United Nuclear Corporation Superfund Site

The United Nuclear Corporation in McKinley County, New Mexico was placed on the National Priorities List (NPL) by the US Environmental Protection Agency (EPA) on September 1983.  The NPL is the list of national priorities among the known releases or threatened releases of hazardous substances, pollutants, or contaminants throughout the United States and its territories.

The NPL is part of the Superfund program, the common name for the United States environmental policy officially known as the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), enacted by the United States Congress on December 11, 1980 in response to the Love Canal and Times Beach disasters. The Superfund law was created to protect people, families, communities and others from heavily contaminated toxic waste sites that have been abandoned. Many of the contaminants at Superfund sites are also regulated under the Resource Conservation and Recovery Act (RCRA).


The United Nuclear Corporation site is located 17 miles northeast of Gallup, New Mexico and on the southern border of the Navajo Indian Reservation. The surrounding lands include Indian Tribal Land, Indian Allotment Land and UNC-owned property. The land use near the Site is primarily grazing for sheep, cattle and horses.

 EPAUnited Nuclear Corporation site Source: EPA

The United Nuclear Corporation operated the site as a uranium mill facility from 1977 to 1982. The mill, designed to process 4,000 tons of ore per day, extracted uranium using conventional crushing, grinding, and acid-leach solvent extraction methods. The Site includes a former ore processing mill and tailings disposal area, which cover about 25 and 100 acres, respectively. The tailings disposal area is subdivided by dikes into three cells identified as the South Cell, Central Cell, and North Cell.

The dewatering of uranium mines, as well as spills and seepage from tailings disposal area caused contamination of three water-bearing units located beneath the site: the Southwest Alluvium, Zone 1 of the Gallup Sandstone, and Zone 3 of the Gallup Sandstone.Underlying the Upper Gallup Sandstone Formation is the Mancos Shale. The Mancos Shale acts as an aquitard to prevent or retard the downward migration of contamination. The primary sources of tailings seepage were the tailings liquids stored in the North Cell.

In July 1979, the dam on the south cell breached, releasing approximately 93 million gallons of tailings and pond water to the Rio Puerco. The dam was repaired shortly after its failure. Cleanup of the resultant spill was conducted according to criteria imposed by state and federal agencies, including the EPA, at that time.


The milling of uranium ore produced an acidic slurry of ground waste rock and fluid (tailings) that was pumped to the tailings disposal area. Uranium milling and tailings disposal were conducted and an estimated 3.5 million tons of tailings were disposed in the tailings impoundments.

 Tailings.infoAnother view of the United Nuclear Corporation site Source: Tailings.info

The contaminants of primary concern in the groundwater are arsenic, cadmium, cobalt, nickel, radium-226/228, selenium, and gross alpha. Other contaminants that were also identified as  exceeding safe levels were aluminum, manganese, molybdenum, nitrate, and total dissolved solids (TDS). Sulfate was among other compounds listed.

Summary of Remedial Actions

The selected remedy included:

  • Containment and removal of contaminated ground water in the Southwest Alluvium and Zones 1 and 3 of the Upper Gallup Sandstone utilizing existing and additional wells.
  • Evaporation of ground water removed from aquifers using evaporation ponds supplemented with mist or spray systems to enhance the rate of evaporation.
  • Implementation of a monitoring program to detect any increases in the areal extent, or concentration of ground water contamination at, and outside of, the boundary of the tailings disposal area.
  • Implementation of a performance monitoring and evaluation program to determine water levels and contaminant reductions in each aquifer, and the extent and duration of pumping actually required outside the tailings disposal area.

The goal of the selected remedy at the Site was to restore ground water outside the tailings disposal area to federal and state standards, or background, to the maximum extent practicable, and to the extent necessary to adequately protect public health and the environment.

Operational results for the Zone 1 and Zone 3 extraction well systems demonstrated significant declines in pumping rates over time due to insufficient natural recharge of the aquifers. The loss in saturation reached levels which did not support pumping and the systems were shut down. The Zone 3 system was also shut down because it was accelerating the migration of the contaminant plume, rather than containing it. In the case of the Southwest Alluvium, the extraction well system provided partial hydraulic containment to tailing-seepage migration, but there was little progress in achieving Site cleanup levels over time. The Southwest Alluvium extraction well system was temporarily shut off to conduct a natural attenuation test.


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