Isotopic Fingerprint Of Uranium Accumulation And Redox Cycling In Floodplains Of The Upper Colorado River Basin
ENVIRONMENTAL SCIENCE & TECHNOLOGY(2019)
摘要
Uranium (U) groundwater contamination is a major concern at numerous former mining and milling sites across the Upper Colorado River Basin (UCRB), USA, where U(IV)-bearing solids have accumulated within naturally reduced zones (NRZs). Understanding the processes governing U reduction and oxidation within NRZs is critical for assessing the persistence of U in groundwater. To evaluate the redox cycling of uranium, we measured the U concentrations and isotopic compositions (delta U-238) of sediments and pore waters from four study sites across the UCRB that span a gradient in sediment texture and composition. We observe that U accumulation occurs primarily within fine-grained (low-permeability) NRZs that show active redox variations. Low permeability NRZs display high accumulation and low export of U, with internal redox cycling of U. In contrast, within high permeability NRZs, U is remobilized under oxidative conditions, possibly without any fractionation, and transported outside the NRZs. The low delta U-238 of sediments outside of defined NRZs suggests that these reduced zones act as additional U sources. Collectively, our results indicate that fine-grained NRZs have a greater potential to retain uranium, whereas NRZs with higher permeability may constitute a more-persistent but dilute U source.
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关键词
uranium accumulation,upper colorado river basin,floodplains
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