The U.S. EPA Report (EPA/ROD/R06-88/044, dated September 30, 1988) indicated that the subject United Nuclear Corporation (UNC) site is located approximately 17 miles northeast of Gallup, New Mexico, in McKinley County. The site operated as a state-licensed uranium mill facility from June 1977 to May 1982. It includes an ore processing mill (about 25 acres) and an unlined tailings pond area (about 100 acres).
The surrounding area is sparsely populated, with the nearest residence located 1.5 miles from the site. In July 1979, approximately 23 million gallons of tailings and pond water were released to a nearby river as a result of a dam breach in the tailings pond area. The site damage was repaired; however, attention was focused on groundwater contamination resulting from tailings seepage. consequently, UNC implemented a groundwater pumping system that withdrew groundwater from the aquifers underlying the site and sent it to an onsite borrow pit for evaporation.
UNC also conducted tailings neutralization from late 1979 to early 1982. Nevertheless, the offsite migration of radionuclides and chemical constituents from uranium milling byproduct materials into the groundwater, as well as to surface water and air, are still principal threats at the site. This remedial action will address onsite ground water contamination. Source control and onsite surface reclamation will be implemented under the direction of the Nuclear Regulatory commission and integrated with this groundwater operable unit.
The primary radioactive contaminants substances of concern affecting the ground water are metals including arsenic, and radioactive substances including radium-226/228 and gross alpha. The selected program remedial action for this site includes: implementation of groundwater monitoring program to detect
any increases in the areal extent or concentration of groundwater contamination outside the tailings disposal area; groundwater pump and treatment using existing and/or new extraction wells to control and remove tailings seepage and remediate groundwater, with discharge to and enhanced mister/pond evaporation system; and implementation of a performance monitoring and evaluation program.
The estimated present worth of cost for the this remedial action is $17,000,000 over a 10-year period, with estimated annual O&M cost of $1,000,000.
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