Pacific Southwest, Region 9: Superfund
Serving Arizona, California, Hawaii, Nevada, the Pacific Islands, and Tribal Nations
Castle Air Force Base
EPA #: CA3570024551
Congressional District: 18
Use the following URL for future direct access to Castle AFB: http://www.epa.gov/superfund/castleafb
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Description and History
NPL Listing History
NPL Status: Final
Proposed Date: 10/15/84
Final Date: 07/22/87
Castle, formerly Castle Air Force Base (AFB), is in central California within the San Joaquin Valley in Merced County. The site is approximately 6 miles northwest of
Merced, near the communities of Winton (to the northwest) and Atwater (to the southwest). The former Castle AFB covered an area of 2,777 acres comprising runway and airfield
operations, industrial areas, housing, recreational facilities, and several noncontiguous parcels of land near the base. The largest noncontiguous parcels were two housing annexes
totaling approximately 206 acres, located to the southwest of the main base area.
Castle AFB was established on September 20, 1941, as the Merced Army Flying School. The base was listed on the U.S. Environmental Protection Agency (EPA) National Priorities List on November 21, 1987, and the base was closed on September 30, 1995, under the authority of the Defense Authorization Amendments, the Base Closure and Realignment Act of 1988, and the Defense Base Closure and Realignment Act of 1990. From closure through 2006, portions of the property were transferred by deeds or federal transfer documents or were subject to lease agreements. By the end of 2006, the Air Force had completed transfer of all property comprising the former Castle AFB to several public and private entities, including the Federal Bureau of Prisons, Merced County, and the City of Atwater.
As of the end of 2015, there were approximately 250 monitoring wells (including inactive extraction and injection wells) within Castle and on nearby properties. In addition to monitoring wells, ten active extraction wells and six active injection wells were associated with Castle groundwater remedial systems. Production wells for water supply operated by Castle and offsite public and private water supply wells are also in the area.
The former Castle AFB also includes two former landfills that are part of a post-closure maintenance and monitoring care program, Landfill 4 (LF-4) (Installation Restoration Program [IRP] Site LF007) and Landfill 5 (LF-5) (IRP Site LF008), as well as a former fire training area that requires cap inspection and maintenance, Fire Training Area 1 (FTA-1) (IRP Site FT001). Inspection and monitoring activities at the closed landfills and FTA-1 and the groundwater remedial system maintenance and monitoring activities are part of ongoing Air Force Installation Restoration Program (IRP) efforts.
The Revitalization of the Former Castle Air Force Base
In the December 2006, the final deeds were signed and the Former Castle Air Force Base has now been completely transferred, meaning the Air Force no longer owns property at the site. In the final round of land transfers, the County of Merced received over 1,900 acres of land including the airfield. Other recipients of the land include: the US Department of Health and Human Services, the US Bureau of Prisons, the City of Atwater, and Castle Gardens Redevelopment.
Through the efforts of the County of Merced, with the assistance of the FAA and the State, Castle Airport has undergone significant renovations. As a result, the airport has been fully converted from a military facility to a certified commercial airport. The FAA approved the airport for commercial use in 2006, and it became fully operational January 30, 2007.
Contaminants and Risks
- Soil and Sludges
- Environmentally Sensitive Area
Groundwater in the shallow and subshallow aquifers in the Main Base is contaminated with trichloroethylene (TCE). There are currently institutional controls to prevent the use of the groundwater until the groundwater is treated to drinking water standards. Additionally, groundwater underlying a residential housing area located adjacent to the southwest portion of the Base (Castle Vista) had been contaminated with cis-1,2-dichloroethylene (DCE), but is now cleaned up to drinking water standards, and treatment systems have been decommissioned. The site also has two landfills and a former Fire Training Area with protective covers and institutional controls requiring actions to prevent exposure to the landfill contents.
List of Site Contaminants
benzene (soil only)
Who is Involved
The US Air Force is the lead agency for this site, with the U. S. EPA and the State of California providing oversight. The FFA for this site was signed in 1990.
