Nitrates in fertilizers may drive uranium mobility into groundwater, a discovery from University of Nebraska–Lincoln researchers
Researchers have long understood that soluble carbon-bearing species in water can interact with uranium in Earth’s crust and help it move through soil and rock. A new study from the University of Nebraska–Lincoln adds another player to this picture: nitrates in fertilizers can also assist uranium in traveling toward groundwater. This work fits into a broader history of how subsurface chemistry governs the fate of uranium in aquifers and surfaces new questions about nutrient management and groundwater protection.
To investigate the mechanism, the team collected two cylindrical rock cores from an aquifer near Alda, Nebraska. Each core is about 5 centimeters in diameter and extends roughly 20 feet in depth. The site features soils with elevated uranium and a groundwater system that naturally flows eastward toward the Platte River. The researchers aimed to recreate natural groundwater movement within the sediment and then test whether the introduction of nitrate would increase the transport of uranium by the moving water.
In the laboratory experiments, a close analogue of groundwater was pumped through the cores at a speed that mirrors subsurface flow. Some runs included an inhibitor to slow the biochemical activity of microorganisms living in the sediment, while other runs introduced nitrates without the inhibitor. The results showed a notable difference: water containing nitrates but without microbial inhibitors carried away about 85 percent of the uranium, compared with roughly 55 percent removal when nitrates were absent, and about 60 percent when nitrates were present with the microbial inhibitor. The trend suggests that nitrates can act as a pathway for uranium to enter groundwater, while microbial activity plays a significant role in transforming uranium into soluble forms that migrate with the water.
Overall, the results support a model in which nitrates help uranium move by creating conditions that favor its dissolution and mobilization in groundwater. The effect appears linked to microbial processes that drive chemical changes in the sediment environment, influencing how uranium binds to minerals or is released from them. As groundwater progresses through nitrate-rich zones, a combination of chemical reactions and microbial activity can shift uranium from a relatively stable state in the crust to a soluble form that travels with the water toward drinking-water sources and natural conduits like rivers. The findings help explain regional observations of uranium variability in groundwater and highlight the importance of managing nutrients to protect aquifers. The study underscores how agricultural practices, soil chemistry, microbial life, and groundwater quality are interconnected, and points to the need for ongoing monitoring of nitrate levels in agricultural regions that rely on groundwater for drinking water and irrigation. Attribution: University of Nebraska–Lincoln