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.