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Shiprock Mill
Location: The former Shiprock mill site is located on the southwest side of the San Juan River and adjacent to the town of Shiprock in San Juan County, New Mexico. The tract of land on which the former mill was located was leased from the Navajo Nation.1
Background: The Shiprock mill processed mainly uranium-vanadium ore from mines located in the Carrizo Mountains, Lukachukai Mountains, and Sanostee Wash areas in northeastern Apache County, Arizona, and adjacent San Juan County, New Mexico, all on Navajo Indian Reservation land. It processed uranium-vanadium products from the Monument Valley upgrader-plant in northeastern Arizona as well as uranium ore from the Velvet Mine, Lisbon Valley district, Utah, and some Poison Canyon mines, Grants district, New Mexico. The Shiprock mill also processed uranium ore from mining districts in western Colorado, after the Durango, Colorado, mill was closed in 1963.
The host rocks for vanadium-uranium deposits in the Four Corners area of northeastern Arizona and northwestern New Mexico are principally sandstone beds of the Salt Wash Member (Morrison Formation, Upper Jurassic). In surface exposures around the Carrizo Mountains, the Salt Wash host rocks are intruded and bowed upward by the Tertiary age igneous rock that was injected as an irregularly shaped, central mass in the mountains and as sills that extend laterally into the sedimentary sequence around the mountains. About 15 miles south of the Carrizo Mountains, the Lukachukai Mountains form a high ridge that extends northwestwards from the Chuska Mountains. Below the crest of the Lukachukais, folded and truncated Salt Wash host rocks are exposed on finger-like “mesas”carved by erosion into the northeast and southwest mountain slopes. Salt Wash strata in the Lukachukai are unconformably overlain by about 700 feet of Tertiary age eolian sandstone that also forms the flat-topped mountain summit.2
In 1918, vanadium-uranium mineralization was discovered on Salt Wash sandstone outcrops exposed on the slopes of the Carrizo Mountains. In the northwest Carrizos area, 20 tons of uranium-bearing radium ore was shipped in November 1920 from a lease owned by Radium Ores Company.3 From the mid 1920s until the mid 1930s, vanadium-ore mining decreased in the Carrizo Mountains area.4 In the mid to late 1930s, however, vanadium mines were being reopened in southwestern Colorado and southeastern Utah.
By 1941, high demand for domestic vanadium for manufacturing of hardened steel for the war effort led to increased mining of vanadium ore in the Carrizo Mountains area. Under a program to study critical materials for the war effort, in late 1942 the U.S. Geological Survey mapped the vanadium mines, compiled data on vanadium production, and prepared estimates of the vanadium resources. The results of the project were given to the Manhattan Engineer District.5
Between 1942-1945, about 21,300 tons of ore containing about 1.0 million pounds V2O5 was shipped from the Carrizo Mountains area. The ore was processed at the Monticello, Utah, and Durango, Colorado, mills.6 The vanadium ore also contained about 108,600 pounds of U3O8.7 When the Manhattan Project was created in August 1942 under the Army’s Corps of Engineers, acquisition of uranium raw materials for weapons production was a key function. It has been estimated that during 1943-1945 some 76,000 pounds U3O8 was secretly recovered from the Carrizo vanadium-uranium ores processed at the Monticello, Utah, and Durango, Colorado, mills for the Manhattan Project for use in the atomic weapons program.
In 1943, Union Mines Development Corporation, under contract to the Manhattan Engineer District, began a field program to study and map carnotite-bearing prospects and mines in outcrops of the Salt Wash Member on the Colorado Plateau.10 In the fall of 1943, a field party investigated Salt Wash outcrops in the southern Carrizo Mountains, Arizona. By December that year it had moved south onto Mexican Cry Mesa, the northernmost of the Lukachukai “mesas.” By the end of the 1943 field season, only small and widespread uranium-vanadium occurrences had been found on the mesa. Field work was not resumed in the Lukachukais the following year, because it was thought that further south into the mountains the Salt Wash Member had been removed during a period of erosion that preceded deposition of the Tertiary sandstone that caps the mountains. The uranium-bearing Salt Wash outcrops in the Lukachukai uranium-ore belt thus were not discovered until early 1949.11
Vanadium demand declined sharply in 1944.12 From 1945 to 1947 fewer shipments of vanadium-uranium ore were made from mines in the Carrizo Mountains area.
