Pacific Southwest, Region 9: Superfund
Serving Arizona, California, Hawaii, Nevada, the Pacific Islands, and Tribal Nations
Riverbank Army Ammunition Plant
EPA #: CA7210020759
City: 10 miles northeast of Modesto
Congressional District: 18
Other Names: NI Industries Inc. Riverbank Plt
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Description and History
NPL Listing History
NPL Status: Final
Proposed Date: 06/24/88
Final Date: 02/21/90
The 173-acre Riverbank Army Ammunition Plant site has been used to manufacture materials, including cartridge cases, grenades, and projectiles, since 1951. The site comprises the 145-acre main facility and four unlined industrial waste treatment ponds in the flood plain of the Stanislaus River, located approximately 1 1/2 miles north of the main facility. In 1942, the Aluminum Company of America built the site plant as an aluminum reduction facility to supply the military. The plant closed in 1944 and reopened in 1951, with Norris Industries, Inc. as the operating contractor. The plant was identified for closure under the 2005 Base Realignment and Closure round. As of 2010, the U.S. Army is negotiating property transfer conditions with the City of Riverbank. As a result of industrial activities, the site has generated corrosive wastes, solvents, and wastewater containing heavy metals. Chromium and cyanide have been released onto the property and have subsequently leached into the groundwater, and over the years the contaminant plumes have migrated beyond the installation boundary. In addition, a variety of waste and debris were burned or disposed of on the ground surface and in trenches in a landfill area at the facility. Studies also have determined that sediments in the waste treatment ponds contain elevated levels of zinc. Overflows from the ponds have dumped into the Stanislaus River, and the river has occasionally overflowed into the ponds during periods of flooding. Approximately 13,700 people obtain drinking water from public and private wells within 3 miles of the site, and at least 3,500 acres of nut and fruit orchards are irrigated partially by groundwater. The Stanislaus River is used for irrigation and recreational activities.
Contaminants and Risks
- Surface Water
- Soil and Sludges
- Environmentally Sensitive Area
Groundwater is contaminated with chromium and cyanide. Soils in the landfill are contaminated with chromium and arsenic. Sediments in the industrial waste treatment ponds contain zinc and petroleum. Potential human health threats include drinking contaminated groundwater and showering or bathing with the water, and accidentally coming into contact with or ingesting contaminated soils from the landfill or sediments from the ponds. Potential ecological threats include exposure of local land-dwelling and migratory species to contaminated soils from the landfill and to sediments from the ponds.
Investigation and Cleanup Activities
This site is being addressed in two stages: immediate actions and a long-term remedial phase focusing on cleanup of the entire site.
Immediate Actions: When chromium was found in off-site domestic water wells in 1989, the Army supplied affected residents with bottled water. The Army subsequently closed the contaminated wells and replaced them with new deeper, clean wells. The Army completed an interim groundwater treatment system that became fully operational in October 1991. This system treats chromium and cyanide contaminants from extracted groundwater. In 1991, the Army began work to extend the existing City of Riverbank's public water supply system to all residents who have been, or could potentially be, affected by the contaminated groundwater. By December 1992, the residents were connected to the Riverbank water system. In 1993, the Army removed petroleum and zinc-contaminated sediments from its industrial waste treatment ponds and disposed of the sediments off-site. Groundwater around the ponds is monitored quarterly as a requirement in the National Pollution Discharge Elimination System (NPDES) permit for the ponds.
Entire Site: A remedy was selected in the spring of 1994 that includes a comprehensive groundwater extraction and treatment system for the contaminated groundwater; stabilization, capping, and monitoring of the landfill; and no further cleanup action for the ponds, which were cleaned up through the immediate actions described above. The Army completed its design of the landfill cap in late 1994. Design and construction of the groundwater extraction and treatment system were completed in November 1996.
Groundwater Extraction and Treatment. The groundwater extraction system and treatment plant is in operations and maintenance (O&M) mode. The groundwater treatment plant and extraction wells are monitored daily. Groundwater treatment influent is sampled monthly. Groundwater from certain monitoring wells are sampled quarterly. A quarterly groundwater report is submitted to the regulatory agencies.
