7-22-15-University of New Mexico scientists and collaborators have documented elevated concentrations of uranium and co-occurring metals in abandoned uranium mine wastes at a site on the Navajo Nation in northeastern Arizona, according to a paper published recently in Environmental Science & Technology, a journal of the American Chemical Society.
Results of the investigation conducted last year at the Claim 28 Mine in the Blue Gap-Tachee Chapter suggest that abandoned mine wastes can be a major source of potential metal exposure to local people and livestock living close to abandoned mine waste sites.
“The Navajo people who live next to the Claim 28 site could potentially be at risk of exposure to uranium and other metal contaminants,” said Johanna M. Blake, a postdoctoral geochemist in the UNM Department of Chemistry and lead author of the article. “The mobilization of uranium from mine wastes to water sources, coupled with the small grain sizes we observed in the wastes in the laboratory, present potentially significant ingestion and inhalation exposures to the people who live nearby.”
The results reported in ES&T have been shared with the Navajo Nation, which has placed the site on two priority lists to accelerate formal assessment and eventual remediation of the site.
The investigation was conducted by a team of scientists from the UNM, Stanford University, and the Southwest Research and Information Center (SRIC), an Albuquerque-based research and education organization, in collaboration with residents of Blue Gap-Tachee Chapter who live near the site. Characterization of the mine wastes and analyses of water samples from local springs were performed at the UNM Department of Earth and Planetary Sciences laboratory. Co-authors of the paper were Sumant Avasarala, Kateryna Artyushkova, Abdul-Mehdi S. Ali, Adrian J. Brearley, Johnnye Lewis, and José M. Cerrato at UNM; Chris Shuey and Paul Robinson with SRIC; Sadie Bill and Christopher Nez of the Tachee Uranium Concerns Committee; Chris Hirani at Central New Mexico Community College; and Juan S. Lezama-Pacheco from Stanford University.
Blake reported that the uranium content of the mine wastes ranged from 2,200 to 6,600 parts per million (milligrams per kilogram), which is on par with uranium-ore grades mined on the Colorado Plateau between the 1940s and 1980s. High concentrations of vanadium, iron and arsenic were also found in the wastes.
Uranium concentrations in water from a seep on the mine site ranged from 163 to 169 micrograms per liter (µg/l, or parts per billion), or more than 5 times the USEPA’s drinking water limit of 30 µg/l, according to the report. Samples collected from a natural spring located 5 kilometers northeast of the mine site had U concentrations ranging from 67 to 135 µg/l, or 2.2 to 4.5 times the limit.
Cerrato, an environmental engineer in the UNM Department of Civil Engineering and principal investigator for the team, said the advanced spectroscopy and microscopy instrumentation available at UNM was essential in identifying the presence of uranyl-vanadate- and arsenic-iron-bearing mineral phases in the Claim 28 mine wastes. He said the work is sponsored by the National Science Foundation (NSF) through the Uranium Group of the New Mexico Experimental Program to Stimulate Competitive Research (EPSCoR) and the Center for Water and the Environment (CREST), which is an NSF Center for Research Excellence in Science and Technology.
“We were astonished to find uranium concentrations in the waste of more than 0.6 percent,” Cerrato said. “We were also surprised to find acidic waters seeping from the mine waste, greatly increasing the potential for mobilization of uranium, vanadium, arsenic and other metals into soils and local drainages.” Laboratory experiments showed that the release of uranium, arsenic, vanadium, and iron from abandoned mine wastes in water was four-fold higher under acidic pH (3.8) conditions compared with more natural alkaline (pH 8.3) conditions, he said.
The Claim 28 site was mined for vanadium and uranium in the 1950s and 1960s. Some reclamation of the site was conducted by the Navajo Nation in the early 1990s, but a waste dump measuring 150 to 200 feet high is spread on a steep slope in Tachee Wash canyon. Several Navajo residences are located at the base of the slope. The research to characterize the waste, which was authorized by the Navajo Nation EPA, came in response to long-standing community concerns.
“When mining began, we were not aware of the related hazards. The community has had concerns about negative health effects over the years,” said Christopher Nez, who was raised less than a mile from the mine site and who assisted researchers in collecting waste and water samples. “Now we know what’s in the rocks, and that it’s hazardous. Now there are some facts and studies to back us up. The community people are very impressed with the scientific data.”
Chris Shuey, SRIC’s environmental health specialist who coordinated the field work, said the team worked closely with the community to ensure that residents participated in the field studies and that the research plan was informed by local knowledge and indigenous perspectives on how the natural world works.
“Without community knowledge, we would not have known about the seep from the mine waste,” Shuey said. “And as a result of these findings, the community has successfully advocated for placement of the Claim 28 site on the Navajo Nation’s priority lists for further assessments and remediation.”
Cerrato said the UNM-SRIC-TUCC team has presented the results of the field and laboratory studies to the community on several occasions in the past year. Blue Gap-Tachee Chapter has adopted resolutions supporting the research and pledging to continue participating in it. Additional soil, waste and water samples collected by the team are being analyzed now at UNM, and with the input and participation of local residents, studies are being proposed to collect dust samples for chemical-mineralogical analyses and for toxicological and inhalation research at the UNM College of Pharmacy.
Note: The article can be accessed at DOI: 10.1021/acs.est.5b01408.