Mr. Curtis Payton
r.c.payton.civ@army.mil
916-267-1246
September 2026
Riverbank Army Ammunition Plant
Sixth Five-Year Review Fact Sheet
What is a Five-year Review?
The purpose of a five-year review is to determine if remedies at a site are/remain protective of human health and the environment. If any issues that affect protectiveness are found during the five-year review, recommendations are made to address them. The report addresses three major questions:
- Is the remedy functioning as intended?
- Are the exposure assumptions, toxicity data, cleanup levels, and remedial action objectives (RAOs) used at the time of remedy selection still valid?
- Has any other information surfaced that could affect the protectiveness of the remedy?
Site History
The Riverbank Army Ammunition Plant (RBAAP) is in the City of Riverbank in Stanislaus County, CA. RBAAP is a 173-acre facility that consists of large production buildings with numerous support buildings. The main plant consists of 145 acres situated in a primarily rural area. Four Evaporation/Percolation (E/P) ponds cover an additional 28 acres and are located adjacent to the Stanislaus River, about 1.5 miles to the north.
RBAAP was originally constructed in 1942–1943 as an aluminum reduction plant supplying the military. RBAAP was closed under the 2005 Base Realignment and Closure (BRAC). RBAAP is currently leased to the City of Riverbank to operate as a “green industry” business park.
Site Chronology
- 1980 – Installation Assessment Study: Identified potential hazardous materials release sites.
- 1987 – Remedial Investigation initiated: Chromium and cyanide identified as contaminants in groundwater.
- 1990 – National Priorities List (NPL): RBAAP placed on the NPL.
- 1990 – Interim Groundwater Treatment System: Construction completed.
- 1992 – Water Supply Extended: City of Riverbank water supply extended to residential areas.
- 1996 – Landfill: Construction of cap completed.
- 1997 – Construction complete. Expanded Groundwater Treatment System complete.
- 2010 – RBAAP closed.
- 2014, 2017, 2020 – In situ chromium injections.
Site Map
Major Developments since Last Five-Year Review
- Groundwater – Long-term monitoring is conducted and reported on a semi-annual basis. In-situ treatment injections were completed and monitored with another round of injections initiated.
- Landfill – The landfill cover is inspected weekly. A landfill cover grade survey, completed in 2023, found the 2% grade was in accordance with the Record of Decision.
- Hexavalent Chromium Standard – The State of California finalized the State Maximum Contaminant Level (MCL) for hexavalent chromium at 10 micrograms per liter in October 2024.
- Explanation of Significant Difference (ESD) #2 was finalized in April 2026.
Issues, Follow-up Actions, and Schedule Dates
The issues listed below do not affect current protectiveness because there is no current exposure to chemicals of concern in groundwater and at the landfill.
- The Groundwater Treatment System is limited by current operation times due to non-functioning extraction well communication systems. The system performance will be increased to full time operation in early fiscal year 2027.
- Damage to the landfill surface water drainage is scheduled for repair in the fourth quarter of 2026. Current damage does not affect the integrity of the landfill cover.
- Phase 2A In-situ treatment is delayed until well replacements and access to off-site wells are secured.
- Prior in-situ treatment was ineffective at reducing hexavalent chromium concentrations in downgradient wells. After Phase 2 treatment is complete, the groundwater model will be modified to support decision making for off-site plume management.
- Further ecological risk assessment for avian aquatic receptors at the E/P ponds may be warranted. Soil sample collection and assessment at the E/P ponds is scheduled for fiscal year 2027.
Protectiveness Summary
- Landfill
- Short-Term Protective
- Groundwater
- Short-Term Protective
- Sitewide
- Short-Term Protective
Year Review
Contact Information
All publicly available documentation is located at:
Stanislaus County Library, Riverbank Branch3442 Santa Fe Avenue, Riverbank, California 95367
(209) 869-7008
http://www.stanislauslibrary.org/
