The small town of Rifle, Colorado, has an interesting history related to uranium and vanadium production. A mineral found near Rifle, called roscolite, contains both vanadium and uranium but was originally mined and milled for its vanadium content. Union Carbide Corporation began milling the ore in 1924 at what is now referred to as the Old Rifle mill site.
The plant was shut down in 1932 due to a poor market, but reopened in 1942 to produce vanadium for the war effort. The mill was left abandoned along with several million tons of unprocessed ore. In 1946, the plant was torn down and a new one built on the site to recover uranium. Vanadium is still used in the process of hardening steel. Additional ores were mined and milled for the uranium content from other sites in Colorado. Concentrated ores from Slick Rock, Colorado, and Green River, Utah, were processed at the Rifle mill in the 1960s at what is referred to as the New Rifle mill site (Merritt, 1971).
Much of the uranium present in economic concentrations occurs in the desert southwest, so most of the products’ mines and mills were located on the Colorado Plateau, which extends from the U.S. Four Corners states north into Wyoming. Many of the mines and mills in this area supported the effort to produce uranium during World War II and the Cold War era. In 1978, Congress recognized the public health hazards associated with access and exposure to uranium mill tailings and the Uranium Mill Tailings Radiation Control Act (UMTRCA) was enacted.
|Old Rifle mill site in operation.|
The principle health threat associated with uranium mines, ore, and tailings is the accompanying colorless, odorless radon gas, which is radioactive and heavier than air, so it settles and accumulates in low places, such as crawl spaces and basements.
Through a number of chemical processes vanadium, and then uranium, were precipitated from solution at the Old Rifle mill. This eventually resulted in yellowcake—a uranium oxide product. The yellowcake was sent to Hanford, Washington, for further processing into plutonium, or to Oak Ridge, Tennessee, to produce uranium metal.
|Old Rifle mill site following cleanup and reclamation by DOE.|
Ammonia was used for neutralization at the Rifle mill sites to create precipitation. Open, unlined ponds were used for slimes settlement and waste solutions storage at many mill sites. Process chemicals leached into the ground and resulted in the groundwater contamination we observe today.
Upon passage of UMTRCA, little thought was given to the groundwater contamination. Instead, radon was seen as the principle threat requiring mitigation. Radon is relatively easy to prevent from escaping into the atmosphere by covering it with a soil layer. So, the main goal at the time became creation of a disposal cell that would be durable for 1,000 years. The protective approach generally taken is to construct an engineered cell to cover the tailings with a low-permeability material such as clay, and then to protect that layer from erosion.
|New Rifle mill (left), pond (middle), and tailing pile (right) during operation.|
The design and construction has evolved considerably since UMTRCA’s inception until the end of the surface program in 1998. The surface program addressed isolation of the tailings in engineered disposal cells. The Colorado sites were addressed late in the program, and at the request of the State, mill sites near the rivers were typically relocated outside of the flood plain to a more remote location. The Rifle disposal cell is approximately 8 miles north of town in the Estes Gulch.
While UMTRCA was enacted in 1978, it was not until 1983 that a regulation addressing groundwater contamination was drafted. The regulation was not finalized until 1996. The U.S. Environmental Protection Agency sets the standards for soil and groundwater cleanup, while the U.S. Nuclear Regulatory Commission (NRC) acts as the regulator to DOE. Under an Agreement State Program, the Colorado Department of Public Health and Environment has delegated authority from NRC to regulate private licensees on permitting, operation, and cleanup of uranium mills. While UMTRCA covers uranium milling, mines were specifically and purposely excluded from the Act.