Mexican
Hat Mill Site
San Juan County, Utah
Years of Operation |
Status of Mill or Plant Site |
Uranium Ore Processed
(Million Short Tons) |
Production
(Million Pounds U3O8) |
1957-1965 |
Decommissioned |
2.20 |
11.38 |
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) |
235 |
72 |
3.48 |
1,800 |
667 |
54.48 |
Location:
The former Mexican Hat uranium mill site is located on Navajo Nation land
1.5 miles southwest of Mexican Hat, San Juan County, Utah. The site is
northeast of the town of Halchita, Utah, which began as the housing area
for the mill's employees.
Background:
Radioactive mineral deposits in the Triassic rocks of the Colorado
Plateau area of Utah were first worked during the early 1900s. In 1912,
a high grade vanadium-uranium deposit was mined in the Temple Mountain
area of Emery County, Utah. During World War I, high grade uranium ore
was mined for radium from deposits in the Chinle Formation (Moss Back
Member) at Temple Mountain. Over the period 1920-1940, the U.S. Geological
Survey conducted a field reconnaissance and mapping project to evaluate
the oil and gas potential in a broad area of eastern Utah and northeastern
Arizona. In that study, uranium, vanadium, and copper minerals in Triassic
rocks were noted in many places throughout the region, especially near
the unconformable contact between the Moenkopi Formation (Early and Middle(?)
Triassic age) and the Chinle Formation (Late Triassic age).
From the early 1930s through 1945, several radioactive deposits in the
region were mined principally for vanadium. The large vanadium-uranium
deposit (discovered in 1921) in the Monument Valley area of northeastern
Arizona was mined for vanadium in 1942. In about 1944, a few carloads
of copper-uranium ore were mined from the Happy Jack mine in the White
Canyon district and shipped to the Garfield smelter in Salt Lake County,
Utah. From 1949-1953, the Vanadium Corporation of America operated a small
mill near Hite, Utah, to process uranium ore from the White Canyon area
(see the White Canyon Mill page). From the late 1940s through the 1980s,
about 130 uranium mines were developed on the uranium deposits in the
White Canyon-Red Canyon district. Most yielded less than 10,000 tons of
ore, but four deposits contained more than 100,000 tons and one contained
more than 600,000 tons of ore.
The White Canyon-Red Canyon uranium district in southwestern San Juan
County lies along the western flank and crest of the Monument Upwarp,
an uplifted region where sedimentary rocks of Permian to Jurassic age
are exposed in the major stream canyons that formed along the upwarps
axis. In the district, which is about 40 miles long by 20 miles wide,
uranium-bearing strata are present in the Shinarump Member, the basal
Chinle Formation. Most of the minable uranium deposits in the Triassic
rocks occur stratigraphically near the Moenkopi-Chinle unconformity. The
uranium ore deposits are irregularly distributed and occur in Shinarump
sandstone and conglomeratic strata that fill stream channels and scours
that were eroded in Middle Triassic time into the underlying sandstone-siltstone
strata of the Moenkopi Formation. Shinarump channels locally are deep
and some cut through the Moenkopi and into the underlying Permian strata.
Uranium present in the uppermost Moenkopi strata was locally mined where
contiguous ore was present in overlying Chinle rocks. The ore deposits
are generally tabular in shape and elongated with the trend of the hosting
paleochannel strata. In most uranium deposits, the grade of the mineralization
varies strongly over short distances, and adjacent deposits within the
same channel commonly are separated by barren rock. The ore grade in mined
deposits averaged between 0.20 and 0.30 percent U3O8, with some deposits
also containing up to 2 percent V2O5 and other deposits up to 2 percent
copper.
In July 1954, the U.S. Atomic Energy Commission (AEC) announced it would
open a uranium ore-buying station in the White Canyon area of Utah near
the Happy Jack mine. The station was opened in August 1954, and through
July 1957 about 180,000 tons of ore containing some 916,000 pounds of
U3O8 were purchased by the AEC and stockpiled at the buying station.
On July 17, 1956, the AEC contracted with Texas-Zinc Minerals Corporation
for a mill to produce uranium concentrate through March 31, 1963. Texas-Zinc
Minerals selected a mill site located on land belonging to the Navajo
Nation, and the firm acquired a lease for the site and built the Mexican
Hat mill. The mill opened in November 1957 with an initial capacity of
775 tons of ore per day (TPD). In 1958, the mill's throughput capacity
was increased to 1,000 TPD. In August 1958 the AEC contract was modified
to extend the final concentrate delivery term to February 1967. The modified
contract also provided for the sale to Texas-Zinc of the AEC's stockpile
of uranium ore at the White Canyon ore buying station. In June 1963, the
mill and mill-site lease were acquired by the Atlas Corporation. Texas-Zinc's
contract with AEC was terminated, and uranium concentrate remaining to
be delivered to AEC was consolidated into the AEC-Atlas contract under
which Atlas was operating its Moab, Utah, uranium mill. AZ Minerals, the
new Atlas subsidiary, took over and continued to operate the Mexican Hat
mill. From 1953-1965, the mill processed nearly 2.2 million tons of ore
that averaged 0.28 percent U3O8. Mill feed came from independent ore producers
(65 percent), mines controlled by Texas-Zinc and Atlas (25 percent), and
from the purchased AEC stockpile (10 percent)1. The mill ceased operations
in February 1965. Over its operating life, the mill achieved an average
ore processing rate of 850 TPD and an average uranium recovery rate of
more than 93 percent. All uranium production, about 11.4 million pounds
U3O8, was delivered to the AEC. The mill was closed in early 1965. An
on-site sulfuric acid plant that had supplied acid for the milling operation
continued to be operated until 1970. When the mill-site lease expired
in 1970, control of the mill site, including its buildings and tailings
piles, reverted to the Navajo Nation.