Evaporation Ponds. A removal action of the contaminated soil was completed in December 1993.
A Record of Decision (ROD) signed in March 1994 documented no further action for the evaporation ponds.
Landfill. Construction of the landfill cap was completed in November 1995. Routine landfill O&M such as mowing and weed control, occasional revegetation, minor erosion and drainage repair is continues.
The Riverbank Army Ammunition Plant is participating in the Installation Restoration Program, a specially funded program established by the Department of Defense (DOD) in 1978 to identify, investigate, and clean up petroleum and hazardous contaminants at DOD facilities. In April 1990, the Army, the EPA, and the State of California signed a Federal Facility Agreement, which established a procedural framework and schedule for the Army to carry out the necessary site cleanup actions.
Cleanup Results to Date
Treating and monitoring the groundwater, and disposing of contaminated sediments have reduced the potential for exposure while final cleanup activities are being planned at the Riverbank Army Ammunition Plant site.
The previous (second) Five-Year Review (FYR) for the Riverbank Army Ammunition Plant (RBAAP) in Riverbank, California was finalized in September 2006. The RBAAP was put on the Base Realignment and Closure (BRAC) list in 2005, and is undergoing closure and transfer to the City of Riverbank Local Redevelopment Authority. The third FYR completed in September 2011 concluded that:
Overall, the groundwater extraction and treatment system and landfill cover remedial actions are functioning as designed and are operated and maintained in an appropriate manner. However, groundwater pump and treat specified in the ROD for chromium and cyanide removal has not been fully successful for removal of residual chromium in some areas (US Army, 2011), but appears to be more effective for cyanide removal. Therefore, Ahtna (AGSC, 2009) has recommended in situ chromium reduction for the site. An Explanation of Significant Differences (ESD #1) has been developed to describe this.
The Army is implementing land use controls (LUCs) / institutional controls (ICs) for the Riverbank Army Ammunition Plant through deed restrictions. An Explanation of Significant Differences (ESD #2) is being developed to describe these controls.
Appropriate health and safety and emergency response protocols are in place at the RBAAP facility and are being implemented properly to control risks. Immediate threats to human health and the environment have been addressed through the implemented groundwater remedy. The groundwater extraction and treatment system is operating and functioning as designed, with the exception of extraction wells EW104 and EW114, access to which has recently been regained. Containment of the contaminated areas has been achieved through establishment of inward gradients that limit migration of the groundwater plumes. Contaminant levels throughout the site have generally decreased. The groundwater remedial action is currently protective of human health and the environment, but deed restrictions are required for institutional control to prevent inappropriate use of the contaminated groundwater while the groundwater remediation is occurring. Presently, groundwater in the vicinity of RBAAP is not used for drinking water, as residents are on the municipal drinking water system. Stanislaus County has stated that applications for domestic water wells in the vicinity of RBAAP will be denied and applications for irrigation wells will be considered individually, but that the County has not received a water well application in the RBAAP area for approximately 10 years.
The deed restrictions would include a restrictive covenant on the landfill site and on groundwater usage. The Industrial Wastewater Treatment Plant will be addressed under the RCRA Permit closure requirements. At the time of transfer of the property, the deed restrictions will be incorporated into the transfer documents.
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.
Online information about the PRPs for the site is not yet available.
Documents and Reports
Public Information Repositories
The public information repositories for the site are at the following locations:
Stanislaus County Library,
3442 Santa Fe Street,
Riverbank, CA 95367
EPA Site Manager
Mail Code SFD
75 Hawthorne Street
San Francisco, CA 94105
EPA Community Involvement Coordinator
Mail Code SFD-6-3
75 Hawthorne Street
San Francisco, CA 94105
EPA Public Information Center
Sacramento, CA 95826-3200
California Water Board
Central Valley Region
11020 Sun Center Drive,
Rancho Cordova, CA 95670
After Hours (Emergency Response)