Riverbank Army Ammunition Plant

Overview

Riverbank Army Ammunition Plant covers 173 acres near Modesto in Stanislaus County, California. The site manufactured ammunition and left groundwater, soils, and sediments contaminated with heavy metals. It was added to the National Priorities List in February 1990, making it eligible for federal Superfund cleanup funding and oversight. The U.S. Army owns and operates the facility and bears primary responsibility for cleanup. The EPA and the California Department of Toxic Substances Control both oversee the work, and a Federal Facility Agreement signed in April 1990 set the cleanup schedule and framework.

Three contaminants pose unacceptable risks at the site. Chromium appears in groundwater across the overall site. Cyanide has been found in both soil and groundwater. These contaminants can reach people through ingestion, inhalation, and direct contact with affected soil, groundwater, and groundwater vapors.

Major cleanup actions started in the late 1980s. Beginning in 1989, the Army gave affected residents bottled water and shut down contaminated wells. By 1992, all residents were connected to the public water supply. An interim groundwater treatment system began running in 1991 to address chromium and cyanide. In 1993, petroleum and zinc-contaminated sediments were removed from industrial waste ponds. A landfill cap was finished in 1995, and the long-term groundwater treatment system completed construction in 1996. Operation and maintenance have continued since September 1997.

The EPA has determined that human exposure is currently under control across the entire site, with no unacceptable exposure pathways present. Contaminated groundwater is stabilized in its original area, and no unacceptable discharge to surface water is expected. Physical construction of the cleanup is complete, but the site has not yet been deleted from the National Priorities List. Additional remedial investigation and feasibility study work is currently underway. New removal work is estimated to begin between October and December 2026, with a new Record of Decision expected between September and November 2028. The most recent five-year review was completed in September 2021, and the next is estimated between September and November 2026.

The site is also moving toward transfer to the City of Riverbank's Local Redevelopment Authority. Community members with questions can contact the EPA Community Involvement Coordinator or the Remedial Project Manager. State agency contacts and Army representatives are also available for specific questions about the site.