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The ore processed at the Mexican Hat mill contained sulfide copper and
other sulfide minerals. Froth flotation was used to recover copper minerals
from the finely ground ore fed to the mill. Flotation concentrates and
tailings were separately acid leached. Leached copper concentrates were
then filtered to obtain the final copper product. The copper-leach filtrate
was combined with the main-circuit-leached tailings slurry (containing
the bulk of uranium in solution). This uranium-pregnant liquor was separated
from the leach solids by countercurrent decantation, and the liquor was
clarified by pressure filtration. Uranium was recovered in a solvent extraction
circuit that used centrifugal contactors. The final uranium oxide product
was precipitated by neutralization of the ammonium nitrate strip solution
with magnesium hydroxide. The uranium precipitate was thickened, washed,
dried, and packed. The mill's copper-concentrate product was shipped to
a smelter for processing.
UMTRA
Surface Remediation: When the Mexican Hat mill ceased operation,
the mill site covered some 550 acres. Two on-site tailings piles covered
about 70 acres and held about 2.2 million tons of material. The tailings
impoundment structures were placed on natural, upper hillside slopes drained
by arroyos that flow northward toward the San Juan River. Earthen dikes
constructed across the toe of each pile to retain the tailings. Maximum
depth of the tailings was 40 feet. When the mill was closed in 1965, the
piles were not stabilized and in the early 1980s wind- and water-eroded
materials were found to contaminate 250 acres adjacent to the mill site.
When Phase I of the DOE remediation work was begun in July 1987, the original
concrete pad for the mill building, several secondary buildings and other
mill structures, and the tailings piles remained at the site. An on-site,
aboveground disposal cell was constructed by moving the "upper"
tailings pile material onto the top of the larger "lower" pile.
The remaining mill structures were demolished, and radioactively contaminated
demolition debris was placed in the cell. In addition, nearly 1.3 million
tons of tailings and contaminated structural debris were transported from
the Monument Valley, Arizona, site and placed in the Mexican Hat Disposal
Cell. (See Monument Valley page.) Fifteen vicinity properties, including
eleven at Mexican Hat and four at Monument Valley, were also cleaned up
during the DOE remediation work. Surface remediation at the Mexican Hat
site was completed in 1995 per the U.S. Nuclear Regulatory Commission
(NRC) approved plan.
Disposal
Area: The final stabilized Mexican Hat Disposal Cell covers
72 acres and contains about 4.4 million tons of radioactively contaminated
material. It rests on sandstone and shale of the Halgaito Formation (Early
Permian age). The cell is capped with a 3.5-foot thick, engineered multilayered
covering that meets U.S. Environmental Protection Agency standards. A
2-foot thick radon/infiltration barrier of compacted silty sand amended
with 10 percent bentonite clay was placed directly on top of the radioactively
contaminated material in the cell. This low-permeability barrier reduces
percolation of rainwater downward into the cell and escape of radon gas
to the atmosphere. A 6-inch thick bedding layer composed of coarse sand
and gravel covers the radon barrier and provides drainage of rainwater
away from the top of the cell. An erosion-protection layer composed of
8 inches of riprap was placed on top of the cell, which has a 2-percent
northward slope. The cell's side slopes and toe aprons are armored with
a 12-inch thick covering of coarse riprap material.
Responsibility
for Remediation: U.S. Department of Energy is responsible for
the long-term custody, monitoring, and maintenance of the Mexican Hat
Disposal Cell site. The site was included under the general license by
the NRC in 1997. Because the land is located on the Navajo Nation, the
Navajo Nation retains title to the land and tailings. The DOE has a permanent
agreement with the Navajo Nation for access to the disposal site.
Stewardship: The
Mexican Hat 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 conduct annual disposal site inspections,
evaluate condition of surface features, monitor groundwater seeps, and
enforce institutional controls at the site. DOE's responsibility for the
site will last indefinately.
Groundwater
Program : Groundwater contamination at the former Mexican Hat
mill site is related mainly to the historical uranium ore processing operations
conducted at the site. The release over time of contaminated pore water
from the saturated tailings pile material during its natural settling
and compaction is a probable secondary source of contaminants. Groundwater
contaminated with nitrate, molybdenum, selenium, uranium, arsenic, chromium,
radium, and net gross alpha radioactivity is present in the perched, ephemeral
groundwater system in the upper beds of the Halgaito Formation. The contamination
affects about 0.55 million cubic yards of groundwater. Active remediation
of the groundwater is not planned. The approved groundwater compliance
strategy includes continuing to monitoring contaminant concentrations
in groundwater discharged from natural seeps downgradient from the disposal
cell structure. Based on intermittent groundwater discharges from the
seeps, the flow rates appear to be decreasing and the flows may eventually
cease over time. Monitoring of flow rates and contaminant levels on groundwater
seeps in the vicinity of the disposal cell will be continued until 2004
to demonstrate the disposal cell's effectiveness in isolating the encapsulated
radioactively contaminated materials from the local groundwater system.
Groundwater in the uppermost aquifer (the lower unit of the Halgaito Formation)
beneath the mill site is not suitable for use because of the presence
of hydrogen sulfide and hydrocarbons.
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