The U.S. Atomic Energy Commission (AEC) was established January 1, 1947, and assigned the functions of uranium studies and uranium procurement for the Federal Government. Under the AEC, several events in 1948 and the early 1950s influenced uranium development in the northeastern Arizona and northwestern New Mexico region. In April 1948, AEC issued the Domestic Uranium Program Circular 3 that provided minimum prices, specifications, and conditions for AEC to purchase different types of uranium ore delivered at the Monticello, Utah, ore-buying station.13 With the sale of mined ore guaranteed, exploration for uranium intensified across the northeastern Arizona and northwestern New Mexico area. Mines controlled by Vanadium Corporation of America (VCA) on the East and West Reservation Leases in the Carrizos were reopened in 1948 and the uranium-vanadium ore was shipped to VCA’s Naturita, Colorado, mill. In 1949, the AEC contracted with VCA to mine a lease on Cove Mesa it had acquired from Union Mines Development Corporation.14
Early in 1949, a prospector working with two Navajo men reported finding uranium minerals on sandstone rim outcrops in the Lukachukai Mountains. Under tribal law, a mining permit could only be issued to a member of the Navajo Tribe, who was then free under Tribal regulations to assign the mining rights. The men applied for mining leases to the Navajo Tribal Council in May 1949. After completing an engineering survey of the discovery areas, leases were issued to the two men by the Department of Interior in December 1949. The leases covered about 1.8 square miles in six parcels located on mesas along the northeast and southwest sides of the Lukachukai Mountains. On the day when the leases were issued, the men assigned a 75 percent interest in their leases to F.A. Sitton, a private firm. Sitton built two dirt-surfaced roads in 1950 from Cove, Arizona, into the high Lukachukais to access the separate lease areas. The first ore shipment from the Lukachukais was in June 1950 to VCA’s mill at Durango, Colorado. Other ore shipments that year went to the AEC uranium ore-buying station at Monticello, Utah.
Between September 1950 and August 1955, the AEC conducted six exploratory drilling projects in the Lukachukai Mountains area. Geological and uranium mineral-occurrence data developed by the drilling projects were placed on an open file by the AEC. The data aided the mining companies in developing additional minable uranium ore reserves.15
In March 1951, Sitton applied to the Navajo Tribal Council to lease a 160-acre tract near Shiprock, New Mexico, for construction of a 100 ton-per-day uranium mill. The lease was approved, But the proposal to build the uranium mill was not approved by the AEC.
Early in 1951, Climax Uranium signed an agreement covering the operation of uranium mines in the Lukachukais for a Navajo lease holder. Climax built an access road up to deposits in the northern part of the uranium ore belt area. The first ore shipment from that area was made in June 1951 to the AEC ore buying station at Monticello, Utah. Other miners also shipped ore out of the Lukachukais using the Climax-built road.
A private effort was begun in summer 1951 to map and sample mineral occurrences in the Salt Wash Member in the ore belt area on the northeastern face of the Lukachukais. The results of the study showed excellent potential for additional ore to be developed in that area. In August 1951, the newly formed Navajo Uranium Company (NUC) of Cortez, Colorado, acquired the F.A. Sitton, Inc., firm and the leases it held by assignment. Data from the new study along with the positive results from the AEC’s exploratory drilling program prompted the NUC to propose a 300 ton-per-day mill at Shiprock, New Mexico. The proposed mill was not approved by the AEC; however, it did approve the building of an ore-buying station at Shiprock. The buying station was opened in January 1952 and was operated by American Smelting and Refining Company under contract to the AEC. The Shiprock station provided a market for all non-VCA controlled uranium ore mined in the northeastern Arizona and northwestern New Mexico areas.
On the Colorado Plateau in the early 1950s, the lack of a system of all-weather roads for transporting ore materials was a major drawback to the Federal uranium procurement program. By 1951, the importance of the Carrizo-Lukachukai deposits to the fledgling Federal uranium procurement effort was recognized. The uranium-exploration areas were at best served by poorly constructed dirt-surfaced roads that could become impassable in wet periods. New mining areas typically had no roads or were served only by primitive jeep trails. Under AEC’s authority to justify construction of roads into uranium mining areas and with the cooperation of the Bureau of Public Roads and various state agencies, the agency began in 1951 a region-wide project to investigate and recommend improvements on the most urgently needed routes. Under this project, the former Indian Service road from US Highway 666 in New Mexico through Red Rock Trading Post, Arizona, to the camp at Cove, Arizona, was improved to an all-weather haulage road. The grading of the old dirt road was improved and gravel was applied. From the Red Rock Trading Post, the former wagon road northward to Oak Springs, which is near the East Reservation leases, was also improved under the AEC program.17
Navajo Uranium Company continued to mine in the Lukachukais into early 1952. It was acquired in May 1952 by Kerr-McGee Oil Industries, Inc.,18 which also acquired the 75 percent interest in the Navajo mining leases. Kerr-McGee also acquired the Shiprock ore-buying station that was leased to AEC by Navajo Uranium. The firm also signed operating agreements with several other Navajos who held individual leases in the Lukachukais. Kerr-McGee opened new mines and soon increased uranium production from its properties. In 1953, Kerr-McGee formed the Navajo Uranium Division that included the Shiprock buying station and its mining operations in the Lukachukai Mountains.
