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Tuba City Mill Site
Coconino County, Arizona
Current Owner: Navajo Nation
| Years of Operation |
Status of Mill
or Plant Site |
Uranium Ore
Processed
(Million Short Tons) |
Production
(Million Pounds U3O8) |
| 1956-1962, 1963-1966 |
Decommissioned |
0.80 |
4.70 |
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) |
| 145 |
50 |
1.40 |
940 |
441 |
34.14 |
Location: The
former Tuba City mill site is located approximately 5 miles east of Tuba
City, Coconino County, Arizona, south of U.S. Highway 160 on the Navajo Indian
Reservation. It is 85 miles north of Flagstaff, Arizona, and near the Hopi
Indian Reservation.
Background: The
uranium ores processed at the Tuba City mill came largely from sites in Arizona:
the Orphan Lode mine within the Grand Canyon National Park, mines in the
Cameron area and adjacent areas, mines in the Monument Valley district, and
ores purchased by the U.S. Atomic Energy Commission (AEC) at the Tuba City
ore buying station and at other AEC ore buying stations1,
including the Globe ore buying station,
Gila County, Arizona.2 3
Orphan Lode mine - Uranium mineralization was identified by
a U.S. Geological Survey geologist in April 1951 in old copper mine workings
at the Orphan mining claim, a lode claim located in 1893 and patented
as a copper prospect in 1906. 4 5The
mine site is near Grand Canyon Village and about 1,100 feet below the south
rim of the Grand Canyon of the Colorado River. Private exploration drilling
in 1955 confirmed a high-grade uranium-copper deposit on the claim. The Orphan
Lode deposit was in the upper reaches of a vertical, nearly circular, solution-collapse
breccia-pipe structure. It is exposed on the canyon wall near the Coconino
Sandstone-Hermit Shale (Permian) contact. 6 The
pipe structure extends downward from outcrop through sedimentary rocks of Permian
and Pennsylvanian ages and bottoms near the middle of the Redwall Limestone
(Mississippian age), a vertical distance of about 1,660 feet. 7 The collapse material in the pipe consists of
poorly consolidated sand fill, sandstone and claystone
breccias with varying calcite cementation, and limey siltstone. 8 9 Uranium
ore bodies associated with the pipe occurred as irregular masses in the collapsed
material that fills the pipe and in rocks associated with the pipe's peripheral,
annular-ring shear zone that defines the border of the pipe. 10 11 Some
ore also extended outside the lode claim boundary and onto land belonging to
the National Park Service.12 The
ore was mined to a depth of about 585 feet, but uranium mineralization associated
with the pipe structure is present below that mine level.
Development work at the Orphan Lode mine began in 1953. A three-tower, aerial
tramway was completed in March 1955 from the canyon rim to service the mine
site. A new 1,800-foot tramway with eight-towers replaced the old one in May
1956. The tramways had limited capability for moving materials to and from
the mine. To expand ore production, a 1,590-foot, 2˝ compartment mine
shaft was sunk on the claim behind the canyon rim from May 1958-January 1959.
The shaft was connected to the mine through a 1,200-foot crosscut at the 400
level. Mine development costs at the Orphan were high, as all mined materials
had to be transported off site. Cut and fill methods, which involved handling
and rehanding waste rock in the underground mine, were used when possible to
save on costs for hoisting and surface storage. Mine waste rock could not be
stored on National Park Service (NPS) land. Some waste rock excavated during
mine-shaft and crosscut construction was used by the NPS for road projects
in the park.
