The U.S. Atomic Energy Commission (AEC) and the Porter Brothers entered into a contract for the purchase of uranium recovered from chemical processing of columbite-euxenite concentrate from the Lowman mill. The contract period was from March 1955 through June 1960, and the first delivery of U₃O₈ to the AEC occurred in August 1956. A total of 365,231 pounds U₃O₈ were purchased under the AEC contract. The Lowman mill was operated by the Porter Brother Corporation from 1956 to 1960. The milling process consisted of wet and dry circuits with mechanical classifiers, electromagnetic and electrostatic separators, and wet and dry tables to separate the various valued mineral products. The electrostatic/electromagnetic separation process was only partially effective in separating the desired mineral grains, and some of the valued mineral material was lost in the mill tailings. On average, about one-half pound of euxenite mineral grains was recovered per cubic yard of gravel material that was dredged. The DOE estimated that about 200,000 tons of black-sand concentrate were processed at the Lowman mill site for recovery of columbite-euxenite and monazite plus the byproduct minerals magnetite, ilmenite, zircon, and garnet.

Porter Brothers shipped the uranium-bearing mill products to the Mallinckrodt Chemical Works at Hematite, Missouri, for chemical treatment under a toll arrangement. At the Hematite facility, niobium and tantalum pentoxide, U₃O₈, rare earths, titanium, and thorium-iron residues were recovered. All of the U₃O₈ produced by Mallinckrodt was purchased by the AEC under its contract with Porter Brothers, and the niobium pentoxide and tantalum pentoxide were sold to the General Services Administration.

The mill was dismantled soon after it ceased operations. The mill buildings were removed from the site, though several support structures and small buildings remained. In the early 1980s, about 90,000 tons of tailings from the upgrading operations and about 5,000 tons of unprocessed jig concentrate residues remained at the site. An unknown amount of mill residues was removed from the mill site prior to the late 1970s and used as construction fill at several commercial sites and in flood-control sand bags.

UMTRA Surface Remediation:   The processing of uranium and thorium ore materials resulted in contamination of the 37-acre Lowman mill site with sand material containing uranium, radium, and thorium. The contaminated areas had been used for mill buildings, processing equipment, ore storage, and evaporation ponds areas. Radioactive black-sand materials composed of resistate mineral grains with low natural leaching characteristics and process residues were stored in 10 piles that covered about five acres of the mill site. In 1992, the Department of Energy (DOE) consolidated the piles of radioactive materials along with radioactively contaminated soils and materials recovered from 38 vicinity properties onto the on-site tailings pile remaining at the mill site.

Disposal Area:   The Lowman Disposal Cell was constructed in 1992 on the mill site property. The Nuclear Regulatory Commission licensed the cell in October 1994. About 129,400 cubic yards of radioactively contaminated materials, including demolished building foundations and process structures, settling pond residues, and some 28,000 cubic yards of vicinity-property materials were stabilized in the disposal cell. These materials were compacted and covered with an engineered cap system that meets Environmental Protection Agency (EPA) standards for longevity, radon emanation control, and groundwater protection. The built-up cap consists of a 1.5-foot thick layer of compacted clay soil that serves as the main barrier against emission of radon from the cell and infiltration of rainwater into the cell. A 6-inch thick layer of sand and gravel covers the barrier and serves as the bedding layer for the overlying erosion-resistant cover. A 12-inch thick layer of crushed basalt riprap material covering the top and side slopes provides protection from erosion by wind and surface water. A rock apron around the cell directs rainwater away from the cell structure and provides further erosion control. In 1998, DOE constructed a water diversion system north of the disposal cell.

Responsibility for Remediation:   U.S. Department of Energy, 90 percent; State of Idaho, 10 percent.

Stewardship:  The former Lowman mill site and disposal cell are being managed under the DOE’s Long-Term Surveillance and Monitoring (LTSM) Program in accordance with the approved site specific plans. DOE assures the integrity of the disposal cell site through annual inspections and by performing maintenance as necessary. The Lowman site is not fenced, though a locked gate across the entrance road restricts vehicular traffic.

Groundwater Program:   A survey of the groundwater on the Lowman mill site property in 1994 found no contamination of groundwater. A groundwater remediation program is not planned, as the ground water is in compliance with applicable water quality standards. To demonstrate structural integrity of the cell, the DOE has conducted monitoring of the groundwater via the site’s six monitoring wells and one surface-water monitoring station since the disposal cell was completed in 1994. Such monitoring will be continued indefinitely or until the surface water infiltration control for the disposal cell is demonstrated.