Investigation and Cleanup Activities
The site was originally divided in to three operable units, two groundwater OUs (OU1 and OU2) and one soil OU. The soil OU is referred to as the Source Control Operable Unit (SCOU). The two groundwater OUs were later combined into one groundwater OU in the Comprehensive Basewide ROD - Part 1.
The Air Force installed two deep wells in 1988 to replace TCE contaminated water supplies: one for the City of Atwater and one to meet on-base needs. In 1989, the Air Force also built a granular activated carbon filtration system to treat TCE contaminated groundwater. Prior to the installation of these filters, the Air Force supplied area residents with bottled water.
In 1989, a study was initiated to evaluate the nature and extent of soil contamination at over 200 sites that comprise the SCOU and to determine the most effective methods for addressing the contamination. A remedial investigation (RI) was conducted from 1993 to 1997 in order to define the nature and extent of contamination at each site. The SCOU Remedial Investigation /Feasibility Study (RI/FS) report was released in 1997. In order to address data daps in the original RI, a follow up SCOU Data Gap Investigation was published in 1999.
The Comprehensive Basewide RI/FS - Part 1 was published in 1996. The field work for this document used data from previous studies including the earlier SCOU investigations in order to scope out the potential areas of groundwater contamination. The reason the SCOU sites are linked to groundwater is because contaminated soil can serve as a source for contamination to the groundwater. As rain water travels to the groundwater aquifer, it can carry with it contaminants within the soil.
The Remedial Investigation for Comprehensive Basewide ROD - Part 2, dated December 2002, centered on integrating the data from both the SCOU RI and CB-Part 1 RI in order to create a comprehensive RI report. This integration of data demonstrated a relatively direct relationship between soil contamination (SCOU RI) and groundwater contamination (CB-Part 1 RI). The organic contaminants of potential concern that were found in both the soil and groundwater included TCE, cis-1, 2 DCE, tetrachloroethene (PCE) and benzene. The majority of organic contaminants of concern that were found exclusively in the soil include polynuclear aromatic hydrocarbons (PAHs), pesticides, and PCBs. The inorganic contaminants of concern for both soil and groundwater were primarily metals.
As of 2006, all final remedies at CAFB were selected.
The Former Castle Air Force Base now has five final Records of Decision (RODs):
1) SCOU ROD - Part 1: This ROD was finalized in 2002 and addressed 169 of the 233 SCOU Sites. The SCOU ROD - Part 1 selected no further action for 137 of the 169 sites. In addition, 32 sites were selected for no further action under CERCLA because they were petroleum-only sites, which are not regulated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). The State of California oversees the petroleum-only sites.
2) SCOU ROD - Part 2: This ROD was finalized in 2003 and covers 53 of the 233 SCOU sites at CAFB. This ROD documented 12 additional sites that were exempt from CERCLA due to petroleum-only contamination. In addition, 14 more sites were selected for no further action. The ROD also selected SVE as the remedy for 21 sites with volatile organic compounds (VOCs) and fuel hydrocarbons. (These sites were not exempt from CERCLA because the fuel hydrocarbons were commingled with VOCs.) The final six sites addressed under this ROD were waste oil tank and oil/water separator (OWS) tank sites. These sites were contaminated with fuel hydrocarbons, semi-volatile organic compounds and metals. The tank sites were all selected for excavation and disposal of both the tanks and the associated contaminated soil.
3) SCOU ROD - Part 3: This ROD addressed the remaining 11 out of 233 SCOU sites. The ROD selected cap maintenance and monitoring as well as institutional controls for the Landfills 4 and 5, and their associated sites (6 sites total). In addition, ecological monitoring was selected for Landfill 5. Also, no further action was selected for Disposal Pit 9. As for the two former skeet shooting ranges ETC-8 and ETC-10, the ROD selected excavation and disposal for ETC-8, and institutional controls and ecological monitoring for ETC-10. The ROD also selected SVE, cap maintenance and monitoring, institutional controls, ecological monitoring and excavation and disposal for the former Fire Training Area (FTA-1). Finally, Landfill 3 and ETC-12 were selected for ecological monitoring. The institutional controls outlined in this ROD prevent tampering with the caps that are protecting the public and ecological receptors from waste left in place at the landfills.