The first shipment of Orphan Lode ore went to the AEC's ore buying station
at the Tuba City mill in April 1956. Most of the ore mined from the Orphan
was hauled to the Tuba City mill for processing. Some ore was shipped in
1958-1960 to mills in Utah and New Mexico, when the Tuba City mill could not
take the ore. In 1959 and 1960, some Orphan Lode ore was also shipped to an
Ambrosia Lake, New Mexico, alkaline-leach mill that could treat the high-lime
ore. 13
Throughout its life the Orphan Lode mine was operated by several owners.14 Cotter
Corporation, the last owner, shipped and processed ore from the mine
at its Canon City, Colorado, mill from late 1967-early 1969. In April 1969
the Orphan Lode mine was permanently closed due to the high cost of shipping
the ore to the Canon City mill. 15
The total Orphan Lode mine ore production (April 1956-April 1969) was 500,432
tons ore containing 4,286,254 pounds U3O8. 16 About
32 percent of the total uranium produced was recovered from ore mined on National
Park Service land. The highest grade monthly ore shipment was 726 tons that
averaged 2.29 percent U3O8. 17 Some 6.68 million pounds of copper and about 107,000 ounces of silver also were
recovered from the Orphan Lode ore .18
Cameron uranium area - Uranium deposits occur in the Cameron area
in an area that extends about 30 miles north-south and 12 miles east-west near
the town of Cameron, Arizona. 19 The
area extends across parts of the Painted Desert and Ward Terrace
in the western part of the Navajo Reservation. In the Ward Terrace area east
of Cameron, uranium minerals were discovered in 1950 on an outcrop of the Kayenta
Formation (Lower Jurassic). 20 From mid
1951 to the spring of 1952, several shipments of uranium ore from small deposits
in this area were sent to the AEC ore buying station at Monticello, Utah. In
early 1952 ore was also shipped to the AEC buying station at Shiprock, New
Mexico. 21
In 1952, uranium was discovered on Chinle Formation (Upper Triassic) outcrops
near Cameron by a Navajo prospector who was conducting surface radiometric
reconnaissance on behalf of the AEC. 22 Other
discoveries were soon made at sites on Chinle outcrops. In 1952, shipments
of uranium ore from Cameron deposits were sent to the AEC's ore buying station
at Bluewater, New Mexico. 23 By
1954, six operators were shipping ore from several Cameron area mines. 24
In December 1954, the Navajo uranium leases held by the Arrowhead Uranium
Company, which had become a major player in the Cameron area, were acquired
the Rare Metals Corporation of America.25
The deposits in the Cameron area occurred mainly in the lower 60 feet of the
Petrified Forest Member and the upper 30 feet of the underlying Shinarump Member
of the Chinle Formation. 26 The ore occurred
mainly in poorly consolidated sandstone and in clay-pellet sandstone and conglomerate
beds in intraformational channel and scour structures. 27 Ore-grade
material occurred in pod- and lens-shaped tabular bodies up to 10-15 feet thick
at depressions and bends along ‘channel and scour' trends and was often associated
with bentonitic clay. Higher grade ore was associated with carbonaceous trash. 28 Uranium-mineralized
fossil logs were mined in some deposits. The deepest open pit mine was 130
feet deep. The average depth for mines in the area ore was about 60 feet. The
largest ore body mined contained about 40,000 tons of ore, and the average
size for the mined deposits was reported to be about 7,000 tons of ore. The ‘channel
and scour' environment accounted for over 95 percent of the ore mined from
the Cameron area.
During 1951-1963 about 289,300 tons of ore mined in the Cameron area averaged
0.21 percent U3O8and 0.04 percent V2O5. 29 The
ore was classified as ‘low vanadium and low lime.' At depth the ore consisted
of uraninite with some sulfide minerals. Ore nearer the surface was oxidized.
In the Cameron area, the deposits generally were of small size, which limited
the depth for economical stripping of the ore. Mines were mainly open pits:
some underground mining was done via adits driven into pit walls to follow
ore trends. The deepest open pit mine was 130 feet deep. The average depth
for mines in the area ore was about 60 feet. Four vertical mine shafts were
dug to reach deposits. The largest ore body mined contained about 40,000 tons
of ore, and the average size for mined deposits was reported to be about 7,000
tons of ore. The small mines depended on the AEC production bonus for profitable
operation. 30 At
the Riverview mine, uranium ore was mined from down-dropped blocks of sandstone
found in a solution-collapse breccia-pipe structure that penetrated the Moenkopi
Formation (Upper Triassic). 31
Tuba City ore buying station and uranium mill - In July 1955 Rare
Metals Corp. signed a contract with the AEC to produce uranium concentrate
at a new mill to be built near Tuba City, Arizona. Construction of the
mill began in August 1955. 32 The
Tuba City mill operated from June 1956-November 1966. 33
In February 1956, the AEC opened a uranium ore buying station at the mill
construction site. 34 Rare
Metals leased the mill's ore sampling plant to the AEC pending the mill's
completion. The AEC purchased ores at the Tuba City site through June 1957,
when Rare Metals began buying the ore. From February 1956 to June 1957, 40,782
tons of uranium ore averaging 0.22 percent U3O8and 0.05
percent V2O5were purchased at the buying station. Rare
Metals purchased the AEC's ore stockpiled at Tuba City as well as ore from
other AEC ore-buying stations. 35
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The original Tuba City mill was designed to process the clayey, low-lime,
high-slime, ores obtained from Cameron area mines. 36 After
its initial startup period, the mill was rated at 300 tons of ore per day.
