Scientists have found that nitrate, a compound common in fertilizers and animal waste, can help transport naturally occurring uranium from underground to groundwater. The study was published in the journal “Environmental Science & Technology”.
Their new research supports a 2015 study led by Weber showing that aquifers contaminated with high levels of nitrate including the High Plains aquifer under Nebraska also contain uranium concentrations far in excess of the Environmental Protection Agency’s threshold. Uranium concentrations above the EPA threshold have been shown to cause kidney damage in humans, especially when regularly consumed in drinking water.
“Most Nebraskans rely on groundwater for drinking water,” said Weber, associate professor in the School of Biological Sciences and Department of Earth and Atmospheric Sciences. “At Lincoln, we rely on it. Many rural communities rely on groundwater. So when you have high concentrations (of uranium), that becomes a potential problem.”
Non-radioactive uranium
Research has already shown that dissolved inorganic carbon could chemically release traces of natural, non-radioactive uranium from underground sediments, eventually preparing it for transport into groundwater. But a 2015 study that found certain areas of the High Plains aquifer contained uranium levels as high as 89 times the EPA threshold convinced Weber that nitrate was also a contributing factor.
So Weber, with the help of 12 colleagues, set about testing the hypothesis. To do this, the team extracted two cylindrical cores of sediment—each roughly 2 inches wide and extending 60 feet deep—from an aquifer near Alda, Nebraska. Scientists knew that the site not only contained natural traces of uranium, but also allowed groundwater to flow eastward into the adjacent Platte River.
Their goal?
Reproduce this flow in sediment samples and then see if adding nitrate to the water would increase the amount of uranium carried with it.
“One of the things we wanted to make sure was that we didn’t change the state of the uranium, the sediments or the (microbial) community when we took the samples,” Weber said. “We did everything we could to preserve natural conditions.”
“Everything” meant immediately sealing and wax-sealing the extracted cores, placing them in airtight tubes, flushing those tubes with argon to dissipate all oxygen, and putting them on ice. Back in the lab, Weber and her colleagues eventually removed 15-inch segments from each of the two cores. These segments consisted of sand as well as silt that contained relatively high levels of uranium.
Later, the team would fill several columns with this mud before pumping simulated groundwater through them at roughly the same speed as it would travel underground. In some cases, this water contained nothing extra. In others, scientists added nitrates. And in other cases, they added both nitrates and an inhibitor designed to stop the biochemical activity of microorganisms living in the sediment.
Water containing nitrates but lacking a microbial inhibitor was able to remove about 85 percent of the uranium—compared to only 55 percent when the water lacked nitrates and 60 percent when it contained nitrates but also an inhibitor. These results showed that both nitrates and microbes in the further mobilization of uranium.
They also supported the hypothesis that a series of biochemical events triggered by the microbes converted the otherwise solid uranium into a form that could easily dissolve in water. First, sediment-dwelling bacteria donate electrons to nitrate, catalyzing its conversion to a compound called nitrite. This nitrite then oxidizes—steals electrons—from the neighboring uranium, eventually turning it from a solid mineral into a hydrous one, ready to surf on a trickle of water seeping through the mud.
After analyzing the DNA sequences present in the sediment samples, the team identified several microbial species capable of metabolizing nitrate to nitrite. Although uranium-mobilizing biochemistry was known to develop in highly contaminated areas — uranium mines, nuclear waste processing sites — Weber said the new study is the first to show that the same mobilization process also occurs in natural sediment.
“When we first funded this project and thought about it, it was a primary contaminant leading to secondary contamination,” she said of nitrates and uranium. “This research confirms that, yes, it can happen.”
Still, as Weber said, “Nitrate is not always a bad thing.” Both her previous research and some upcoming studies indicate that nitrate mobilizes uranium only when the compound approaches its own EPA threshold of 10 ppm.
“If we think about what we’ve published before, this data suggests that there is a tipping point. The important thing,” she said, “is not to have too much.”
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