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EPA's Region 6 Office

Serving: Arkansas, Louisiana, New Mexico, Oklahoma, Texas, and 66 Tribal Nations

Contaminant Sources and Impacted Areas

Figure 2

Figure 2 from Draft Regional Groundwater Assessment of Impacts from Historic Releases of the NECR Mine and UNC Mill Facilities Navajo Nation dated September 2011 by EPA and Engineering/Remediation Resources Group, Inc.

Mine Discharge Water into the Unnamed Arroyo and Pipeline Arroyo

At its peak, mine water from NECR and neighboring Quivira Mine was discharged at 5,000 gallons per minute (gpm) to the unnamed arroyo which fed into the Pipeline Arroyo .

Mine discharge water was untreated until January 1975. After that time, mine water was treated in settling ponds followed by ion-exchange to meet NPDES permit requirements of maximum uranium concentration of 2 milligrams per liter (mg/L) and dissolved radium-226 at 30 picocuries per liter (pCi/L). The dissolved radium-226 standard was subsequently lowered in 1977 to 3.3 pCi/L.

The USGS estimates that approximately 140 million cubic meters of mine water discharge (37 billion gallons) and 600 tons of uranium were released into the Pipeline Arroyo/Rio Puerco from the discharges conducted from 1967 through 1985 when the UNC mining operations ceased.

Tailings Liquor into UNC Tailings Disposal Cells

The mill used conventional acid leach, solvent extraction methods to extract uranium. The acid-waste tailings mix was pumped to three disposal cells located adjacent to the Pipeline Arroyo. Acidic waste water seeped into two underlying Gallup sandstone formations and the Alluvium material underneath the Pipeline Arroyo.

A sustained release in the form of seepage from the tailings disposal cells occurred, when UNC discharged an estimated 820 million gallons of acidic mine water and sludge into unlined tailings disposal cells located adjacent to the Pipeline Arroyo. Of the estimated 820 million discharged, an estimated 380 million gallons were lost to evaporation during this period and 94 million gallons were lost in the dam failure, leaving approximately 346 million gallons that seeped into the underlying formations or were retained in the tailings sludge.

Whenever possible during closure of the tailings disposal cells, UNC removed excess liquid and mixed lime into the disposal pits in an attempt to neutralize the remaining acidic material. The Closure Report for the Mine Site stated that the tailings were no longer discharging into the underlying units.

1979 Tailings Dam Breach into the Pipeline Arroyo

In July 1979, the dam on the south disposal cell failed and an estimated 94 million gallons and 18,000 tons of suspended solids were released into the Pipeline Arroyo, and ultimately in the Rio Puerco.

The release increased flows in the Rio Puerco and carried mine tailings as far as 80 miles downstream into the State of Arizona. The release left deposits of tailings sludge along the Pipeline Arroyo which contained radioactive thorium, uranium and other metals. Under oversight of the State of New Mexico (NMEID), UNC conducted a cleanup of tailings containing high levels of thorium-230 along approximately 8 miles on the Pipeline Arroyo and Rio Puerco downstream of the spill. Sediment samples collected after the cleanup indicated that most Thorium-230 levels were below NMEID standards.

A comprehensive human health assessment of the spill was conducted by NMEID, NRC, and US EPA, and included water samples, sediment samples, air monitoring, and human and animal tissue analyses. The study found increased levels of radionuclides, specifically uranium, in animal tissue and bone radioactivity, although the high levels could not be directly associated with the 1979 spill, but may have been associated with the mine water discharges (Centers for Disease Control [CDC], 1980).

Mine Discharge Water into Mine Settling Ponds

There were also ponds on the NECR and Quivira Mine sites as part of the NPDES treatment process. The mine water pumped from the Westwater Canyon Formation was held for settlement prior to treatment and discharge into the Unnamed Arroyo. Theoretically, mine water could seep into the underlying formation that is located at the surface. The formation that outcrops at the NECR Mine is the Dalton sandstone which is a non-producing formation in the area.

