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Gunnison Mill Site
Gunnison County, Colorado
| Years of Operation |
Status of Mill
or Plant Site |
Uranium Ore
Processed
(Million Short Tons) |
Production
(Million Pounds U3O8) |
| 1958-1962 |
Decommissioned |
0.54 |
1.45 |
Mill/Plant Area
(Acres) |
Disposal Cell
Area
(Acres) |
Disposal Cell
Radioactive
Waste
Volume
(Million Cubic Yards) |
Disposal Cell
Total Radioactivity
(Ci, 226Ra) |
Disposal Cell
Average Tailings
Radioactivity
(pCi/g, 226Ra) |
UMTRA Project
Final Cost
(Million Dollars) |
| 61 |
29 |
0.74 |
175 |
314 |
58.92 |
Location: The Gunnison uranium mill site is located just southwest of the City of Gunnison in Gunnison County, Colorado.
Background: In 1954-1955, uranium deposits were discovered in the Cochetopa, Tallahassee Creek, and Marshall Pass uranium districts in the Southern Rocky Mountains of central Colorado. Development of the deposits soon followed. In November 1956, the Atomic Energy Commission (AEC) signed a contract with the Gunnison Mining Company (GMC) for a small uranium mill at Gunnison, Colorado. The mill was built on a 62-acre tract just south of Gunnison, and its first delivery under GMC’s only contract was made in February 1958. In late 1961, GMC’s assets were acquired by Kermac Nuclear Fuels Corporation (KNF). The mill continued to operate under KNF until April 1962, when it was closed down. Originally designed for a throughput capacity of 200 tons of ore per day (TPD), the mill was operated over its life at an average of 350 TPD, due to the lower than anticipated grade of ore received. Ore deliveries in 1958 averaged 0.24 percent U3O8, but the grade declined to about 0.12 percent during 1960-1962. The mill feed consisted mainly of uraninite in silicified and brecciated elastic rocks and came mainly from company mines in the Cochetopa district, but a small amount of ore was purchased from independent miners. From January 1958 to April 1962, the mill processed about 540,400 tons of ore that averaged 0.15 percent U3O8 and recovered 1.45 million pounds U3O8 in concentrate. All concentrate production was sold to the AEC. Uranium was leached from the ground ore using sulfuric acid and sodium chlorate as the oxidant. A four-stage classifier-thickner circuit then separated the pregnant solution from the washed solids, which were sent to tailings. Uranium was concentrated from the pregnant solution using solvent extraction. Sodium carbonate solution was then used to strip the uranium from the solvent. This alkaline stripping formed an iron precipitate that was filtered from the solution and saved as an iron cake. From the clarified solution, uranium was precipitated and filtered. The final yellow cake product was dried and packed for shipment to the AEC’s Grand Junction, Colorado facility. After the mill was closed, the 39-acre, partially filled, tailings impoundment pond was reclaimed in accordance with plans approved by the Colorado Department of Health: the pond area was contoured, covered with soil material from a local gravel pit, and was planted with a mixture of grasses. A narrow, 3.5-acre parcel of the mill property was deeded to Gunnison County in 1966 for use in expanding the local airport facility. After 1964, several different owners held the remaining mill site property. In 1990, the mill site was acquired by the State of Colorado, which later assigned site ownership to Gunnison County.
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UMTRA Surface Remediation: Environmental cleanup and stabilization at the Gunnison site were performed in two phases between 1991and 1994. In Phase I, the mill buildings and supporting structures remaining on the site were demolished. Phase II included the cleanup of contaminated areas at the mill site, backfilling with uncontaminated soil to blend with the surrounding terrain, and final grading to assure proper drainage and success of revegetation. During the mill site remediation, cleanup work was also completed at 12 vicinity properties in the Gunnison area. Overall, a total of 719,000 cubic yards of contaminated materials, including the mill tailings, building and other structural debris, and radioactive materials from the vicinity properties were relocated to the Gunnison Disposal Cell.
Disposal Area: Construction on the Gunnison Disposal Cell was begun in 1992 by the Department of Energy (DOE) on an isolated site at Chance Gulch, located about 6 miles southeast of Gunnison. The engineered cell contains some 740,000 cubic yards of contaminated waste materials, including radioactive materials that were relocated to the cell during 1993-1994. The cell was closed in 1995. The cell site, which occupies about 29 acres of the 115-acre tract, is located on a low drainage divide. Its base rests below grade on slope-wash material that is underlain by a sequence of lower-permeability volcaniclactic sand and gravel strata interbedded with ash fall tuffs. The cell is about 60 feet vertically from its lowest to highest points, and its final top slope rises about 50 feet higher than the surrounding natural surface. The cell’s cap is a multilayered structure: a 1.5-foot thick layer of low permeability silty-clay-sand soil amended to 5 percent with bentonite covers the radioactive materials and serves to minimize escape of radon and infiltration of rainwater; a drainage layer made up of sand and gravel provides a capillary break and a pathway to divert downward percolating water away from the radon barrier; a 6-foot thick freeze-thaw barrier of compacted clayey, silty sand overlies the drainage layer; a 6-inch thick bedding layer of coarse-grained crushed rock provides erosion protection for the frost barrier and supports the overlying riprap layer that is 6 inches thick on cell top slopes and 12-inches thick on side slopes. An apron of riprap drains rain runoff away from the cell. Areas surrounding riprap cover were revegetated with native grasses. The cell was licensed by the Nuclear Regulatory Commission in 1997.
Responsibility for Remediation: U.S. Government, 90 percent; State of Colorado, 10 percent.
Stewardship: The disposal cell site is being managed under the DOE’s Long-Term Surveillance and Monitoring (LTSM) Program in accordance with the approved site specific plan. DOE will be responsible in perpetuity for the safety and integrity of the Gunnison Disposal Cell site.
Groundwater Program: Groundwater below the Gunnison mill site was found to be contaminated with residual materials generated during the uranium ore processing at the site. Monitoring has shown that several times since 1990 the groundwater has locally exceeded maximum concentration limits for net gross alpha, radium, and uranium. Other metal contaminants present in the groundwater also pose a potential health hazard. To prevent local populations from being threatened by the groundwater contamination, an alternate domestic water system was constructed during the Gunnison mill site UMTRA project. Residences located downgradient from the mill site where water wells could be impacted by the aquifer contamination are now served by the alternate water system. The groundwater contamination has not degraded the quality of water in the Gunnison River, the main channel of which flows within one-quarter mile of the mill site. Studies of the groundwater contamination plume have been completed, and the groundwater compliance strategy was under review in January 2001. The DOE is also responsible for periodic monitoring of groundwater conditions in wells at the Gunnison Disposal Cell site in order to determine compliance with applicable groundwater protection standards and to assure the integrity of the cell structure. Groundwater at the disposal cell site is not contaminated.
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