Connecting the dots between agriculturally derived nutrients and Uranium in surface waters- Disentangling geogenic sources from anthropogenic inputs

The effects of modern agricultural practices on surface water quality are well documented. Justifiably, the nitrogen and phosphorus cycles receive most of the focus in studies examining the elemental loading of surface waters at regional spatial scales, but other elemental cycles are also being greatly altered by large scale fertilizer use. Because phosphate-rich fertilizers are commonly mined from phosphate and carbonate rocks, trace elements that readily substitute into associated mineral phases often occur at elevated concentrations in fertilizers. Rock derived fertilizers therefore have the potential to widely release these elements to agricultural landscapes and receiving waters. Critically, these trace elements are often toxic to both human health and deleterious to the aquatic environment. Uranium in particular, has been shown to accumulate in agricultural soils in long term studies, but evidence of uranium loading in surface waters is limited. We combine two existing databases of uranium concentrations in soils and natural waters (both archived by U.S. Geologic Survey) with new agricultural soil extractions and surface water samples in Ohio to separate contributions from geogenic and fertilizer-derived sources of uranium. Uranium concentrations in surface soils in the glaciated portion of Ohio are elevated compared to average crustal composition (NURE data set), particularly in soils associated with row-crop agriculture. Soil extractions of soils with phosphate fertilizer application show no significant enrichment in U relative to control plots, suggesting U is mobilized rapidly. Our recent surface water sampling campaigns in 2020-21 suggest that the weathering of these soils and/or direct transport of U contaminants in fertilizers leads to higher U concentrations in receiving waters. Our analysis of surface waters shows dissolved levels of U in agriculturally dominated watersheds ranging from ~ 1 to 16 nM, with a mean of 7 nM, exceeding the global mean river value of 0.78 nM. There was a strong association of U with nutrients (NO3, PO4), Ca, and alkalinity in our river waters, particularly with spring runoff, suggesting a fertilizer source.