The Rocky Mountain Arsenal was a United States chemical weapons manufacturing center located in the Denver Metropolitan Area in Commerce City, Colorado. The site was completed December 1942 and then operated by the United States Army throughout the latter 20th century, remaining controversial among local residents until its closure in 1992.
Much of the site is now protected as the Rocky Mountain Arsenal National Wildlife Refuge.
History After the attack on Pearl Harbor and the United States' entry into World War II, the U.S. Army began looking for land to create a chemical manufacturing center. Located just north of Denver, in Commerce City and close to the Stapleton Airport, the U.S. Army purchased . The location was ideal, not only because of the proximity to the airport, but because of the geographic features of the site, it was less likely to be attacked. The Rocky Mountain Arsenal manufactured chemical weapons including mustard gas, napalm, white phosphorus, lewisite, chlorine gas, and sarin. In the early 1960s, the U.S. Army began to lease out its facilities to private companies to manufacture pesticides. In the early 1980s, the site was selected as a Superfund site and the cleanup process began. In the mid-1980s, wildlife, including endangered species, moved into the space. Later, the land was designated a protected wildlife refuge, partly open to the public.
Policy The environmental movement began in the United States in the 1960-1970s. The U.S. Congress responded to the movement in 1980 with the creation of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), most commonly referred to as a Superfund. CERCLA was a tax imposed on chemical and petroleum industries. CERCLA also gave the Federal government the authority to respond to the release of life-threatening hazardous materials.
In 1984, after 42 years of chemical manufacturing, the United States Army began to inspect the level of contamination at Rocky Mountain Arsenal (RMA). The site was placed on the National Priorities List (NPL), a list of the most contaminated areas in the United States. Rocky Mountain Arsenal, among other post-military sites, was a top priority, establishing RMA as a Superfund site. This was further exacerbated when the U.S. Army discovered an endangered species, the bald eagle. After the bald eagles were captured, tested, and found to be healthy, the National Wildlife Federation (NWF) worked with policymakers to transition RMA to a wildlife refuge. In 1992, Congress passed the Rocky Mountain Arsenal National Wildlife Refuge Act (RMANWR Act). Included in the RMANWR Act, areas within RMA that were still contaminated were still owned by the U.S. Army, however, the vast majority of the land that was deemed clean would be managed by the U.S. Fish and Wildlife Service (USFWS or FWS).
Tensions arose between the United States Environmental Protection Agency (USEPA), the State of Colorado, United States Army, and the chemical industries as they partnered to clean up the site and create the RMANWR. This led the State of Colorado to take legal action over who has legal authority over RMA remediation efforts, payment of natural resource damages (NRDs), and reimbursement of costs expended for cleanup activities (response costs).
Site selection The Arsenal's location was selected due to its relative distance from the coasts (and presumably not likely to be attacked), a sufficient labor force to work at the site, weather that was conducive to outdoor work, and the appropriate soil needed for the project. It was also helpful that the location was close to Stapleton airfield, a major transportation hub.
In 1942, the US Army acquired of land on which to manufacture weapons in support of World War II military activities at a cost of $62,415,000. Additionally, some of this land was used for a prisoner of war camp (for German combatants) and later transferred to the city of Denver as Stapleton Airport expanded. A lateral was built off the High Line Canal to supply water to the Arsenal.
Manufacturing operations
Weapons manufactured at RMA included both conventional and chemical munitions, including white phosphorus (M34 grenade), napalm, mustard gas, lewisite, and chlorine gas. RMA is also one of the few sites that had a stockpile of Sarin gas (aka nerve agent GB), an organophosphorus compound. The manufacturing of these weapons continued until 1969. Rocket fuel to support Air Force operations was also manufactured and stored at RMA. Subsequently, through the 1970s until 1985, RMA was used as a demilitarization site to destroy munitions and chemically related items. Coinciding with these activities, from 1946 to 1982, the Army leased RMA facilities to private industries for the production of pesticides. One of the major lessees, Shell Oil Company, along with Julius Hyman and Company and Colorado Fuel and Iron, had manufacturing and processing capabilities on RMA between 1952 and 1982. The military reserved the right to oust these companies and restart chemical weapon production in the event of a national emergency.
Deep injection well RMA contained a deep injection well that was constructed in 1961. It was drilled to a depth of . The well was cased and sealed to a depth of , with the remaining left as an open hole for the injection of Basin F liquids. For testing purposes, the well was injected with approximately 568,000 US gallons (2150 m³) of city water prior to injecting any waste. The injected fluids had very little potential for reaching the surface or usable groundwater supply since the injection point had of rock above it and was sealed at the opening. The Army discontinued use of the well in February 1966 because the fluid injection triggered a series of earthquakes in the area. The well remained unused until 1985 when the Army permanently sealed the disposal well.
