Pacific Southwest, Region 9: Superfund
Serving Arizona, California, Hawaii, Nevada, the Pacific Islands, and Tribal Nations
Mather Air Force Base
EPA #: CA8570024143
City: 12 miles east of Sacramento
Congressional District: 11
Other Names: AC & W Disposal Site, Mather Field
The Fourth Five Year Review Report was completed on September 30, 2015.
On this page
Description and History
NPL Listing History
NPL Status: Final
Proposed Date: 10/15/84
Final Date: 07/22/87
The U.S. Air Force built Mather Air Force Base (MAFB) in 1918 to serve as a flight training school. After World War II, MAFB was the sole aerial navigation school for U.S. military and its allies. In 1958, the Strategic Air Command B-52 squadron was assigned to Mather, a position it kept until 1989. Up to 1993, when it was decommissioned as an active air base under the Base Realignment and Closure Act (BRAC), MAFB's primary mission was training of military personnel. At the time of closure, the base encompassed 5,845 acres, including 129 acres of easements. Most of the base was ruled surplus to the needs of the federal government and has been transferred or leased to various entities, primarily the County of Sacramento. In 1995, Mather Airport was officially reopened as a 2,675-acre cargo airport and another 1,432 acres became the Mather Regional Park. Other areas of the former AFB have been developed for housing, a business park, the Veterans Administration Medical Center, and the Federal Aviation Administration’s Northern California TRACON facility.
Environmental investigations began at MAFB in 1982 and continued for several years. A total of 89 areas with significant contamination were identified. Most of these Installation Restoration Program (IRP) sites had contaminated soils/sediments and included fire training areas, drainage ditches, waste pits, oil/water separator sites, spill sites, landfills and a sewage treatment plant. Soils are contaminated from toxic and hazardous materials, such as petroleum, oils, lubricants, solvents and protective coatings used during routine maintenance and operation of Mather. In addition, groundwater is contaminated beneath portions of MAFB with five groundwater plumes identified. One of the groundwater plumes is in the Aircraft Control and Warning (AC&W) Disposal Area, located on the east-central part of the base between family housing and the aircraft alert apron. The AC&W groundwater plume contains trichloroethene (TCE). The AC&W Disposal Area was listed on the National Priorities List (NPL) in July 1987. Another groundwater plume, the Site 7 plume, begins at the southern edge of MAFB and extends off-base; it is associated with the Site 7 Disposal Area. Landfills in the northeastern area of the base are believed to be the source of the Northeast plume that has low concentrations of chlorinated solvents in proximity to two closed landfills. The Main Base/Strategic Air Command (SAC) Area plume was migrated over a mile offbase. Approximately 10,000 people live within a 1-mile radius of the site, and approximately 60,000 people within a 3-mile radius of the site depend on groundwater for their main drinking water supplies. The entire base was listed on the NPL in November 1989.
Contaminants and Risks
- Soil and Sludges
- Environmentally Sensitive Area
Soil and groundwater contain various volatile organic compounds (VOCs), gasoline, diesel fuel, metals, pesticides, and other contaminants. This site was originally listed because of the potential for human exposure to these hazardous chemicals by accidentally ingesting, inhaling, or coming into direct contact with contaminated soil or groundwater.
Who is Involved
This site is being addressed through Federal actions, with the Air Force as the lead agency.
Investigation and Cleanup Activities
This site is being addressed in five stages: immediate actions and four long-term remedial phases focusing on cleanup of the AC&W Disposal Area, the landfills, groundwater and soils, and basewide soils sites.
The Air Force took action to clean up three soil areas using soil excavation, bioventing, and air sparging. In addition, the Air Force provided alternate sources of drinking water to residents along the western boundary of Mather whose domestic wells had been contaminated by base operations. Initially, the Air Force provided bottled water to residents of Old Placerville Road and Happy Lane and in 1986 the Air Force connected the Citizens Utility District water supply to four residences on Old Placerville Road and the Camelia Mather Mobile Home Park (37 units).
AC&W Disposal Area: In 1989, the Air Force began investigating the nature and extent of contamination at the AC&W Disposal Area. Late in 1993, the Air Force selected a cleanup remedy for the area that consisted of extracting the contaminated groundwater; treating the extracted groundwater by air stripping, and reinjecting the treated groundwater into the ground. This system became operational in 1995. In 1997, the disposal option changed to discharge of the treated groundwater to Lake Mather. The AC&W plume is contained and TCE concentrations are declining.
