NWC researchers publish work in water quality

New publication considers the connection between uranium and nitrate in groundwater

A Nebraska Water Center project funded by the USGS 104G competitive grant program has been published in the journal Environmental Science and Technology. This manuscript, led by Karrie Weber, Associate Professor in UNL’s School of Biological Sciences and Department of Earth and Atmospheric Sciences and Director of the Microbiology Program, synthesizes all the work from the 104G project.

Uranium is naturally occurring in Nebraska. While the amounts are low enough to make any mining, milling, or other forms of extraction unproductive, they are high enough to be measurable. In stable systems, some of the naturally occurring uranium is consumed by microbes. The microbes form solid uranium minerals and that do not readily leach into the groundwater. The mineral can be dissolved by oxygen or nitrate. Some of dissolved uranium can bind to the surfaces of iron oxide minerals present in the sediments. The naturally occurring levels of uranium generally aren’t a concern to Nebraskans, but interesting interactions can occur when the existing system is changed by human activities.

“We know that in the state of Nebraska we do have buried uranium.” Karrie Weber said. “It’s just high enough in the sediments that as we change things in the subsurface, we’re also changing what and how uranium is behaving in the groundwater.”

The challenges occur as nitrogen is added to the system. The additional nitrogen changes the way the microbes interact with the existing uranium. When more nitrogen is available to the microbes, they’re likely to breathe the nitrate and turn it into nitrite rather than breathing the uranium. This can mean the uranium concentration remains higher because it isn’t being consumed by the microbes. The production of nitrite can further increase groundwater uranium concentrations.

This interaction between microbes, uranium, and nitrogen has been studied by Weber and others. In 2015, Weber’s research group published a correlation between nitrate and uranium concentrations in groundwater in the High Plains Aquifer. The recently published manuscript takes the research a step further.

“The project in this publication is testing the hypotheses generated by the previous study.” Weber shared. “What we wanted to know was in one of these regions where we had observed high uranium concentrations as well as high nitrate, could nitrate be driving the control? Our hypothesis was that in these alluvial aquifer sediments, the uranium gets buried and it’s in a mineral form that is solid that nitrate can react with and change it to the soluble species.”

In Nebraska and beyond, human activities have changed the nitrogen levels in soil. The research hypothesized that increased nitrate contamination in groundwater has the potential to periodically mobilize and release naturally sequestered uranium into the groundwater.

Two sites within the Platte River drainage basin in the High Plains aquifer located in Central Nebraska were used to collect field data for Weber’s research. Weber and the team collected sediment cores to collect data on nitrogen concentration, uranium concentration, and sediment type. Aquifer water level was also measured at each core site.

Weber and the team found that microbes are reducing the nitrate to nitrite, which changes the uranium to the soluble form. Some of this can bind to the iron oxide surfaces and some will increase groundwater uranium concentrations. But this result is specific to shallow alluvial aquifers. Weber’s ongoing research indicates that it is more complicated than it seems. “Nitrate can influence uranium concentrations, but it doesn’t mean that if there is high nitrate there will be high uranium, or if there’s high uranium it’s because of high nitrate.” Karrie Weber shared. “It’s not that simple.”

A systems-based approach is required to understand the connection between microbes, nitrate, and uranium. The researchers are looking to improve our understanding in their future work. The next phase of their research is focused on identifying the tipping point where nitrogen and uranium interact in order to improve management of the alluvial aquifer systems for the long term. By better understanding the interactions through the system as a whole, management of these systems can be improved because the microbes, nitrogen, and uranium are acting in expected ways.

Manuscript abstract

Groundwater uranium (U) concentrations have been measured above the U.S. EPA maximum contaminant level (30 μg/L) in many U.S. aquifers, including in areas not associated with anthropogenic contamination by milling or mining. In addition to carbonate, nitrate has been correlated to uranium groundwater concentrations in two major U.S. aquifers. However, to date, direct evidence that nitrate mobilizes naturally occurring U from aquifer sediments has not been presented. Here, we demonstrate that the influx of high-nitrate porewater through High Plains alluvial aquifer silt sediments bearing naturally occurring U(IV) can stimulate a nitrate-reducing microbial community capable of catalyzing the oxidation and mobilization of U into the porewater. Microbial reduction of nitrate yielded nitrite, a reactive intermediate, which was further demonstrated to abiotically mobilize U from the reduced alluvial aquifer sediments. These results indicate that microbial activity, specifically nitrate reduction to nitrite, is one mechanism driving U mobilization from aquifer sediments in addition to previously described bicarbonate-driven desorption from mineral surfaces, such as Fe(III) oxides.

Read the full manuscript at https://doi.org/10.1021/acs.est.2c07683

New publication on legacy nitrogen, arsenic, and uranium in the vadose zone

A new study from a Nebraska Water Center research team discusses the relationship between legacy nitrogen and inputs on arsenic and uranium in the vadose zone. Titled “Interplay of legacy irrigation and nitrogen fertilizer inputs to spatial variability of arsenic and uranium within the deep vadose zone”, this article was published in Science of the Total Environment and continues the work the research team has been conducting on the vadose zone in Nebraska.

The vadose zone is the interval between the crop root zone and the water table, and is a critically important region for storage, transport, and transformation of chemicals that can impact groundwater quality.

Key findings of the study include:

  • Unsaturated/vadose zone is a dynamic reservoir for nitrogen and trace metals.
  • Vadose soil nitrate and ammonium are found to be influenced by surface irrigation.
  • Amount of water input can be a key driver to vadose zone reactivity.
  • Nitrate in the vadose zone seems to affect soil uranium occurrence.
  • Vadose zone monitoring can be critical to protecting groundwater resources.

The research team includes Arindam Malakar, Chittaranjan Ray, Matteo D’Alessio, Jordan Shields, Craig Adams, Marty Stange, Karrie A. Weber, and Daniel D. Snow.

Read the full study at https://www.sciencedirect.com/science/article/pii/S0048969723039220?dgcid=author.

USDA NIFA-funded vadose zone study targets water quality concerns due to intensive agriculture in Nebraska    

Nitrate pollution in the groundwater resources of Nebraska is a looming challenge. The research team is led by Dr. Arindam Malakar, a research assistant professor at Nebraska Water Center and School of Natural Resources. The team includes experts in the field – Dr. Daniel Snow, a research professor in the School of Natural Resources and Director of the Water Science Laboratory; Dr. Erin Haacker, an assistant professor in the Department of Earth and Atmospheric Science; Dr. Chittaranjan Ray, director of the Nebraska Water Center; USDA-ARS scientists Dr. Dan Miller and Dr.  Tim Green; and graduate student  Yvon Ukwishaka.   

They are conducting a statewide study, funded through the USDA Agriculture and Food Research Initiative (AFRI) program, to investigate nitrogen biogeochemical transformation in the vadose zone—the portion of Earth above groundwater. To understand the anthropogenic effect on various nitrate transformation processes within the vadose zone, the researchers collected soil cores from a pristine native prairie area at Homestead National Historical Park. These soil cores will be used to create columns representative of the vadose zone and divided into three groups simulating different irrigation regimes: Pivot, Gravity irrigation, and Dryland. Columns will be supplied with N-15 labeled fertilizers, and corn will be grown. The chosen prairie area serves as an essential baseline, allowing researchers to analyze changes in vadose zone biogeochemistry when prairies are converted to croplands. The study particularly aims to shed light on the complex interplay of factors influencing nitrogen transformation in the vadose zone, including the impact of fertilizer type and various irrigation practices.