Isotopic Fingerprint of Uranium Accumulation and Redox Cycling in Floodplains of the Upper Colorado River Basin

Pierre Lefebvre, Vincent Noël, Kimberly V. Lau, Noah E. Jemison, Karrie L. Weaver, Kenneth H. Williams, John R. Bargar, Kate Maher

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

Abstract

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 (δ 238 U) 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 δ 238 U 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.

Original languageEnglish (US)
Pages (from-to)3399-3409
Number of pages11
JournalEnvironmental Science and Technology
Volume53
Issue number7
DOIs
StatePublished - Apr 2 2019

All Science Journal Classification (ASJC) codes

  • Chemistry(all)
  • Environmental Chemistry

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