Environmental issues
In 1984, the Army began a systematic investigation of site contamination in accordance with the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA), commonly referred to as Superfund. In 1987, the RMA was placed on the National Priorities List (NPL) of Superfund sites. As provided by CERCLA, a Remedial Investigation/Feasibility Study (RI/FS) was conducted to determine the extent of contamination. Since 1985, the mission at RMA has been the remediation of the site.
Contaminants The primary contaminants include organochloride pesticides, organophosphate pesticides, carbamate insecticides, organic solvents and feedstock chemicals used as raw products or intermediates in the manufacturing process (e.g., chlorinated benzenes), heavy metals, chemical warfare material and their related breakdown products and biological warfare agent such as TX. Additionally, ordnance (including incendiary munitions) was manufactured and tested, and asbestos and polychlorinated biphenyls (PCBs) were used at RMA. Today, it is considered a hazardous waste site according to the Colorado Department of Public Health and Environment (CDPHE).
Groundwater contamination The contamination of the underlying alluvial aquifer occurred due to the discharge of waste into unlined basins. The following data were derived from the United States Nuclear Regulatory Commission (NRC). From 1943 to 1956, the US Army and Shell discharged wastes into the unlined basins resulting in the contamination of the South Platte River outside the Arsenal. Farmers in the vicinity complained about the damage to crops due to the water pumped from the shallow alluvial aquifer. In response, the Army constructed an asphalt-lined impoundment for the disposal of wastes in 1956. Further, in 1961, the Army constructed a 12,000-foot deep injection well for the disposal of wastes. This resulted in subsequent earthquakes in Denver area. In 1975, Colorado Department of Public Health and Environment ordered the Army and Shell to stop the non-permitted discharge of contaminants, to control the contaminated groundwater leaving the site, and to implement a monitoring plan. The Army and Shell took remedial actions to prevent the contamination that includes the installation of the groundwater barrier system which treated approximately 1 billion gallons of water every year. The deep injection well was closed in 1985 and Basin F was closed in 1988 According to a National Resource Damage Assessment (NRDA), although the contamination has been reduced by the treatment efforts, the water in and around the arsenal may never be fully clean. A volume of approximately 52,500 acre-feet (65 million cubic metres) of the alluvial aquifer is not usable for human consumption.
Wildlife Injuries The NRDA found several injuries to wildlife. It was estimated by the U.S. Fish and Wildlife Service that at least 20,000 ducks died in a 10-year span during the 1970s. Mallard carcasses found to have higher levels of Dieldrin. Many mammals and birds were found dead and may have suffered lower reproduction rates or birth defects.
Safety concerns for neighboring residents Because of the Superfund site status and the dramatic cleanups, many residents in neighborhoods surrounding the RMA voiced concern about ongoing health risks of living within the close vicinity of the site. In September 2017, the state of Colorado filed a lawsuit to sue the United States government for the right to control the contaminated areas of the RMA. Though the cleanup of the site was considered complete in 2010, soil and groundwater monitoring practices occur every five years to ensure the effects of the clean-up remain. Restrictions on well water use, residential development, consumption of fish and game from the arsenal, and agricultural use of the arsenal will exist in perpetuity until further scientific research is completed at the site.
Water Many of the surrounding neighborhoods have been provided with potable tap water from other areas of Adams County because of the potential effects of contaminated groundwater from wells.
The carcinogenic chemical 1,4-dioxane has been found in nearby private groundwater wells and, in trace amounts, in source water wells for the area. The contamination has been confirmed by the county as sourcing from the RMA Superfund. The average concentration of 1,4-dioxane in district drinking water is well below the EPA health advisory level (0.71 parts per billion as of 2025, versus an EPA health advisory level of 35 parts per billion).
Soil As part of the clean up of the RMA, much of the soil, up to 10 feet below the surface was removed from the site. This soil is contained in hazardous waste landfills. Contaminated areas of soil remain in the Rocky Mountain Arsenal, but are contained in basins and containment structures.
Air quality During the cleanup of the RMA, concern for air pollution from the hazardous materials was raised. The Colorado Department of Public Health and Environment established monitoring systems throughout various locations of the RMA. Throughout the decades of cleanup, the air monitors revealed there was no safety hazard to public health as no arsenal chemicals had been released into the air.
Epidemiological studies