Soil Sites: Three sites are being treated with SVE systems which extract chlorinated solvents and other volatile compounds from soil.
Groundwater: There are four other groundwater plumes in addition to the AC&W groundwater plume -- the Site 7 plume, the Northeast plume, and the Main Base/SAC Area plume. The Site 7 plume is associated with the Site 7 disposal pit, which was capped in 1999. The Site 7 plume is being remediated through a groundwater extraction and treatment system (ie., "pump and treat"). Contaminated groundwater is extracted and run through an air stripper that removes the contaminants; the treated water is reinjected into the groundwater system. The Site 7 plume extends offsite into a gravel mining area, which has made remediation difficult since wells have had to be abandoned and then redeveloped as mining progressed. The system went back on line in late 2006 after a more than three year down period. The Northeast plume is in the vicinity of landfills 3 and 4. Because of the low concentrations of contaminants in the plume, the plume is being monitored to see if the contaminant concentrations decline or increase over time. Concentrations in the plume have declined such that the plume currently being monitored is in close proximity to the former landfills. The Main Base/SAC Area plumes have comingled and are being treated as one, using extraction and air stripping. The Main Base/SAC plume has migrated over a mile offsite to the west/southwest of the MAFB, affecting three municipal supply wells. As a result, the Air Force installed well-head treatment systems at these three public drinking water supply wells in 1997. Because of the size of this plume, the Air Force has taken a phased approach to its remediation using pump and treat methods. The first three phases focused on "hot spot" removal, by installing 27 extraction wells primarily on the former base property. Phase IV, completed in 2002, added 8 extraction wells to control plume migration. An additional extraction well was installed at the southwest toe of the plume in late 2004 and a final extraction well was installed at the toe of the southern lobe of the plume in late 2007. The extracted groundwater is treated by air stripping and the treated water is currently reinjected into the groundwater system. Due to injection well capacity failures, the Air Force has proposed to begin surface discharge of treated water to a local ditch in 2008.
Landfills: In 1996 the Air Force excavated and consolidated contaminated soil from three landfills into another landfill. Two landfills, including the consolidation landfill, received low permeablity caps, which eliminated the potential for human contact with the refuse and reduced infiltration. The capped landfills are monitored for potential releases to groundwater and air.
Soil Sites: Of the 82 IRP sites identified for soils clean up, 69 have completed remediation. Most of these sites are relatively small areas such as ditches, motor pool wash areas, paint shops, fire training areas and oil/water separator sites. Some contaminated soils have been excavated and either bioremediated or disposed of offsite. Other areas of contaminated soil have been treated in-situ with soil vapor extraction (SVE) systems and bioventing. One area was capped.
Mather Air Force Base is participating in the Installation Restoration Program (IRP), a specially funded program established by the Department of Defense (DOD) in 1978 to identify, investigate, and control the migration of hazardous contaminants at military and other DOD facilities. Mather Air Force Base was approved for closure under the 1988 Defense Base Closure and Realignment Commission and Congressional Action, which established special accounts for funding the environmental cleanup at closing bases.
Cleanup Results to Date
All potential exposure to contaminated soils and groundwater has been eliminated at Mather AFB. Soil Vapor Extraction (SVE) will continue to operate at several sites with residual soil contamination until cleanup levels are achieved. Groundwater pump and treat systems will continue to operate until all groundwater cleanup levels are achieved.
Potentially Responsible Parties
Potentially responsible parties (PRPs) refers to companies that are potentially responsible for generating, transporting, or disposing of the hazardous waste found at the site.
The Air Force is acting on behalf of the DOD to clean up the former Mather AFB.
Documents and Reports
Public Meetings: Restoration Advisory Board (RAB) has formally been disbanded.
Public Information Repositories
The public information repositories for the site are at the following locations:
Online versions are available through the U.S. Air Force website. Records of Decision can also be found at this link: http://afcec.publicadmin-record.us.af.mil/Search.aspx
EPA Site Manager
75 Hawthorne Street
San Francisco, CA 94105
EPA Community Involvement Coordinator
75 Hawthorne Street
San Francisco, CA 94105
EPA Public Information Center
Mather BRAC Environmental Coordinator
After Hours (Emergency Response)