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| Titel |
Uranium Bio-accumulation and Cycling as revealed by Uranium Isotopes in Naturally Reduced Sediments from the Upper Colorado River Basin |
| VerfasserIn |
Pierre Lefebvre, Vincent Noël, Noah Jemison, Karrie Weaver, John Bargar, Kate Maher |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250130979
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| Publikation (Nr.) |
 EGU/EGU2016-11320.pdf |
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| Zusammenfassung |
| Uranium (U) groundwater contamination following oxidized U(VI) releases from weathering
of mine tailings is a major concern at numerous sites across the Upper Colorado River Basin
(CRB), USA. Uranium(IV)-bearing solids accumulated within naturally reduced
zones (NRZs) characterized by elevated organic carbon and iron sulfide compounds.
Subsequent re-oxidation of U(IV)solid to U(VI)aqueous then controls the release to
groundwater and surface water, resulting in plume persistence and raising public health
concerns. Thus, understanding the extent of uranium oxidation and reduction within
NRZs is critical for assessing the persistence of the groundwater contamination. In
this study, we measured solid-phase uranium isotope fractionation (δ238∕235U) of
sedimentary core samples from four study sites (Shiprock, NM, Grand Junction, Rifle and
Naturita, CO) using a multi-collector inductively coupled plasma mass spectrometer
(MC-ICP-MS). We observe a strong correlation between U accumulation and the extent of
isotopic fractionation, with Δ238U up to +1.8 ‰ between uranium-enriched and low
concentration zones. The enrichment in the heavy isotopes within the NRZs appears to be
especially important in the vadose zone, which is subject to variations in water
table depth. According to previous studies, this isotopic signature is consistent
with biotic reduction processes associated with metal-reducing bacteria. Positive
correlations between the amount of iron sulfides and the accumulation of reduced
uranium underline the importance of sulfate-reducing conditions for U(IV) retention.
Furthermore, the positive fractionation associated with U reduction observed across all sites
despite some variations in magnitude due to site characteristics, shows a regional
trend across the Colorado River Basin. The maximum extent of 238U enrichment
observed in the NRZ proximal to the water table further suggests that the redox
cycling of uranium, with net release of U(VI) to the groundwater by non-fractionating
oxidation, is occurring within this zone. Thus, release of uranium from the NRZs may
play a critical role in the persistence of groundwater contamination at these sites. |
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