Portions of the Navajo Nation are on geologic formations rich in radioactive uranium ores. Beginning in the 1940s, widespread mining and milling of uranium ore for national defense and energy purposes on Navajo tribal lands led to a legacy of abandoned uranium mines. The Skyline AUM (the Site) is one of approximately 520 AUMs located on the Navajo Nation. Nearby residents have expressed concern to the Navajo Nation Environmental Protection Agency (NN EPA) regarding the potential for wind-blown and water-borne radioactive particles to migrate from the Site and impact their health as well as the health of their livestock and the environment. During a site inspection performed by the NN EPA in July 2005, gamma radiation activity greater than two times the background level were detected in approximately 80 locations at the Site. NN EPA requested assistance from the EPA in performing an investigation to determine the nature and extent of the contamination and to mitigate any potential impacts to human health and/or the environment.
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The Site encompasses the former Skyline Mine on top of Oljato Mesa (the Mesa), mine tailings on the eastern edge of the Mesa directly below the mine, mine tailings in the Skyline AUM Waste Piles (SMWP), and associated residential and pastoral areas east and down gradient from the foot of the Mesa. The SMWP are located approximately 700 feet below the former mine at the eastern base of the Mesa and are characterized by talus slopes covered with waste rock and/or waste ore that was pushed over or fell from the top of the Mesa where the mine was located. Portions of the cliff directly below the mine are visibly stained a grey-green color from this activity. As part of regular mine operations, a gondola was used to transport ore down to the foot of the Mesa where it was loaded into trucks for transport to an offsite mill. Portions of the gondola, including steel cables, remain at the foot of the Mesa.
Approximately five homesites with residential structures are located between 1,490 and 1,800 feet east and northeast of the SMWP. Land use in the project area is characterized by low density of single-family residences surrounded by open grazing land. The top of the Mesa and the SMWP area are used seasonally during the spring, summer, and fall by local residents as pastureland for grazing sheep and cattle and for gathering traditional herbs and plants.
During the late 1990’s, portions of the Skyline AUM on top of the Mesa were closed by the Navajo Nation Abandoned Mine Land (NN AML) program. This work focused on removing immediate physical hazards including sealing the mine portals, consolidating loose accessible mine waste, and capping it with uncontaminated fill material. Due to the steep terrain, inaccessible mine tailings at the eastern edge of the Mesa directly below the Mine were not removed during NN AML activities.
In November 2008, EPA conducted radiation assessments at the Site in coordination with NN EPA. A background area was established 1.35 miles east-southeast of the SMWP in an area with no suspected impacts from mining. First, 1-second total gamma radiation activity measurements using a GPS integrated ratemeter were collected in the background and SMWP areas. Two areas were identified as having elevated gamma activity, and designated the talus slope decision area (TSDA) and the trans-loading decision area (TLDA). Next, sampling grids were established that encompassed the TSDA, TLDA, and the background area respectively. From July 2009 to March 2010, additional soil sampling and total gamma activity surveys were conducted to further delineate the lateral and vertical extent of contamination in the TLDA, the TSDA, and the unnamed arroyo that receives runoff from the Skyline AUM and the north side of the TSDA.
Current Site conditions pose ongoing releases and the threat of future releases of hazardous substances, namely: uranium and its progeny (i.e., radium-226 and radon) and ionizing gamma and alpha radiation associated with those progeny. The likelihood of direct human exposure, via ingestion and/or inhalation of hazardous substances, and the threat of future releases and migration of those substances, pose an imminent and substantial endangerment to the public health or welfare or the environment.
No State or Tribal actions have taken place at the Site to address the SMWP or the Arroyo. Formal consultations with the Navajo Nation for a broad range of AUM-related issues have been ongoing for several years, and formal consultations for this removal action began in late 2009. In September 2010, the Navajo Nation Department of Justice requested in writing that EPA conduct a federal removal action at this Site. NN EPA has been and will continue to be closely involved in the planning and execution of this removal action.