In August 1953, Kerr-McGee Oil Industries, Inc., signed a contract with the AEC for a uranium processing mill to be built at Shiprock near the AEC ore-buying station. Construction began in the fall of 1953 and the mill started up in November 1954. When the mill was started up, the firm also assumed operation of the AEC buying station. Kerr-McGee eventually purchased from the AEC a total of 186,035 tons of ore averaging 0.31 percent U3O8 and 1.02 percent V2O5, including the AEC-purchased ore stockpiled at the Shiprock buying station. The U3O8 recovered from this ore in the Shiprock mill was sold to the AEC under the agency’s uranium concentrate procurement contracts.19
In March 1963, Vanadium Corporation of America (VCA) purchased the Shiprock mill and the Lukachukai mines of Kerr-McGee and continued its operation until August 1967, when VCA was merged into Foote Mineral Company. Foote then operated the mill (then known as the Navajo mill) until it was closed in May 1968.20
Originally designed as a 300 tons-per-day (TPD) throughput-capacity mill, the Shiprock mill’s capacity was later raised to 500 TPD by addition of new equipment and process modifications. The ore was crushed and ground and then leached in a two-stage bath of hot sulfuric acid plus oxidant to solubilize uranium and vanadium. Pregnant solution was recovered from leached solids by countercurrent washing in classifiers and thickeners, then treated by separate solvent extraction circuits to first recover uranium and then vanadium. Precipitation of uranium from the strip solution was accomplished by increasing the acidity and boiling to expel the carbonate, followed by neutralization with magnesia.21 Over the mill’s operating life, a total of about 1.53 million tons22 of ore was processed, and some 7.42 million pounds U3O8 and 20.46 million pounds V2O5 in concentrate were produced. Uranium recovery averaged about 94 percent and vanadium about 58 percent.23 Nearly all (99.98 percent) of the uranium concentrate product was purchased by the AEC. The vanadium concentrate was sold into the commercial market.
Over its history, the mill treated raw uranium ore, dried processed slime concentrates, chemical precipitates, bulk uranium-vanadium precipitates from heap-leach piles, stockpiled vanadium-bearing sand tailings, purchased vanadium liquor, and cleanup materials from the Durango and Naturita mills after they were shut down. Contractor-controlled mines in the Navajo Nation and other widely located areas of the Colorado Plateau region contributed slightly more than one-half of the uranium ore milled at the Shiprock mill. About one-third of the ore was purchased by the mill’s operators from independent mining firms, and the remainder came from contractor-controlled mines and through purchases the uranium ore that had been stockpiled at AEC buying stations.24
When Shiprock milling operations ended in 1968, the mill, offices and other buildings, a raffinate pond area, and about 1.7 million tons of mill tailings contained in two adjacent piles near the San Juan River remained at the 230-acre mill site. The larger pile (46 acres) was not stabilized. The smaller pile (26 acres), stabilized with 0.5 to2.0 feet of pit-run soil and gravel, eventually developed a natural vegetative cover. Earthen dikes were constructed across the toe of the piles to prevent the spreading of tailings toward the San Juan River.
Control of the former mill site reverted to the Navajo Nation in 1973, when the Foote Mineral Company’s lease for the land expired. The Navajo Engineering Construction Authority (NECA) then used about 40 acres of the site, including mill offices and buildings, as a training facility to instruct students to operate and maintain earthmoving heavy equipment. The larger tailings pile was used as a practice ground for earthmoving equipment.25 In January 1975, NECA began decontamination of the former mill site under an Environmental Protection Agency plan in order to reduce the radiological exposure to NECA employees and trainees. Tailings material that had been moved by wind action was loaded and returned to the piles. NECA removed contaminated material from the mill site and ore-storage areas and placed it in the larger tailings pile. That pile was then covered with earthen material taken from the site during the decontamination activities. A broadcast-type irrigation system was installed on the relatively flat top of the larger pile in an attempt to reduce further wind erosion of the tailings material.
