EPA Sign-Off Clears Way For 4,000-acre Transfer At Former Mather AFB

  • Published
  • By Susan Wolbarst
  • Air Force Real Property Agency
The U.S. Environmental Protection Agency (EPA) has signed off on cleanup certification paperwork that clears the way for transferring more than 4,000 acres of the former Mather Air Force Base property to the public.

The signed document certifies that all groundwater treatment facilities are operating properly and successfully at Mather, a condition known as OPS.

"This is a big deal," said Phil Mook, senior representative of the Air Force Real Property Agency, Western Region. The Agency has overseen Mather's environmental cleanup since the former base closed in 1993. "A significant milestone in transferring more than 4,000 acres, including the airfield, to Sacramento County has been reached," Mook said.

The Air Force, which is financially and legally responsible for the cleanup, has spent more than $162 million to clean up Mather. More will be spent for ongoing monitoring and operation of groundwater treatment and soil cleanup systems, and maintenance and monitoring of landfill sites. Of the 89 sites identified at Mather, 75 sites are clean. Fourteen of the remaining sites are being cleaned or awaiting final closure documentation.

Mather Air Force Base, earlier called Mather Field, operated from 1918 to 1993, although there were breaks in service prior to World War II. To perform its mission, Mather's military workforce used chemicals, including fuels, solvents and oils. Over the years while the base was open, some chemicals leaked into the ground from storage tanks. Some were washed down drains or spilled during transportation and use. Chemical disposal also contributed to soil and groundwater contamination. Such disposal practices, legal in the past, are now known to cause environmental contamination and are no longer used.

In 1979, contamination was detected in water supply wells near Mather. The primary source was solvents such as perchloroethylene (PCE), tetrachloroethene (TCE) and carbon tetrachloride, and from petroleum hydrocarbons (fuels and oils). More extensive testing followed in the 1980s, and 89 sites were identified as needing further study or cleanup. These included former landfills and sites with contaminated soil, groundwater, or both.

Environmental cleanup began in the 1980s, years before Mather closed. The cleanup primarily involves removing contaminants from the soil and groundwater beneath the land surface. Part of Mather was added to the EPA's National Priorities List (NPL) in July of 1987 and the remainder was added in June of 1989. Adding Mather to the NPL ensured that the appropriate parties are involved in the cleanup effort, including the U.S. EPA, the Department of Toxic Substances Control, Regional Water Quality Control Board, and the Integrated Waste Management Board. Community members have provided input through the Mather Restoration Advisory Board since 1994.

The contaminated water is not used for drinking water. Drinking water is supplied by municipal water purveyors. Their wells providing drinking water are tested regularly, and annual water quality reports are provided to water customers as required by law. In addition, the Air Force samples the water from hundreds of monitoring wells and several drinking water wells to ensure that the contaminated water doesn't impact drinking water supplies.

To date, more than 12 billion gallons of groundwater have been pumped out of the ground and treated at Mather. Some 4,050 pounds of solvents have been removed from the water and the cleaned water is injected back into the ground or used to maintain the level of Mather Lake during the dry season.

Soil vapor extraction, which vacuums chemical vapors from the spaces between the grains of soil above the water table, has removed an estimated 1,118,200 pounds of petroleum products from the soil. Soil vapor extraction has also removed some 7,410 pounds of solvents from the soil.

Bioventing pumps air underground so oxygen moves through the soil to promote the destruction of contamination by indigenous microorganisms such as bacteria and fungi.

More than 600 monitoring and treatment wells are in use at Mather as part of the groundwater cleanup. Thirty-four wells extract water for treatment, eight are injection wells that return cleaned water back into the ground, and the rest are monitoring wells, used to track and measure contamination.

Former landfills at several sites were dug up and their waste was consolidated in a single landfill. That landfill along with two other landfills were covered with impervious barriers.. Such barriers keep rainwater from percolating through the waste and transporting it into the groundwater. Landfills are monitored to make sure the waste is contained.

All cleanup systems are in place and operating properly. Most of the groundwater cleanup will be finished in about five years, while getting the last remaining amount out of groundwater could take approximately 50 years. The Air Force will continue managing the cleanup process until the cleanup is finished.