The milling process for the low-lime ores included ore crushing and grinding,
sulfuric-acid leaching, sand-slime separation, and the basket resin-in-pulp
ion exchange process for uranium recovery. 37
By the early 1960s, many mines in the Cameron area that had shipped low-lime
ores to the Tuba City mill were depleted, and the area’s ore production
was in decline. Shipment of Orphan Lode ore to the Tuba City mill began in
early 1956. The Orphan ore had higher lime content than Cameron-type ores,
and for the mill’s acid-leach process the Orphan ore was blended at the
mill with lower lime ores. Cameron area mine production had fallen to only
a few hundred tons per year by 1962, and Orphan Lode ore was then set to become
the main feed for the mill.38
After an accident closed the Orphan Lode mine in December1961, the Tuba
City mill was forced to temporarily suspend operations. At the Orphan mine,
hoisted ore was stored in an ore bin attached to the head frame, which was
erected over the 2˝-compartment mine shaft. The ore bin collapsed in
late December 1961 and dumped ore back down the mine shaft. The mine was forced
to shut down for several months for repairs. By January 1962, the ore on hand
at the mine at the time of the accident was shipped to the Tuba City mill.
With Orphan Lode ore temporarily not available and declining ore production
in the Cameron area (less than 300 tons of ore in 1962), there was not enough
ore feed to keep the mill supplied. In May 1962 the mill closed.39
In July 1962 Rare Metals Corporation was merged into the El Paso Natural Gas
Company (EPNG). EPNG signed a new contract in November 1962 with the AEC for
the sale of uranium concentrates produced from Orphan Lode ore at the Tuba
City mill through December 1966. 40
The need to convert the milling process at the Tuba City mill to handle the
higher lime Orphan Lode ore was foreseen as early as 1961, and planning was
begun for an alkaline-leach capability at the mill. 41 Addition
of the new circuit was completed during the 1962 mill-closure period. Some ‘used'
equipment obtained for the conversion was purchased under competitive
bidding when the AEC's Monticello, Utah, mill was dismantled. 42 In
April 1963, the Tuba City mill was again operational with a new design capacity
of 200 tons ore per day. 43
The conversion to alkaline-leach included installation of new ore grinding
equipment, a flotation circuit to separate sulfides after grinding, pressure-leach
vessels, liquid-solids separation equipment, precipitation tanks, and re-carbonation
equipment for leach-solution recycle. 44 Sulfide
flotation tailings were sent to the alkaline circuit and the bulk flotation
concentrate was processed in an acid-leach circuit. 45 Uranium
was recovered from the flotation concentrate by ion exchange. 46
From June 1956-September 1966 the Tuba City mill processed a total of 796,489
tons of uranium ore with an average grade of 0.33 percent U3O8. 47 The
mill recovered 4,696,402 pounds U3O8in concentrates,
a 90 percent recovery rate. 48 All
uranium concentrate produced was delivered under contract to the AEC.
Copper products from the Tuba City mill were shipped to the smelter at Inspiration,
Arizona. 49
When the Tuba City mill was closed in September 1966 control of the mill site
reverted to the Navajo Nation. Three connected mill-tailings piles containing
some 800,000 tons of material and three evaporation ponds remained on the mill
site. 50 Earthen dikes protected the tailings510
piles from surface water runoff.
In 1968, under a cooperative program with the U.S. Bureau of Mines, the surface
of the tailings piles was treated with a chemical-binder material to prevent
the wind transport of fine tailings materials onto the surrounding grazing
lands. By 1974, the crust formed on the piles by the binding agent had become
damaged, and it was no longer effective.51
A gamma-radiation survey of the Tuba City mill site and adjacent area was
made in 1972 by an AEC contractor under an interagency agreement with the U.S.
Environmental Protection Agency. Anomalous gamma radioactivity was identified
at 14 sites. Seven of these were found to have uranium tailings materials.
The contamination was attributed to dispersal of tailings material by wind
action rather than by human intervention. Anomalous gamma radiation was found
to decrease to background levels at a distance of 0.25 miles from the piles.
Along the prevailing wind direction, the gamma radiation decreased to background
levels 0.7 miles from the piles. 52
The former Tuba City mill site is situated on a natural terrace about 5,100 feet
above sea level. The natural soil cover is unconsolidated sand and gravel overlying
the Navajo Sandstone (Lower Jurassic). Natural vegetation is sparse in the area.
