Superfund Information Systems: Site Profile

Superfund Site:


Cleanup Activities

On this page:

On related pages:


The 125-acre United Nuclear Corp. site is located near Gallup, New Mexico. The site includes a former uranium ore processing mill and tailings disposal area. Two underground uranium mines formerly operated near the site. United Nuclear Corporation (UNC) operated the former Northeast Church Rock mine, located northwest of the site. Quivira (formerly Kerr-McGee) operated a mine north of the site. The uranium mill on site operated from 1977 to 1982. The facility processed uranium ore using a combination of crushing, grinding and acid-leach solvent extraction methods. Milling produced acidic slurry of ground rock and fluid tailings. Disposal of about 3.5 million tons of tailings took place in on-site impoundments. Facility operations contaminated soil and groundwater. Cleanup activities and monitoring are ongoing.







The United Nuclear Corporation National Priority List Site (UNC Site) is located 17 miles northeast of Gallup, on the southern border of the Navajo Indian Reservation in Church Rock, McKinley County, New Mexico. The Site includes a former uranium ore processing mill (25 acres) and tailings disposal area (100 acres). The tailings disposal area is subdivided into three cells identified as the South Cell, Central Cell, and North Cell. The surrounding lands include Indian Tribal Land, Indian Allotment Land and UNC-owned property. There are two operable units within the UNC Site: the ground water operable unit (OU01) and the surface soil operable unit (OU02). The area is sparsely populated, with the nearest residence located 1.5 miles north of the Site. The land use near the Site is primarily grazing for sheep, cattle and horses.

Four water wells are within a 4-mile radius, the nearest being 1.7 miles northeast of the Site; however, nearby residents generally have used bottled water.

Ground Water Operable Unit (OU01)

EPA signed the Record of Decision for the ground water operable unit (OU01) on September 30, 1988. The contaminants of concern in ground water are acidic mill tailings seepage, total dissolved solids, sulfate, thorium, radium, aluminum, ammonia, and iron. The selected remedy included: containment and removal of contaminated ground water in three shallow ground-water zones utilizing existing and additional wells, evaporation of ground water removed from aquifers, and implementation of performance monitoring and evaluation programs. The tailings cells have been capped with an interim radon barrier cover as part of the reclamation activities directed by the Nuclear Regulatory Commission. Two evaporation ponds have been constructed on top of the cells as part of the EPA’s ground-water remedy.


The remedy consists of extraction wells to pump contaminated ground water and evaporation ponds for water disposal. Currently, groundwater migration is not under control. The only ground-water extraction system being operated is for Zone 3. The extraction systems for Zone 1 and the Southwest Alluvium are shut off. The Zone 1 extraction system was shut down in 1999 because it had reached its limit of effectiveness in achieving the cleanup levels established by EPA in its 1988 EPA Record of Decision (ROD). Operational results for Zone 1 demonstrated significant declines in pumping rates over time due to insufficient natural recharge of the aquifers. The loss in saturation reached levels that did not support pumping and the systems were shut down. For the Southwest Alluvium, the extraction system provided partial hydraulic containment to tailing-seepage migration, but there was little progress in achieving some Site cleanup standards over time and the system was temporarily shut-off to perform a natural attenuation test.

The Zone 3 system was shut down in 2000 for the same reasons as Zone 1 and because it was accelerating the movement of the contaminated water rather than containing it. However, over the last few years, UNC conducted other tests to enhance the performance of the extraction system for Zone 3 and stop the migration of the tailing seepage-impacted ground water. Those tests were unsuccessful, but operation of extraction wells at the leading edge of the seepage-impacted front was found to slow its advancement. Hence, UNC has continued to operate those wells and, in 2008, drilled new wells further down-gradient to continue to slow the seepage-impacted ground water, to the extent possible.

Due to the dysfunction of the remedy, in 2006, UNC was required to complete a Site Wide Supplemental Feasibility Study (SWSFS). As an interim measure, while the remedy was under re-evaluation, additional extraction wells were installed in 2010 at the leading edge of the advancing seepage-impacted front to continue to collect contaminated water and slow contaminant migration to the maximum extent practicable. Alkalinity amended Mill water was injected in one of the new wells to neutralize tailings seepage fluid. Three of the seven new Zone 3 Extraction wells were taken off line during this latest five-year period due to fouling. Four extraction wells and one Plume Boundary well were operational in 2012. The remedy enhancements in Zone 3 are meant to buffer, intercept, slow down, direct, and extract impacted ground water. The configuration and pumping scheme of the injection-extraction well arrays tries to minimize the withdrawal of background water and the tendency to draw it westward while maximizing the volume of impacted water that is extracted. Extraction wells in Zone 3 have a life expectancy of one to two years due to fouling and extremely low yield (less than one gallon per minute (gpm)).



Surface Soil Operable Unit (OU02)

During the 2000s, the Navajo Nation requested EPA take the lead for the NECR Cleanup; EPA then oversaw UNC/GE investigations of the Mine Site and performed initial time-critical cleanup actions to remove approximately 200,000 tons of contamination found in the residential area. EPA completed and put out for public comments the Engineering Evaluation/Cost Analysis. EPA decided on a cleanup plan to consolidate the NECR mine waste on the nearby UNC Mill Site and in 2013, published the remedy in a Record of Decision Amendment. Since then, EPA has overseen UNC/GE complete numerous field investigations in preparation of the Remedial Design. The remedial design must be submitted and approved by the Nuclear Regulatory Commision (NRC) as a license amendment. As part of the license amendment review, the NRC will submit for public comment a Safety Evaluation Report and Environmental Impact Statement.

National Priorities Listing (NPL) History
NPL Proposed Date: 12/30/82
NPL Final Date: 9/08/ 


Top of Page

What Has Been Done to Clean Up the Site?

