Uranium groundwater contamination at old uranium-ore processing sites is persisting despite remediation of contaminated surface materials two decades ago. The contamination is persisting at levels high enough to pose significant risks to human health — as per recent studies.
It had previously been assumed that, over these last ~20 years, the remaining uranium at the site (in the ground) would “flush out” via natural groundwater flow across the sites, but this hasn’t turned out to be true.
“Uranium dissolved in groundwater flows slowly into nearby rivers, where it becomes diluted below the uranium concentrations naturally present in river water,” stated John Bargar, SLAC’s project lead and researcher at the lab’s Stanford Synchrotron Radiation Lightsource (SSRL), a DOE Office of Science User Facility. “However, studies have shown that the groundwater contamination is unexpectedly long-lived.”
(Author’s note: Well, unexpectedly to some perhaps, not to everyone.)
As part of new work being done by the Department of Energy’s SLAC National Accelerator Laboratory, though, researchers are looking to now uncover the mechanisms behind the longer-than-expected contamination — in cooperation with the DOE Office of Legacy Management.
“Our collaboration is motivated by the need to better understand the geochemical and biological factors influencing uranium mobility and transport,” stated William Dam, a hydrologist and site manager at the Office of Legacy Management. “We want to understand the way uranium gets from the ground into the groundwater, creating a plume of contamination in which uranium concentrations stay above regulatory safety requirements.”
A bit of background via the SLAC National Accelerator Laboratory:
The contaminated sites, on floodplains in the upper Colorado River basin, operated from the 1940s to the 1970s to produce “yellowcake,” a precursor of uranium fuel used in nuclear power plants and weapons.
Previous field research by Bargar’s team and collaborators at Lawrence Berkeley National Laboratory (LBNL) at the site of a former uranium mill in Rifle, Colorado, has provided a possible explanation for the longevity of the uranium contamination. It revealed that up to 95% of the subsurface uranium is concentrated in zones of organic-rich sediment — the buried remains of plants and other organisms along former Colorado River stream banks — generally located 10 to 30 feet underground. These organic substances appear to store large amounts of uranium, restricting its mobility and releasing it very slowly into the surrounding water over many years. Current estimates predict that the contamination will not flush away for at least another 100 years at several sites.
“Our model for Rifle predicts that organic-rich zones may generally influence uranium mobility throughout the upper Colorado River basin and therefore could also play an important role at other sites,” stated Bargar.
The new work will include 5 other sites in Colorado, Wyoming, and New Mexico — with field work having begun last fall, and continuing again this spring.
Commenting on the partnership with the Office of Legacy Management, Bargar noted: “Access to those sites is regulated, and some of them are in very remote locations. Our partners from the Office of Legacy Management, as well as LBNL, provide us with site access and logistical support. They also carry out the drilling operations required to take sediment and water samples.”
These samples are then sent to SSRL, where the researchers are analyzing them with a variety of X-ray techniques. Via this approach, the researchers will be able to ascertain the specific chemical form of uranium in various samples taken from various depths.
The study can reveal how the presence of particular chemical forms in organic-rich zones affects overall uranium mobility at the contaminated sites. The study will also investigate the types of organic carbon present in the ground to help understand how it influences uranium behavior. The researchers will combine the X-ray data with studies of how bacteria affect uranium chemistry.
“We know that microbes strongly influence the chemical form of uranium and, hence, its mobility,” Bargar noted. “By collecting information about microbial populations present in the sediments, we hope to gain information about how and when bacteria do that, and how bacteria couple subsurface carbon chemistry to uranium behavior.”
This research could possibly lead to better remediation practices — something potentially of great use considering the large number of contaminated sites around the world.
That’s what always comes into my mind when listening to the proponents of nuclear energy as they go over their talking points — if the relatively limited deployment of nuclear energy seen over the last century resulted in so much contamination, then can you imagine what a full-scale buildout would look like?
Image Credit: SLAC National Accelerator Laboratory; John Bargar
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