater contamination has not yet stabilized, and cleanup construction is not complete across all operable units. The most recent five-year review was completed in September 2021. The City of Mesa installed five large public supply extraction wells about five miles downstream to protect the water supply. The site has not yet been deleted from the National Priorities List.
Community members can contact the Air Force at (916) 643-1250 extension 257 or by email at AFCEC.CIB.WREC@us.af.mil. The EPA's Community Involvement Coordinator can be contacted with questions. For state-related questions, contact the Arizona Department of Environmental Quality.
