Mather Air Force Base covers 5,845 acres in Sacramento, California. It opened in 1918 as a flight training facility and closed in 1993 under the Base Realignment and Closure Act. Environmental investigations starting in 1982 found 89 areas of significant contamination. The site was proposed for the National Priorities List in October 1984 and finalized in July 1987. The Air Force leads cleanup work, and the EPA serves as the lead regulatory agency.
Over 150 chemical contaminants have been identified across soil, groundwater, sediment, surface water, and air. These include volatile organic compounds such as benzene, trichloroethene, tetrachloroethene, and vinyl chloride. Heavy metals include lead, arsenic, chromium, cadmium, mercury, and nickel. The site also contains petroleum products, pesticides such as DDT and chlordane, polycyclic aromatic hydrocarbons, and chlorinated dioxins and furans. Contamination is concentrated in the groundwater, landfill, and final basewide operable units.
Cleanup has moved through several phases. Early actions included soil excavation and providing alternate drinking water to residents whose wells were affected. A groundwater extraction and air stripping system at the AC&W Disposal Area became operational in 1995 and discharges treated water to Lake Mather. Of 82 identified soil sites, most have been remediated using excavation, bioremediation, soil vapor extraction, or capping. Physical construction across the site was completed in September 2009. Groundwater treatment systems and soil vapor extraction units continue to operate. The most recent five-year review was completed September 30, 2025.
Two concerns remain open. First, vapor intrusion at the site poses human health risks, and the Air Force has not fully characterized those risks. Second, human exposure pathways cannot yet be fully assessed because response activities have not yet produced sufficient data to evaluate whether exposures are currently controlled. A sitewide investigation of PFAS, per- and polyfluoroalkyl substances, is underway and expected to finish between December 2027 and February 2028. A separate investigation of Building 4260 has a record of decision expected between September and November 2027. The site is not yet ready for anticipated use sitewide.
Much of the former base has already been redeveloped. Sacramento County received most of the surplus land. Reuses include Mather Airport, a regional park, housing, a business park, and federal facilities. Community members with questions can contact the EPA Community Involvement Coordinator or the Remedial Project Manager. For state-related questions, contact the California Central Valley Regional Water Quality Control Board or the California Department of Toxic Substances Control.
