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
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Description and History
NPL Listing History
NPL Status: Final
Proposed Date: 10/15/84
Final Date: 07/22/87
The 2,777-acre former Castle Air Force Base (CAFB) was originally used as an aircrew training facility in 1941. Wastes generated were disposed until 1977 when the base was officially closed. In 2006, CAFB achieved the major milestone of "construction completed" under the Superfund Cleanup Process and the property was transferred to other governmental agencies. Currently, a certified commercial airport and a federal prison operate on the site.
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. In addition, the county has purchased/installed the equipment to reactivate the airport control tower and hired a tower operator company to manage flight operations. The FAA approved the airport for commercial use in 2006, and it became fully operational January 30, 2007. The county has also been awarded a grant to build a passenger terminal for future flight operations.
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). Additionally, groundwater underlying a residential housing area located adjacent to the southwest portion of the Base is contaminated with cis-1,2-dichloroethylene (DCE). There are currently institutional controls to prevent the use of the groundwater until the groundwater is treated to drinking water standards. The site also has two landfills with institutional controls 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 one groundwater pump-and-treat system that treats water from multiple extraction wells, three smaller point-source pump and treat systems, which treat water from one or two extraction wells, one site with soil vapor extraction and bioventing systems, and two permanent landfill caps.
The groundwater program includes monitoring of approximately 125 groundwater monitoring/extraction wells and 20 domestic/irrigation/production wells at various scheduled intervals, and the operation of the groundwater treatment plants. The Phase 3 groundwater treatment system pumps approximately 350 gallons per minute from four extraction wells. The system treats groundwater as it flows through one pair of carbon vessels which remove trichloroethene. The water is reinjected through four injection wells.
The three current point-source extraction systems are located at monitoring wells EW11/EW12 (OU-2 system) and MW951 (MW951 system), which treat groundwater for trichloroethene, and MW1046 (Castle Vista system), which removes cis-1,2-dichloroethene. Flow rates at these systems vary from 2 gallons per minute to approximately 120 gallons per minute when the units are operating. Process water is treated through granular activated carbon beds and reinjected into wells located near each system.
The one 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 of the PFFA system is undergoing review by the State Agencies.
The two permanent and closed landfills maintained at Castle are Landfill 4 and Landfill 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.
Cleanup Results to Date
Landfills: The Air Force completed 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 will involve 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. Also, the monitoring of the plume at Landfill 1 is complete (plume is no longer present in excess of cleanup levels).
Soil Vapor Extraction (SVE): With the exception of the Petroleum, Oils, and Lubricants Fuel Farm Area (PFFA), 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. The AF will provide an investigation plan for PFCs in 2015.
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
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)
Sacramento Field Office
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)