4) Comprehensive Basewide (CB) ROD - Part 1: This ROD, finalized in 1997, combined the two groundwater operable units, OU-1 and OU-2. This ROD supersedes the interim ROD for OU-1 and the ROD for OU-2. A three-phase pump and treat system was selected for the Main Base Plume with the goal of treating TCE and other contaminants until they reached the maximum contaminant levels (MCLs). As for the East Base Plume, the selected remedy involved the sealing and abandonment of wells to protect against further cross-contamination of hydrostratigraphic zones. (The groundwater underneath CAFB is divided into discrete layers, known as hydrostratigraphic zones, which normally don't mix much; however, wells can act as conduits allowing contaminated water to more easily move vertically from one discrete layer to another discrete layer of groundwater.) The selected remedy for the Castle Vista Plume was a pump-and-treat system to capture the contaminated plume and treat the groundwater to MCLs. By treating to MCLs, the groundwater could be restored to a beneficial use. This ROD also decided that the North Base, Landfill 1 and Landfill 4 Plumes did not require active remediation. The selected remedy for these plumes was long term monitoring of the groundwater and institutional controls to prevent the installation of groundwater supply wells until MCLs were achieved.
5) Comprehensive Basewide (CB) ROD - Part 2: This ROD, signed in 2006, selected the final remedies for CAFB. The ROD added institutional controls for the existing groundwater plumes. These institutional controls are designed to prevent the use of the groundwater while it still exceeds MCLs. In addition, treatment or provisions for alternate water supplies was selected for public and private wells to prevent public exposure to groundwater at levels exceeding the MCLs. The CB ROD - Part 2 also selected local treatment to reduce groundwater contamination where plume capture with the existing pump-and-treat system was impractical.
Current remedial actions at CAFB include the Main Base Plume groundwater program as well as maintenance and monitoring of two landfill caps and a cap at the former Fire Training Area 1 Site.
The groundwater program includes monitoring of hundreds of groundwater monitoring, extraction, and injection wells and dozens of domestic/irrigation/production wells at various scheduled intervals, and the operation of the groundwater treatment plants. The treatment plants consist of two groundwater pump-and-treat systems (Phase 3 and OU-2) that treat water from multiple extraction wells, and one smaller point-source wellhead treatment system (MW951). As of the end of 2015, the OU-2 treatment system consisted of four operating extraction wells (EW11, EW12, MW806A, and MW948), one injection well (IW02), and a GAC treatment plant with two pairs of 2,000-pound vessels. As of the end of 2015, the Phase 3 treatment system consisted of five operating extraction wells (EW19, EW20, EW24, EW34, and EW36), four injection wells (IW27, IW29, IW30, and IW31), and a GAC treatment plant with one pair of 10,000-pound vessels. The wellhead GAC treatment system at MW951 consisted of a single pair of 2,000-pound vessels and one injection well (IW37).
The two permanent and closed landfills maintained at Castle are Landfill 4 and Landfill 5 (LF-4 and LF-5). Both were closed with impermeable caps in 1999, and are equipped with settlement monuments, landfill gas monitoring probes and landfill gas vents. The landfills are inspected and maintained to ensure cap integrity and monitored for potential methane gas migration semiannually. Institutional controls are in place on the landfill parcels limiting use of the property, prohibiting groundwater use, preventing contact with the waste or gases, protecting the integrity of the remedial systems and minimizing the potential for migration of contaminants to the vadose zone.
A third impermeable cap exists at the former Fire Training Area 1 site, which was put in place to prevent exposure to soil contaminants and to maximize effectiveness of a soil vapor extraction system at that location. The site is monitored and maintained to ensure cap integrity.
