In 1989, wildlife managers released 714 prairie dogs onto a former chemical weapons production site roughly 10 miles outside Denver. The land, part of the Rocky Mountain Arsenal, had been contaminated by decades of industrial activity and was considered biologically dead. The prairie dogs were introduced with a modest goal: to stabilize the soil and reduce dust. Their effect, however, went far beyond that original task.
The Rocky Mountain Arsenal was established in 1942, shortly after the United States entered World War II. The Army selected a large parcel of flat land in Commerce City, Colorado, that was far enough inland to be out of reach of enemy bombers. Approximately 200 farming families were relocated, and the land was converted into a major chemical weapons manufacturing facility. During the war, the site produced mustard gas, nerve agent precursors, napalm, and white phosphorus.
After the war, the Army leased large portions of the facility to Shell Oil, which used the infrastructure to produce commercial pesticides including Aldrin, Dieldrin, Endrin, and Chlordane. For roughly 30 years, both industries operated on the same property, disposing of chemical runoff into unlined trenches and open ponds. One containment area, known as Basin F, held over 200 million gallons of liquid waste in a pit lined only with a thin layer of asphalt. Over time, toxic compounds seeped into the groundwater and formed plumes of contamination that moved toward municipal water wells serving the Denver area.
In the early 1960s, the Army attempted to dispose of waste by pumping 165 million gallons of toxic liquid into a well drilled 12,000 feet into the bedrock. The injected fluid increased pressure on ancient fault lines, triggering a series of earthquakes in the Denver area, including several with magnitudes around 5. The pumping was stopped, but the seismic activity continued for years as pressure dissipated through the fractured rock.
By the time the EPA designated the site a Superfund location in 1987, more than 600 distinct chemical compounds had been identified in the soil and water. The central areas of the arsenal were too contaminated to support plant or animal life. For years, the site remained fenced off from the public while cleanup strategies were debated.
While the contamination was extensive, the security perimeter that surrounded the arsenal had an unintended effect. For 40 years, the buffer zones between the industrial core and the outside world were left untouched. There was no farming, no development, and no pesticide application. This allowed small patches of native shortgrass prairie to survive, along with colonies of black-tailed prairie dogs that had gone undisturbed for decades.
In the 1980s, biologists also discovered that bald eagles were wintering in the arsenal’s buffer zones. The eagles were drawn to the mature cottonwood trees and the prey base supported by the prairie dog colonies. This finding led to local conservation groups and Audubon societies advocating for the site to become a wildlife refuge rather than being sold for development. Congress agreed, and the land was designated for transfer to the U.S. Fish and Wildlife Service.
The cleanup itself was a massive undertaking. Over 15 years, the Army and Shell Oil spent approximately $2.1 billion removing contaminated topsoil and sealing it in two double-lined landfills on site. In areas where contamination extended deeper than 10 feet, the soil was capped with engineered barriers. Five groundwater treatment plants were built to pump and filter contaminated water, processing up to a billion gallons per year.
The engineered caps presented a problem for the refuge’s future, however. Prairie dogs dig, and their burrows can extend deep into the ground. If colonies established themselves over the capped zones, they could tunnel through the barriers and bring toxic material back to the surface. To prevent this, engineers incorporated an 18-inch layer of crushed concrete into the caps, sized and compacted so that no burrowing mammal could get through it.
After the engineering work was completed, the surface of the refuge was technically safe but ecologically barren. Heavy machinery had removed the topsoil and destroyed the native seed bank, fungal networks, and soil structure. In 1989, managers began reintroducing prairie dogs to restart the ecological process. The 714 animals released that year came from three sources: 134 were trapped within the arsenal itself, 534 were rescued from Denver-area construction sites by volunteers, and 46 came from a radio tracking study.
The prairie dogs’ effect on the soil was significant. Their burrowing created channels that allowed rainwater to soak into the ground instead of running off the surface. They brought mineral-rich material from depth up to the surface and mixed it with organic matter. Microbial communities began to recolonize the loosened earth, and the soil started cycling nutrients again.
The prairie dogs also altered the vegetation. Their grazing clipped plants low to the ground, suppressing tall grasses and woody shrubs while favoring short, fast-growing, broadleaf species. Within a few years, the vegetation around the colonies shifted into a patchwork of closely cropped, nutrient-dense plants and bare ground.
This new habitat attracted species that had been absent from the area for decades. Burrowing owls moved into abandoned tunnel entrances, using the underground cavities for nesting. Mountain plovers, ground-nesting birds that require heavily grazed, bare earth to breed, also arrived. Raptors such as ferruginous hawks and red-tailed hawks established territories over the expanding colonies, drawn by the concentrated prey. Bald eagles continued to winter in the area, roosting in the cottonwoods and hunting the prairie dog towns.
In 2007, wildlife managers introduced a herd of wild bison to the refuge. The bison naturally gravitated to the prairie dog zones, where the short, protein-rich forage was similar to what they historically grazed on the Great Plains. The bison’s grazing reinforced the open structure the prairie dogs needed, and the two species operated as a functional unit.
The prairie dogs also faced a recurring threat. Sylvatic plague, caused by the same bacterium responsible for bubonic plague and transmitted by fleas, swept through the colonies six times between 1989 and 2002. Each outbreak killed up to 95% of the animals in the affected zones, stalling the chain of ecological recovery. The refuge responded with insecticide dusting in burrows, applied by hand at a rate of about six acres per hour, and later developed a peanut butter-flavored vaccine bait that protected prairie dogs from plague without requiring them to be handled. The baits were dyed green and laced with a fluorescent marker so biologists could confirm which animals had eaten them.
By 2015, the colony density and prey abundance in the refuge had reached the point that it could support black-footed ferrets. The species is the rarest mammal in North America. It was declared extinct in the wild in 1979, but a small population of 18 animals was found near Meeteetse, Wyoming, in 1981. All black-footed ferrets alive today descend from that group. They eat almost exclusively prairie dogs, needing between 100 and 150 per year, and live inside prairie dog burrows.
On October 5, 2015, roughly 30 black-footed ferrets were released into the refuge’s prairie dog colonies. By 2018, the population had grown to approximately 90 individuals breeding in the wild.
The Rocky Mountain Arsenal now spans over 15,000 acres and is the largest urban wildlife refuge in the United States, fully surrounded by the Denver metro area. It supports more than 330 documented species of plants and animals and receives over 340,000 visitors a year. The EPA has removed nearly 16,000 acres from its list of contaminated sites, certifying them clean for unrestricted public use. More than 900 of those acres were sold to Commerce City and now hold a high school, a civic center, a soccer stadium, and public parks.
The cleanup cost roughly $2.1 billion, but the restoration of the living surface was driven largely by the ecological activity of the prairie dogs. By restructuring the soil, shifting the vegetation, attracting grazers and birds, and supporting a growing food chain, the animals rebuilt a functioning ecosystem on land that had been chemically sterilized. The release of black-footed ferrets in 2015 marked the completion of that chain, from soil microbes to an apex predator, on ground that had been closed to public access for half a century.
