EPA ID# ID3572124557
EPA Region 10
Elmore County
Mountain Home

2nd Congressional District

Other Names:
Last Update: September, 2012

Hide details for Site DescriptionSite Description

Mountain Home Air Force Base (AFB), established in 1943, is located on approximately nine square miles of land on a plateau southwest of Mountain Home, Idaho. The base has been under the control of the Tactical Air Command since 1965.

Hazardous materials have been used by the base for such activities as aircraft maintenance and industrial operations. As a result, hazardous wastes have been generated from operations at Mountain Home AFB. Base wastes were historically disposed of by several methods, including landfilling of solid wastes, discharge of liquid wastes to sanitary sewers, and the use of waste oil in fire training exercises and road oiling.

Approximately 30 areas were investigated at the base, including two abandoned landfills, several abandoned fire training areas, several industrial operation areas, and an entomology shop, where pesticides had been rinsed from application equipment. Wastes disposed of or spilled at these locations included petroleum, waste oils, solvents, and pesticides.

Wells within three miles of the base irrigate agricultural land and supply drinking water to approximately 14,000 people. On-base water supply wells are the sole source of drinking water for base residents and workers.

Site Responsibility: This site is being addressed through federal actions.

NPL Listing HistoryDates
Proposed Date:07/14/1989
Removed Date:
Withdrawal Date:
Final Date:08/30/1990
Deleted Date:

Hide details for Threats and ContaminantsThreats and Contaminants

Media Affected: Groundwater
Regional groundwater below the base contains widespread trichloroethylene (TCE) contamination. Levels are generally below EPA's safe drinking water level of 5 parts per billion (ppb), with the exception of two monitoring wells in the industrial core of the base which contain TCE above this level. In addition to TCE, jet fuel is present in an area of perched groundwater under the flightline and in the regional groundwater at one monitoring well location. The benzene drinking water standard of 30 ppb is significantly exceeded in the perched zone and was exceeded for a time at the well in the regional groundwater.

A vapor plume consisting of TCE and other volatile organic compounds is present in the vadose zone (in fractured bedrock) between groundwater and the ground surface, apparently centering on two likely TCE source areas. The vapor plume is a potential cause of the widespread TCE detections in base groundwater.

Several small sites and landfills had and in some cases have soil concentrations above levels that allow for unrestricted access and unlimited use.

Hide details for Cleanup ProgressCleanup Progress

Cleanup decisions at the base addressed sites grouped based on similar environmental issues, for example abandoned landfills, the fire training areas, the regional groundwater, and other miscellaneous sites across the base.

Abandoned Landfills: The Air Force conducted an investigation and risk assessment for the B Street Landfill and the Lagoon Landfill. At the time, it was determined that neither landfill posed an unacceptable risk to human health and the environment and no remedial action was selected for the landfills. The sites were further evaluated to determine if they were a potential source of contamination to groundwater, and no remedial action was called for in the 1996 groundwater ROD. However, the RODs were based on assumed future uses of the base for industrial/commercial purposes, and in the 2001 Five Year Review, these landfill sites were identified as needing institutional controls to prevent other land uses. In September 2006, the two relevant RODs were amended to include institutional controls to prevent land uses and disturbances that could pose unacceptable human health risks. At LF-23, a former debris dump site, the Air Force conducted studies to assess the potential for a removal action for PAH-contaminated soil. Currently, the Air Force is assessing whether extensive coal ash discovered in the area poses a human health risk that would require remedial action or institutional controls. As of May 2010, it appears that an ESD will be issued, as a portion of the area requires institutional controls to ensure industrial land use.

Fire Training Areas: The Air Force conducted investigations and risk assessments at seven fire training areas. It was determined that these areas did not pose an unacceptable risk to human health and the environment, so no action remedies were selected for these sites. Fire Training Area 8 was considered a potential source of TCE contamination to groundwater, however, and was further evaluated in the regional groundwater investigation. The 1996 groundwater ROD called for no action. In the 2001 Five Year Review, some of the Fire Training Area sites were identified as needing additional data collection, active cleanup, and/or institutional controls. Based on additional data collection, active soil cleanup and removal of deeper contamination through vapor extraction were considered necessary for Fire Training Area 8. A ROD amendment calling for soil vapor extraction at FT-08 was issued in September 2009. Vapor extraction work is ongoing and appears to be effective.

Miscellaneous sites: The Air Force investigated several other areas, including the entomology shop, the former auto hobby shop, the flightline storm drain, the vehicle wash rack, the munitions disposal/popping furnace, the drum accumulation pad, and the underground storage tank beneath Fire Training Area 8. None of these areas presented an unacceptable risk to human health and the environment, assuming industrial uses, and no action remedies were initially selected for these areas. The 2001 Five Year Review indicated that several of these sites needed additional data collection, cleanup and/or institutional controls to ensure protectiveness in these areas. Some small sites have since been addressed through soil removal. Removal actions have eliminated the need for institutional controls in the majority of these areas.

