B002-0020
Utilizing Coupled 234U/238U and 87Sr/86Sr to Understand Fate of Uranium and Strontium from Phosphate Fertilizers in an Agricultural Watershed

Monday, 7 December 2020
Poster
Mary Reinthal1, Michael Forgeng1, Lin Ma2, Andrew R Shaughnessy1 and Susan L Brantley1,3, (1)Pennsylvania State University Main Campus, Department of Geosciences, University Park, PA, United States, (2)University of Texas at El Paso, El Paso, TX, United States, (3)Pennsylvania State University Main Campus, Earth and Environmental Systems Institute, University Park, PA, United States
Abstract:
In agricultural areas, phosphate fertilizer acts as a major source of non-point source pollution. These fertilizers may contaminate surface and groundwaters by accumulating heavy metals such as uranium and strontium. Because fertilizers are not the only source of these heavy metals in watersheds, chemical data alone are not able to discriminate between different sources of pollution, and subsequently struggle to indicate corrective action. Coupled isotope ratios 234U/238U and 87Sr/86Sr can help provide a useful and novel approach to identifying and quantifying the origins of heavy metal contamination.

We collected samples in Shaver’s Creek, a mixed land-use watershed that is part of the Susquehanna Shale Hills Critical Zone Observatory (SSHZCO) in central Pennsylvania. Time-series field measurements were initiated this year in Shaver’s Creek watershed, which sits atop Paleozoic sedimentary rocks, including carbonates and shales. Surface and groundwater samples analyzed for trace element concentrations using inductively coupled plasma mass spectrometry range from 0.02 µg L-1 to 0.37 µg L-1 for U and from 13.9 µg L-1 to 1420 µg L-1 for Sr.

We observe a strong positive correlation between U concentration and agricultural land-use in Shaver’s Creek watershed (R2 = 0.54, p < 0.0001) with the lowest concentrations observed in areas without agriculture. On the other hand, we also observe a strong positive correlation between uranium and bicarbonate concentrations (R2 = 0.94, p <0.0001) suggesting watershed lithology may be influencing riverine U concentrations. Preliminary isotope measurements show significant variability in both the 234U/238U of the surface water, which ranges from 1.37 to 2.05, and the 87Sr/86Sr of the surface water, which ranges from 0.7091 to 0.7146. Groundwater from Shale Hills (shale-forested), Garner Run (sandstone-forested), and Cole Farm (shale-agriculture) sub-catchments had 234U/238U values of 1.73, 1.63, and 1.58, respectively. Combining concentration data with a coupled 234U/238U and 87Sr/86Sr isotopic ratio approach could reveal the evolution and fate of fertilizer-derived heavy metals in this complex watershed. Laboratory experiments aimed at determining uranium and strontium isotopic compositions for bedrock, colloids, and sediment samples, are forthcoming.

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