Utilizing 234U/238U and 87Sr/86Sr to understand the provenance and fate of uranium in Shaver’s Creek, Pennsylvania
Open Access
- Author:
- Reinthal, Mary
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- November 20, 2020
- Committee Members:
- Susan Louise Brantley, Thesis Advisor/Co-Advisor
Christopher Howard House, Committee Member
Timothy Bralower, Committee Member
Lin Ma, Special Signatory
Mark E Patzkowsky, Program Head/Chair - Keywords:
- radiogenic isotopes
uranium
fertilizer
agronomy
CZO
critical zone
Shale Hills
Cole Farm
hydrogeology - Abstract:
- In agricultural areas, fertilizer acts as a significant source of nitrate and phosphate pollution into waterways. In addition, these fertilizers may contaminate surface and groundwaters with heavy metals such as uranium. Because fertilizers are not the only source of this uranium in watersheds, chemical concentrations alone cannot discriminate between different sources of pollution. We hypothesized that using the isotope activities ratio 234U/238U, along with another isotopic tracer, 87Sr/86Sr, would provide a useful and novel approach to identifying and quantifying the origins of heavy metal contamination from fertilizers. 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. Shaver’s Creek watershed sits atop Paleozoic sedimentary rocks, including limestones, dolostones, sandstones, and shales. Surface and groundwater samples were collected and analyzed for trace element concentrations using inductively coupled plasma mass spectrometry. We observed concentrations ranging 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 also observed a strong positive correlation between U concentration and the fraction of agricultural land use per sub-catchment drainage area 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 observed a strong positive correlation between uranium and bicarbonate concentrations (R2 = 0.94, p < 0.0001). Given that the agricultural land use fraction in the watershed sub-catchments varies with the carbonate content of the underlying rock type, lithology could be a hidden variable that influences riverine U concentrations. Isotope measurements in surface water samples show significant variability in both the 234U/238U activity ratio (AR), ranging from 1.37 to 2.05, and 87Sr/86Sr, which ranges from 0.7091 to 0.7146. Groundwater from three sub-catchments, Shale Hills (a forested sub-catchment on Rose Hill Formation shale), Garner Run (a forested sub-catchment on Tuscarora Formation sandstone), and Cole Farm (a sub-catchment and working farm on Wills Creek Formation calcareous shale and Bloomsburg and Mifflintown Formations interbedded shale and limestone bedrock) were characterized by (234U/238U) of 1.73, 1.63, and 1.58 respectively. Observations of uranium and strontium contents in rock and water samples from sub-catchments of Shaver’s Creek show that the Rose Hill shale is the largest source of uranium. Uranium activity ratios document that the rocks are close to secular equilibrium; in contrast, Sr isotopes vary from non-radiogenic in the Tuscarora to highly radiogenic in the Rose Hill shale. Combining concentration data with isotopic ratios reveals that fertilizer use in the overall watershed has led to uranium concentrations above background in groundwater and surface water. While analyses show no waters above the U.S. Environmental Protection Agency’s Maximum Contamination Level of 30 µg L-1, the isotopic approach documents that surface application of fertilizers has increased the overall uranium concentration in surface and groundwaters.