210-3 Using Rust on Irrigation Center Pivots to Predict Concentrations of Redox Sensitive Groundwater Contaminants in an Agricultural Aquifer

Session: Environmental Geochemistry and Health

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Degradation of groundwater quality from anthropogenic and geogenic contaminants is common in agricultural areas. Some common groundwater contaminants, including nitrate, uranium, arsenic, and manganese, are strongly impacted by oxidation-reduction (redox) processes.  However, it is not always clear what areas have high concentrations of these redox sensitive groundwater contaminants from the surface. This is especially important for the estimated 3 million Americans that live in rural areas with no municipal water treatment systems who rely on private wells for drinking water. Local farmers in Central Nebraska have observed that areas with ‘rusted’ irrigation center pivots have lower concentrations of groundwater nitrate than those without ‘rusted’ pivots. The ‘rust’ is a coating of Fe-oxides formed when Fe-rich groundwater sprays onto the surface of the pivot. Thus, the pivot rust could be a qualitative indicator of reducing conditions where Fe is soluble in groundwater. To expand this concept, redox sensitive contaminants (nitrate, arsenic, uranium, iron, and manganese) were sampled from irrigation wells associated with 29 pivots (15 rusted and 14 non-rusted) in Central Nebraska. Samples of the rust were also collected from pivots. Significantly higher (p<0.05) concentrations of iron and manganese and statistically lower concentrations of nitrate and uranium were associated with rusted vs non-rusted pivots, thus demonstrating the link between pivot rust and redox sensitive elements in groundwater. Raman spectroscopy and X-Ray Fluorescence identified the pivot rust as Fe-oxide minerals including hematite, goethite, and magnetite with elevated concentrations of arsenic and uranium. Sequential extraction experiments on pivot rust revealed that the highest concentrations of As are in the recalcitrant metal oxide fraction while up to 40% of the U is in the labile adsorbed and carbonate bound fractions. Batch reactors amended with pivot rust and artificial groundwater medium demonstrated the potential to remobilize As and U in pivot rust back into irrigation water. Overall, this research highlights the utility of pivot rust as a rapid, non-invasive groundwater redox indicator. This work also provides local stakeholders with a scientific explanation of how a simple feature could be used to explain variations in trace metal concentrations in groundwater. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7575

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