Background: The Shiprock mill processed mainly uranium-vanadium ore from mines located in the Carrizo Mountains, Lukachukai Mountains, and Sanostee Wash areas in northeastern Apache County, Arizona, and adjacent San Juan County, New Mexico, all on Navajo Indian Reservation land. It processed uranium-vanadium products from the Monument Valley upgrader-plant in northeastern Arizona as well as uranium ore from the Velvet Mine, Lisbon Valley district, Utah, and some Poison Canyon mines, Grants district, New Mexico. The Shiprock mill also processed uranium ore from mining districts in western Colorado, after the Durango, Colorado, mill was closed in 1963.
The host rocks for vanadium-uranium deposits in the Four Corners area of northeastern Arizona and northwestern New Mexico are principally sandstone beds of the Salt Wash Member (Morrison Formation, Upper Jurassic). In surface exposures around the Carrizo Mountains, the Salt Wash host rocks are intruded and bowed upward by the Tertiary age igneous rock that was injected as an irregularly shaped, central mass in the mountains and as sills that extend laterally into the sedimentary sequence around the mountains. About 15 miles south of the Carrizo Mountains, the Lukachukai Mountains form a high ridge that extends northwestwards from the Chuska Mountains. Below the crest of the Lukachukais, folded and truncated Salt Wash host rocks are exposed on finger-like “mesas”carved by erosion into the northeast and southwest mountain slopes. Salt Wash strata in the Lukachukai are unconformably overlain by about 700 feet of Tertiary age eolian sandstone that also forms the flat-topped mountain summit.2
In 1918, vanadium-uranium mineralization was discovered on Salt Wash sandstone outcrops exposed on the slopes of the Carrizo Mountains. In the northwest Carrizos area, 20 tons of uranium-bearing radium ore was shipped in November 1920 from a lease owned by Radium Ores Company.3 From the mid 1920s until the mid 1930s, vanadium-ore mining decreased in the Carrizo Mountains area.4 In the mid to late 1930s, however, vanadium mines were being reopened in southwestern Colorado and southeastern Utah.
By 1941, high demand for domestic vanadium for manufacturing of hardened steel for the war effort led to increased mining of vanadium ore in the Carrizo Mountains area. Under a program to study critical materials for the war effort, in late 1942 the U.S. Geological Survey mapped the vanadium mines, compiled data on vanadium production, and prepared estimates of the vanadium resources. The results of the project were given to the Manhattan Engineer District.5
Between 1942-1945, about 21,300 tons of ore containing about 1.0 million pounds V2O5 was shipped from the Carrizo Mountains area. The ore was processed at the Monticello, Utah, and Durango, Colorado, mills.6 The vanadium ore also contained about 108,600 pounds of U3O8.7 When the Manhattan Project was created in August 1942 under the Army’s Corps of Engineers, acquisition of uranium raw materials for weapons production was a key function. It has been estimated that during 1943-1945 some 76,000 pounds U3O8 was secretly recovered from the Carrizo vanadium-uranium ores processed at the Monticello, Utah, and Durango, Colorado, mills for the Manhattan Project for use in the atomic weapons program.
In 1943, Union Mines Development Corporation, under contract to the Manhattan Engineer District, began a field program to study and map carnotite-bearing prospects and mines in outcrops of the Salt Wash Member on the Colorado Plateau.10 In the fall of 1943, a field party investigated Salt Wash outcrops in the southern Carrizo Mountains, Arizona. By December that year it had moved south onto Mexican Cry Mesa, the northernmost of the Lukachukai “mesas.” By the end of the 1943 field season, only small and widespread uranium-vanadium occurrences had been found on the mesa. Field work was not resumed in the Lukachukais the following year, because it was thought that further south into the mountains the Salt Wash Member had been removed during a period of erosion that preceded deposition of the Tertiary sandstone that caps the mountains. The uranium-bearing Salt Wash outcrops in the Lukachukai uranium-ore belt thus were not discovered until early 1949.11
Vanadium demand declined sharply in 1944.12 From 1945 to 1947 fewer shipments of vanadium-uranium ore were made from mines in the Carrizo Mountains area.