Westwater Canyon Formation

The mining operations and subsequent closure may have impacted or altered the Westwater Canyon aquifer in the area of the mine. An oxidation/reduction environment is required for uranium to leach into the groundwater. Dewatering the mine workings and exposing the ore to air may have accelerated oxidation of the uranium ore, and once groundwater was allowed to fill the mine workings when the mine closed operations a larger oxidation/reduction environment may have been created than previously existed. In addition, waste rock from the mining and milling processes was placed in the mines to fill the workings and remove the waste rock from the surface. The waste rock also may have added oxidized and partially processed ore to the subsurface environment also increasing the oxidation/reduction environment in the mine area.

Groundwater from the Westwater Canyon formation is used for drinking water up gradient in the aquifer (south of the mine) and near Crownpoint, New Mexico, approximately 40 miles cross gradient. The Westwater Canyon aquifer is deep in the mine vicinity and wells for assessing water quality are limited to the NECR Mine Well (abandoned in 2004) and the UNC Mill Well.

Surface Soil from Windblown Tailings

Remediated in 1990:
Section 1: An area of Navajo Trust Land approximately five acres in the northwest corner of Section 1 was identified as exceeding the criteria. Some of the trees and all of the shrubs within these areas were removed using a dozer and transported to the south end of Borrow Pit #2 where they were to be burned. Earth-moving equipment was then used to remove at least the upper six inches of soil. The soil was disposed in Borrow Pit #2 of UNC's Churchrock mill tailings facility. In order to ensure that no contamination was pushed beyond the Section 1 border into Section 36, the border area of Section 36 was re-cleaned. The areas cleaned were 6.7 acres in Section 1 and 0.7 acres in Section 36.

Verification sampling was performed and the data has verified that the 6.7 acres in Section 1 and the 0.7 acres in Section 36 are clean and meet the criteria of less than 6.0 pCi/gm of Ra-226 in the soil (5pCi + Background 1.0 pCi).

Remediated in 1989 and 1990:
Section 2 and 36: An area of UNC property was identified as exceeding the criteria. Trees and large shrubs within these areas were removed using a dozer and transported to the east side of the North Cell where they were burned. Earthmoving equipment was then used to remove at least the upper six inches of soil which was placed into the North Cell as fill during recontouring. In areas of steep topography, the soil was pushed with a dozer or excavated with a front-end loader into stockpiles which were then trucked to the North Cell.

The areas cleaned were then resurveyed. Soil from the few areas which required additional excavation as a result of verification surveys was placed into the southwest corner of Barrow Pit #2. The material placed in Barrow Pit #2 was sprayed with a polymer binder to prevent dispersal by wind. Placement into Barrow Pit #2 was required because the North Cell had been filled to design grade at the time these additional soils were excavated.

Please see Verification Reports of the Windblown Tailings Cleanup 06-658224.pdf and 06-6582225.pdf.

NECR Mine Surface Water, Soil and Sludges, and Air

Through investigations beginning in 2006, US EPA has detected radium contamination above safe levels both on the mine site (see Figure 3) and in the surrounding areas on the Navajo Reservation immediately downwind and downstream of the mine site (see Figure 4).

Contamination discovered on the Navajo Reservation and in the residential areas was addressed through a series of short-term cleanup actions in 2007, 2009, and 2012. Contamination on the mine site is being addressed over a longer time period with an anticipated three year design phase and four years of construction. In 2009/2010, US EPA and Navajo EPA directed the UNC/GE to remove contaminated soil that had migrated from the NECR mine onto nearby reservation lands (also called the "step-out area", see Figure 4, Map Area A). This effort was called the Interim Removal Action (IRA).

After the initial IRA cleanup efforts during the winter of 2009/2010, two areas still contained contamination above the EPA cleanup level. One area was at the southeast corner of the step-out area, the second area was adjacent to the toe of the NECR waste pile (Figure 4, Map Area B). Both areas were addressed in late 2010. To address the first area, UNC/GE removed additional soils above the cleanup level. The second area was temporarily covered with clean soil to prevent any exposure. This second area also includes buried contamination from an old diesel spill that happened when the mine was operating (see Figure 4, Map Area C). This second area will be excavated to remove any remaining soil above the cleanup level at the same time a treatment system is installed to clean up the diesel spill.

Quivira Mines Surface Soil

300,000 cy total of radium impacted soil; similar in nature and concentrations to the NECR waste. Waste is currently in a pile onsite with an inadequate soil cover. Waste pile is adjacent to an arroyo and to the RWPR community and can be transported off-site via wind and water transport mechanisms.


Figure 3


Figure 4

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