The site is being addressed through federal, state and potentially responsible party (PRP) actions.

The UNC mill site surface reclamation actions by UNC under the direction and oversight of the U.S. Nuclear Regulatory Commission (NRC) at the site between 1988 and 1996 have stabilized the mill tailings and have protected the Rio Puerco from contamination spills like the one that occurred in 1979. The OU0 remedy is primarily directed at removing mine waste from the nearby Northeast Church Rock mine and placing that mine waste on the tailings repository currently existing at the site. The OU01 remedy, while currently protective of human health, is being reassessed in a sitewide supplemental feasibility study for long-term effectiveness.

Potential Risks and Exposure Pathway(s)
There is no current human exposure to ground water at the Site except during the quarterly ground water sampling conducted by UNC personnel. There is limited potential for future exposure to contaminants in ground water below the UNC-owned property because no ground water supply wells drawing from any of the three hydrostratigraphic units will be allowed on UNC property. The same restriction will apply once the NRC Source Materials License is transferred to the DOE for long-term surveillance monitoring. The remedy at OU01 (the final source remedy) currently protects human health and the environment in the short term. Actions taken have minimized potential human exposures to contaminants found in the ground water and reduced the potential for the repository tailings to act as a source of ground water contamination.

The remedy at OU02 is protective of human health and the environment. The remedy described in the 2013 OU2 ROD, which provides for the disposal of NECR mine waste at the UNC Site Tailings Disposal Area is also expected to be protective of human health and the environment upon completion. At present, remedial design activities have been completed which will adequately address all exposure pathways that could result in unacceptable risks in these areas.

Because the remedial actions at both OUs are currently protective of human health and the environment, the Site’s remedy is and remains protective in the short term.

EPA has conducted several five-year reviews of the site’s remedy. These reviews ensure that the remedies put in place protect public health and the environment, and function as intended by site decision documents. The most recent review concluded that response actions at the site are in accordance with the remedy selected by EPA and that the remedy continues to be protective of human health and the environment in the short term. Continued protectiveness of the remedy requires completing cleanup activities that will adequately address all exposure pathways that could result in unacceptable risks.


Top of Page

What Is the Current Site Status?

EPA divided the site into two areas, or operable units (OUs), for cleanup: groundwater (OU01) and surface soil (OU02).


Ground Water Operable Unit (OU01) - Groundwater remedial activities are being conducted by the United Nuclear Corporation (UNC) in accordance with an EPA Unilateral Administrative Order under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA). The groundwater remedial activities were designed to control tailings seepage in three shallow ground-water zones at the UNC Church Rock Superfund site (Site); the alluvium (referred to as the Southwest Alluvium) and Zones 1 and 3 of the Upper Gallup Sandstone Formation. Currently, groundwater is only being pumped from Zone 3 into the evaporation ponds. The Zone 3 groundwater plume is decreasing in areal size and thickness as the groundwater is removed. There is no recharge to the Zone 3 groundwater plume.




The long-term remedy for OU01 includes containment and removal of contaminated groundwater in the Southwest Alluvium and Zones 1 and 3 of the Upper Gallup Sandstone using existing and additional wells, and evaporation of groundwater removed from aquifers using evaporation ponds. It also includes a monitoring program to detect any increases in the spread or concentration of groundwater contamination, as well as a monitoring and evaluation program to determine water levels and contaminant reductions in each aquifer, and the extent and duration of pumping required outside the tailings disposal area. Current pumping and evaporation is restricted to Zone 3.

The long-term remedy for OU2 includes transportation of mine waste from the Northeast Church Rock mine and the receipt, consolidation and disposal of this mine waste at the mill site within the tailings disposal area. A remedial design for the construction of the tailings repository was completed on September 24, 2018. .

Top of Page

Activity and Use Limitations

At this site, activity and use limitations that EPA calls institutional controls are in place. Institutional controls play an important role in site remedies because they reduce exposure to contamination by limiting land or resource use. They also guide human behavior. For instance, zoning restrictions prevent land uses – such as residential uses – that are not consistent with the level of cleanup.

For more background, see Institutional Controls.

The 1988 ROD did not formally establish any ICs; however certain enforcement documents, governmental controls, and informational controls are in place. UAO, Docket No. CERCLA 6-11-89 (issued on June 29, 1989), remains in force and it requires ground water remediation. In addition, the UNC Site Source Materials License No. SUA-1475 remains in effect. It requires that the UNC Site be managed to prevent contaminant exposure, including exposure to those contaminants in the ground water. Restrictions to the use of the on-site ground water will continue after the License is terminated by the NRC and the property is turned over to the DOE for long-term care and surveillance monitoring. However, there are currently no ICs restricting the use of seepage-impacted ground water that has advanced beyond the NRC Licensed Site boundary in Sections 2, 10, 3 and on Navajo Trust land to the north of Section 36. Informational controls such as signs are found near the Tailings Disposal Area. Barbed-wire fence (with “No Trespassing” signs) surround the UNC Site.

No proprietary controls establishing land use restrictions are in place. However, discussions continue regarding their potential utility and effectiveness. It is likely that some form of land and/or ground water use control will become necessary to ensure long-term protectiveness, by preventing exposure to contaminated ground water that has migrated off-Site.

Top of Page

Sampling and Monitoring

UNC has submitted an annual corrective action report at the end of each operating year since 1989. This report includes groundwater quality analyses and groundwater elevations for the first through fourth quarters. Semi-Annual Quality Assurance reports are submitted containing the quality assurance quality control data including the field sampling data sheets, chain of custody forms, laboratory quality control, and data validation. These reports are required pursuant to the Nuclear Regulatory Commission Source Materials License 1475, Condition 30C.

Top of Page