Flightline Fuel Spill (ST-11): Fuel line leaks in the 1950s and 60s beneath the aircraft parking apron in the west-central portion of the base released approximately 65,000 gallons of fuel to a perched water zone below the flightline. The perched water zone is believed to extend 250 feet by 500 feet, but may exist as smaller isolated pools within this range. Perched water at the site was found to contain benzene at concentrations up to 7,800 parts per billion (ppb), which greatly exceed EPA-established safe drinking water levels. The perched water was believed to be stable and the spilled fuel contained under the flightline. For this reason, the ROD signed in 1995 did not require active remediation but called for institutional controls and long-term monitoring of the perched water at the Flightline Fuel Spill. The institutional controls were designed to prevent the use of contaminated perched water and prevent an accidental release of contaminated perched water to the regional groundwater from penetration of the contaminated zone. The 2001 Five Year Review suggested the potential for fuel contamination to migrate to the regional groundwater. In 2004, the ROD was amended to clarify the institutional controls objectives and requirements while an evaluation of potential active remedies for this area continued. Soil vapor extraction testing will be performed in 2010 and is likely to support a ROD amendment for active remediation. A proposed plan for a ROD amendment was issued in 2010, and a ROD is expected to follow later this year. The preferred alternative is vapor extraction.

Regional Groundwater: In 1992, the Air Force did an investigation to determine the type and extent of regional groundwater contamination. Since it was not known what sites were contributing to the groundwater contamination, the groundwater beneath all sites was addressed by this investigation. The investigation was completed in 1994 and the remedy was selected in the Fall of 1995. Long-term groundwater monitoring was selected to confirm that TCE contamination would not rise above EPA-established safe drinking water levels and to ensure that benzene contamination was not migrating to the regional groundwater from the perched water zone where the flightline fuel spill occurred. Institutional controls were required for the perched zone. Long-term monitoring and additional investigations have indicated two wells with TCE above the drinking water standard and the presence of a vapor plume centering on two TCE source areas: Fire Training Area 8 and the Liquid Oxygen Loading Plant. Pilot studies for vapor extraction are being planned for 2007, and a Record of Decision amendment is likely to call for active remediation.

In 2010, EPA anticipates a ROD amendment that will specify where active remediation to remove groundwater contaminant sources is required (primarily for ST-11). It will also provide an update for sites where post-ROD removal actions have addressed potential groundwater contaminant sources or soils containing contaminants above levels that allow for unrestricted exposure and unlimited use. Site SD-24, where a soil removal was completed, is undergoing vapor extraction treatability testing. Depending on the results of the tests, which have so far indicated significant reductions in levels of TCE, a remedy may not be needed.

Since the ROD:

The MHAFB site was designated "construction complete" in September 1998. This designation signifies that all required activities at the site have been implemented. Following remedy selection, the first five years of monitoring of the flightline fuel spill and regional groundwater indicated stable benzene levels in the well in the perched water zone, and the institutional controls required by the ROD were implemented. Regional groundwater monitoring showed continued TCE concentrations below the safe drinking water level of 5 ppb. Since the remedy leaves contaminants in place above levels that would allow for unrestricted uses, Superfund law requires a review of the remedies every five years from the start of remedial action to ensure they remain protective of human health and the environment. The base completed its first 5-year review report on schedule in June 2001.

The 2001 Five-Year Review (2001 Review) confirmed that water supplied to base personnel had not exceeded the EPA-established safe drinking water levels for TCE. However, expected decreases in TCE concentrations were not observed in the groundwater. In addition, monitoring of perched groundwater at the flightline indicated water level fluctuations and the presence of free-phase fuel, which suggested that the spilled fuel could migrate to regional groundwater. The 2001 Review called for additional investigation of potential TCE sources, additional wells to better assess regional and perched groundwater conditions, and investigation or institutional controls to address "No Action" areas where unrestricted use would result in risks above EPA's acceptable risk threshold. The 2001 Review indicated that clearer and more comprehensive institutional controls language was needed to enhance controls established through the 1995 ROD.

The additional monitoring implemented as a result of the 2001 Review have significantly altered the base's understanding of site conditions. New wells to the regional aquifer indicate that TCE exceeds safe drinking water levels in some areas, and the base has performed additional investigations of potential source areas. Volatile organic contaminants have been discovered in a vapor plume that is likely to be affecting TCE concentrations in regional groundwater. New wells in perched groundwater at the flightline fuel spill and nearby in the regional groundwater indicate that the fuel may have reached the regional aquifer. Additional sampling at "No Action" sites identified areas such as landfills that need institutional controls, as well as small, discrete areas that can be easily addressed through soil excavation in areas where maintenance or site development is planned.

In 2004, the base adopted changes to the ROD in an Explanation of Significant Differences (ESD) which improved the institutional controls for the flightline fuel spill site. In 2006, institutional controls were adopted in ESDs for the B-Street and Lagoon Landfills. Samples were collected of air in selected housing and work spaces in 2006, and the results indicate that vapor intrusion is not of concern for residences. A remedial investigation report addendum and focused feasibility study are being prepared to address remediation options for the flightline spill area and the two primary sources of the TCE vapor plume, in preparation for a ROD amendment. Cleanup of several localized areas of soil contamination are planned for 2007. RAB meetings are held twice a year to inform members of the advisory group.

The 2006 Five Year Review summarizes the recent source investigations, groundwater and vapor monitoring, focused soil cleanup work, and institutional controls, and includes recommendations, as appropriate. While the majority of the sites at the base are now protective due to institutional controls, soil cleanups and removal actions, remaining issues such as the vapor plume require EPA to find the sitewide status not protective. The base continues to take measures to resolve these issues.

Hide details for Regional ContactsRegional Contacts

PHONE NUMBER:509.376.3883
Information pertaining to this site is housed at the following location(s):
Mountain Home Public Library (Administrative Record)
790 North 10th,
East Mountain Home, ID 84647

366th Civil Engineering Squadron, Environmental Flight (Administrative Record)
1030 Liberator Street
Mountain Home AFB, ID 83648