Rocky Mountain Arsenal (USARMY)

Overview

Rocky Mountain Arsenal spans nearly 27 square miles in Adams County, Colorado, just outside Denver. The U.S. Army built it in 1942 to produce chemical warfare agents and incendiary munitions. After World War II, Shell Chemical Company leased parts of the site and produced agricultural pesticides there from 1952 to 1982. Those decades of military and industrial use left widespread contamination across the property. The site was listed on the National Priorities List (NPL) in July 1987, and final remedial action began in 2025.

EPA identified over 200 contaminants of concern at the site. They appear in soil, groundwater, soil gas, surface water, sediment, debris, buildings, and waste materials. Key contaminants include volatile organic compounds like trichloroethene and benzene, chlorinated solvents, metals such as arsenic and lead, pesticides including aldrin and DDT, polychlorinated biphenyls (PCBs), and chemical warfare agents such as sarin and VX found in buildings and waste disposal areas. The site is divided into 30 operable units (OUs), which are distinct zones each targeting specific contamination problems.

Cleanup is managed under a Federal Facility Agreement among the Army, EPA, and Shell. Active measures include three groundwater containment and treatment systems, slurry walls around disposal areas, two capped landfills, and alternative water supplies for south Adams County residents. Technologies used across the site's operable units include excavation, air stripping, carbon adsorption, soil vapor extraction, and solidification. Several operable units were partially deleted from the NPL between 2003 and 2010, reflecting completed work in those areas. Physical construction across the entire site is not yet complete.

Current assessments show that human exposure is under control across the whole site, with no unacceptable exposure pathways. Contaminated groundwater migration is also stabilized, with no unacceptable discharge to surface water, though monitoring continues. The most recent five-year review, completed in September 2021, found that remedies are expected to protect public health and the environment upon completion. The next five-year review is planned for fall 2026.

Portions of the site have been transformed into the Rocky Mountain Arsenal National Wildlife Refuge, designated by Congress in 1992. A visitor center opened in May 2011. Community members can follow site progress through the Summer 2025 Newsletter, available in English and Spanish. Administrative records are available at the Joint Administrative Records and Design Facility in Commerce City, at the Colorado Department of Public Health and Environment (CDPHE) Records Center in Denver, and through the EPA Superfund Records Center in Denver.