An SVE system to address petroleum associated contamination was operated at the PFFA, under State oversight. The system extracted soil vapor from shallow wells at a flow rate of approximately 140 standard cubic feet per minute and removes contaminants via a granular activated carbon unit. Influent and effluent soil vapor concentrations are monitored to assure that air discharge standards are met. AF closure and decommissioning of the PFFA system was approved by the State Water Board in 2016.
Cleanup Results to Date
Landfills: The Air Force completed initial remediation of the Base's seven landfills under removal action authority. Landfill 2 and Castle Vista Landfills A and B were excavated during Fall 1997 through Summer 1998; Landfills 1 and 3 were excavated during Summer and Fall 1999. The excavated wastes were determined to be non-hazardous and were disposed in on-site consolidation Landfills 4 and 5. Landfills 4 and 5 have been capped and will be monitored by the Air Force for a minimum of 30 years. The monitoring involves periodically testing groundwater in wells adjacent to the landfills to ensure that contaminants are not leaching from the disposed wastes into the groundwater, monitoring for potential landfill gas migration, and checking the integrity of the landfill caps to ensure that waste is not exposed to the environment and water is not infiltrating.
Removal actions: Removal actions have been conducted on approximately sixteen sites with soil contamination to expedite cleanup and property reuse. At three of those sites, the Firing Range, Skeet Shooting Range (ETC-10), and PCB Sites, the Air Force excavated lead contaminated soils from the former firing range in 1999; lead contaminated soils from the former skeet shooting range in 1997; and polychlorinated biphenyl (PCB) contaminated soils from PCB site 9 in 1998. Wastes containing lead at concentrations exceeding cleanup levels were transported for off-site disposal at an approved facility. All other wastes were disposed on-site at Landfills 4 and 5.
Groundwater Plumes: Well abandonment and sealing for the East Base Plume has been complete. Castle Vista remediation is complete and the system was decommissioned in 2016. The monitoring of the plume at Landfill 1 is complete (plume is no longer present in excess of cleanup levels). Capture of the Phase 3 system is complete, however complete capture of the OU-2 system is uncertain. The AF continues to evaluate extent of TCE and capture at OU-2. Since 1996, the areal extent of the Main Base Plumes have been reduced over 70 percent.
Soil Vapor Extraction (SVE): All SVE remedies have been completed.
Five-Year Review: Five-year reviews are required as long as contamination remains on the site to ensure that the selected cleanup remedies remain protective of human health and the environment. The third Five-Year Review for CAFB was finalized in 2009, and the remedies were determined to be protective. The fourth Five-Year Review was completed in March 2014, and the protectiveness of the remedies was deferred by EPA due to the need to investigate an emerging contaminant (Perfluorinated Compounds-PFCs) that were used in fire suppression activities, such as fire training areas. In 2015, the AF conducted an investigation for PFCs at the former Fire-Training Area and developed a Preliminary Assessment of other potential PFC sources.
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 the PRP for the site.
Documents and Reports
No documents found
Public Meetings: The Castle Restoration Advisory Board (RAB) was adjourned in January 2007. Thank you to all RAB and community members committed to be involved in the cleanup of the site.
Public Information Repositories
The public information repositories for the site are at the following locations:
Merced County Library,
2100 O Street,
Merced, CA 95430
For Air Force updates of the Administrative Record files for Castle Air Force Base, please click here.
Information about real property and facility leasing or purchase Castle Airport Commerce Center at
Information about Castle Air Museum:
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
Christopher Cochrane, R.G. (Water Board)
Environmental Restoration & Brownfields
8800 Cal Center Drive
Sacramento, CA 95826-3200
State of California Environmental Protection Agency
Central Valley California Water Quality Control Board
11020 Sun Center Drive, #200
Rancho Cordova, CA 95670
Air Force BRAC Program Manager
2261 Hughes Ave Suite 155
JBSA Lackland TX 78236-9853
2222 M. Street
Merced, CA 95340
Bureau of Prisons
After Hours (Emergency Response)