Southward from the site, the ground surface descends about 400 feet in elevation
over a distance of about 1.5 miles towards the Moenkopi Wash drainage. This stream
provides some water for livestock and agricultural use. Occasional livestock
grazing is the main land use nearby the mill site, and some land nearby is used
for dry and irrigation farming. The supply of surface water available from natural
precipitation in the area is limited and highly variable. The Navajo Sandstone
contains an extensive multiple aquifer system that is an important regional water
resource. The aquifer system is vast and encompasses all of the Navajo Sandstone
units. 53
UMTRA Surface Remediation: DOE
began the surface remediation work at the former Tuba City mill site in 1988
and it was completed by December 1990. Demolition of the remaining buildings
and site preparation began in 1986. All of the residual radioactive materials
at the mill site, the contaminated windblown materials from surrounding properties,
and debris from the demolished buildings were placed in a disposal cell over
the existing uranium mill tailings materials and stabilized by compaction. 54
Disposal Cell: The
disposal cell covers an area of about 50 acres, is generally triangular in
shape, and rises to a height of about 45 feet above the surrounding ground.
About 2.3 million tons of contaminated materials with a total activity of 940
curies of 226 Ra are entombed in the cell. 55 The
cover encapsulating the disposal cell is an engineered, multiple-layer structure.
A low-permeability radon barrier composed of clayey soil was placed directly
on the contaminated, radioactive cell contents. The 42-inch thick soil layer
is designed to minimize the emission of radon to the air and the downward percolation
of water through the cell and potentially into the underlying aquifer. A layer
of granular bedding material overlies the radon barrier and is designed to
promote the rapid runoff from any precipitation that falls on the cell structure.
Two layers of rip-rap material totaling 18 inches in thickness cap the disposal
cell to protect it against erosion from wind and storm water. A rock apron
surrounds the cell to control runoff water that is shed from the encapsulating
cover. On its up-slope sides, the cell is protected by a drainage ditch that
collects surface-runoff water and directs it away from the completed cell structure. 56
Responsibility for
Remediation: U.S
Department of Energy, 100 percent
Stewardship: The
U.S. Department of Energy (DOE) is responsible for long-term stewardship at
the former Tuba City mill site, including the groundwater cleanup program and
the subsequent long-term site monitoring. In 1996, the NRC included the Tuba
City Disposal Cell under a general license. Under this license, the DOE has
long-term custody, monitoring, and maintenance responsibilities at the remediated
site. The DOE maintains data relating to site characterization, design parameters
for site restoration, and historical and current site monitoring. The Long-Term
Surveillance and Maintenance Program at the DOE's Grand Junction Office will
assure the disposal site's safety and integrity.57 The
Navajo Nation retains title to the land and tailings.58
Ground Water Program: Groundwater
contamination in the Navajo Sandstone aquifer at the Tuba City site resulted
from the milling of uranium ores. The water used by the mill to process uranium
ore was discharged with the tailings slurry into the original tailings impoundment
structures at the Tuba City mill site. The water contained high concentrations
of dissolved materials, such as nitrate, sulfate, sodium, calcium, and low
amounts of uranium resulting from the ore-milling process. The original tailings
impoundment structures were not lined with an impermeable material to prevent
the downward percolation of pore water from the pile, and the tailings materials
are directly in contact with the natural soil cover. The soil cover is composed
mainly of permeable eolian sand that formed on the underlying strata of the
Navajo Sandstone.
The contaminated groundwater plume identified by the DOE covers an area
of about 320 acres and extends downgradient to the south and southwest from
the former mill site. It is estimated that some 3.8 million cubic yards of
groundwater are contaminated. 59 Of
18 contaminants identified in the groundwater at the Tuba City mill site,
the concentrations for 13 do not exceed the maximum limits established by
the U.S. Environmental Protection Agency (EPA) and do not pose a risk to
human health or the environment. No remediation for these 13 is planned by
the DOE. The groundwater concentrations for four contaminants (molybdenum,
nitrate, selenium, and uranium) exceed the respective EPA maximum concentration
limits. In addition, the contaminant sulfate is present in the groundwater
plume at levels that can cause a potential health risk. The DOE groundwater
remediation strategy includes active remediation to reduce the concentrations
of these five contaminants in the groundwater plume. 60
The DOE designed program for clean-up of the Navajo Sandstone aquifer
at the former Tuba City mill site includes the extraction of contaminated
groundwater from the plume and removal of the contaminants by distillation.
About 90 percent of the treated groundwater will be returned to the aquifer
via injection wells and infiltration ditches. The extraction of contaminated
water from the aquifer and re-injection of clean water will contain the
further spreading of the five contaminants. A monitoring program will
continue throughout the groundwater cleanup phase to assure that the
aquifer restoration standards established for the project are met. 61
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