The U.S. Atomic Energy Commission (AEC) was established January 1, 1947, and assigned the functions of uranium studies and uranium procurement for the Federal Government. Under the AEC, several events in 1948 and the early 1950s influenced uranium development in the northeastern Arizona and northwestern New Mexico region. In April 1948, AEC issued the Domestic Uranium Program Circular 3 that provided minimum prices, specifications, and conditions for AEC to purchase different types of uranium ore delivered at the Monticello, Utah, ore-buying station.13 With the sale of mined ore guaranteed, exploration for uranium intensified across the northeastern Arizona and northwestern New Mexico area. Mines controlled by Vanadium Corporation of America (VCA) on the East and West Reservation Leases in the Carrizos were reopened in 1948 and the uranium-vanadium ore was shipped to VCA’s Naturita, Colorado, mill. In 1949, the AEC contracted with VCA to mine a lease on Cove Mesa it had acquired from Union Mines Development Corporation.14
Early in 1949, a prospector working with two Navajo men reported finding uranium minerals on sandstone rim outcrops in the Lukachukai Mountains. Under tribal law, a mining permit could only be issued to a member of the Navajo Tribe, who was then free under Tribal regulations to assign the mining rights. The men applied for mining leases to the Navajo Tribal Council in May 1949. After completing an engineering survey of the discovery areas, leases were issued to the two men by the Department of Interior in December 1949. The leases covered about 1.8 square miles in six parcels located on mesas along the northeast and southwest sides of the Lukachukai Mountains. On the day when the leases were issued, the men assigned a 75 percent interest in their leases to F.A. Sitton, a private firm. Sitton built two dirt-surfaced roads in 1950 from Cove, Arizona, into the high Lukachukais to access the separate lease areas. The first ore shipment from the Lukachukais was in June 1950 to VCA’s mill at Durango, Colorado. Other ore shipments that year went to the AEC uranium ore-buying station at Monticello, Utah.
Between September 1950 and August 1955, the AEC conducted six exploratory drilling projects in the Lukachukai Mountains area. Geological and uranium mineral-occurrence data developed by the drilling projects were placed on an open file by the AEC. The data aided the mining companies in developing additional minable uranium ore reserves.15
In March 1951, Sitton applied to the Navajo Tribal Council to lease a 160-acre tract near Shiprock, New Mexico, for construction of a 100 ton-per-day uranium mill. The lease was approved, But the proposal to build the uranium mill was not approved by the AEC.
Early in 1951, Climax Uranium signed an agreement covering the operation of uranium mines in the Lukachukais for a Navajo lease holder. Climax built an access road up to deposits in the northern part of the uranium ore belt area. The first ore shipment from that area was made in June 1951 to the AEC ore buying station at Monticello, Utah. Other miners also shipped ore out of the Lukachukais using the Climax-built road.
A private effort was begun in summer 1951 to map and sample mineral occurrences in the Salt Wash Member in the ore belt area on the northeastern face of the Lukachukais. The results of the study showed excellent potential for additional ore to be developed in that area. In August 1951, the newly formed Navajo Uranium Company (NUC) of Cortez, Colorado, acquired the F.A. Sitton, Inc., firm and the leases it held by assignment. Data from the new study along with the positive results from the AEC’s exploratory drilling program prompted the NUC to propose a 300 ton-per-day mill at Shiprock, New Mexico. The proposed mill was not approved by the AEC; however, it did approve the building of an ore-buying station at Shiprock. The buying station was opened in January 1952 and was operated by American Smelting and Refining Company under contract to the AEC. The Shiprock station provided a market for all non-VCA controlled uranium ore mined in the northeastern Arizona and northwestern New Mexico areas.
On the Colorado Plateau in the early 1950s, the lack of a system of all-weather roads for transporting ore materials was a major drawback to the Federal uranium procurement program. By 1951, the importance of the Carrizo-Lukachukai deposits to the fledgling Federal uranium procurement effort was recognized. The uranium-exploration areas were at best served by poorly constructed dirt-surfaced roads that could become impassable in wet periods. New mining areas typically had no roads or were served only by primitive jeep trails. Under AEC’s authority to justify construction of roads into uranium mining areas and with the cooperation of the Bureau of Public Roads and various state agencies, the agency began in 1951 a region-wide project to investigate and recommend improvements on the most urgently needed routes. Under this project, the former Indian Service road from US Highway 666 in New Mexico through Red Rock Trading Post, Arizona, to the camp at Cove, Arizona, was improved to an all-weather haulage road. The grading of the old dirt road was improved and gravel was applied. From the Red Rock Trading Post, the former wagon road northward to Oak Springs, which is near the East Reservation leases, was also improved under the AEC program.17
Navajo Uranium Company continued to mine in the Lukachukais into early 1952. It was acquired in May 1952 by Kerr-McGee Oil Industries, Inc.,18 which also acquired the 75 percent interest in the Navajo mining leases. Kerr-McGee also acquired the Shiprock ore-buying station that was leased to AEC by Navajo Uranium. The firm also signed operating agreements with several other Navajos who held individual leases in the Lukachukais. Kerr-McGee opened new mines and soon increased uranium production from its properties. In 1953, Kerr-McGee formed the Navajo Uranium Division that included the Shiprock buying station and its mining operations in the Lukachukai Mountains.
In August 1953, Kerr-McGee Oil Industries, Inc., signed a contract with the AEC for a uranium processing mill to be built at Shiprock near the AEC ore-buying station. Construction began in the fall of 1953 and the mill started up in November 1954. When the mill was started up, the firm also assumed operation of the AEC buying station. Kerr-McGee eventually purchased from the AEC a total of 186,035 tons of ore averaging 0.31 percent U3O8 and 1.02 percent V2O5, including the AEC-purchased ore stockpiled at the Shiprock buying station. The U3O8 recovered from this ore in the Shiprock mill was sold to the AEC under the agency’s uranium concentrate procurement contracts.19
In March 1963, Vanadium Corporation of America (VCA) purchased the Shiprock mill and the Lukachukai mines of Kerr-McGee and continued its operation until August 1967, when VCA was merged into Foote Mineral Company. Foote then operated the mill (then known as the Navajo mill) until it was closed in May 1968.20
Originally designed as a 300 tons-per-day (TPD) throughput-capacity mill, the Shiprock mill’s capacity was later raised to 500 TPD by addition of new equipment and process modifications. The ore was crushed and ground and then leached in a two-stage bath of hot sulfuric acid plus oxidant to solubilize uranium and vanadium. Pregnant solution was recovered from leached solids by countercurrent washing in classifiers and thickeners, then treated by separate solvent extraction circuits to first recover uranium and then vanadium. Precipitation of uranium from the strip solution was accomplished by increasing the acidity and boiling to expel the carbonate, followed by neutralization with magnesia.21 Over the mill’s operating life, a total of about 1.53 million tons22 of ore was processed, and some 7.42 million pounds U3O8 and 20.46 million pounds V2O5 in concentrate were produced. Uranium recovery averaged about 94 percent and vanadium about 58 percent.23 Nearly all (99.98 percent) of the uranium concentrate product was purchased by the AEC. The vanadium concentrate was sold into the commercial market.
Over its history, the mill treated raw uranium ore, dried processed slime concentrates, chemical precipitates, bulk uranium-vanadium precipitates from heap-leach piles, stockpiled vanadium-bearing sand tailings, purchased vanadium liquor, and cleanup materials from the Durango and Naturita mills after they were shut down. Contractor-controlled mines in the Navajo Nation and other widely located areas of the Colorado Plateau region contributed slightly more than one-half of the uranium ore milled at the Shiprock mill. About one-third of the ore was purchased by the mill’s operators from independent mining firms, and the remainder came from contractor-controlled mines and through purchases the uranium ore that had been stockpiled at AEC buying stations.24
When Shiprock milling operations ended in 1968, the mill, offices and other buildings, a raffinate pond area, and about 1.7 million tons of mill tailings contained in two adjacent piles near the San Juan River remained at the 230-acre mill site. The larger pile (46 acres) was not stabilized. The smaller pile (26 acres), stabilized with 0.5 to2.0 feet of pit-run soil and gravel, eventually developed a natural vegetative cover. Earthen dikes were constructed across the toe of the piles to prevent the spreading of tailings toward the San Juan River.
Control of the former mill site reverted to the Navajo Nation in 1973, when the Foote Mineral Company’s lease for the land expired. The Navajo Engineering Construction Authority (NECA) then used about 40 acres of the site, including mill offices and buildings, as a training facility to instruct students to operate and maintain earthmoving heavy equipment. The larger tailings pile was used as a practice ground for earthmoving equipment.25 In January 1975, NECA began decontamination of the former mill site under an Environmental Protection Agency plan in order to reduce the radiological exposure to NECA employees and trainees. Tailings material that had been moved by wind action was loaded and returned to the piles. NECA removed contaminated material from the mill site and ore-storage areas and placed it in the larger tailings pile. That pile was then covered with earthen material taken from the site during the decontamination activities. A broadcast-type irrigation system was installed on the relatively flat top of the larger pile in an attempt to reduce further wind erosion of the tailings material.
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