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

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.

Persistence of Uranium in Old and Cold Subpermafrost Groundwater Indicated by Linking 234U-235U-238U, Groundwater Ages, and Hydrogeochemistry

“…Although geochemical abundance data suggest this pool of secondary U(IV) is likely smaller than secondary U(VI), it could nonetheless constitute a source of U release if transitionfacies rocks are exposed to oxidizing conditions owing to the rapid reoxidation kinetics of secondary U(IV) phases. 41 4.2. Sulfide and Carbonate Mineral Weathering Drives U(VI) Mobilization in Groundwater by Promoting CaUC Complexation.…”

“…This fractionation is mostly driven by the nuclear field shift (NFS) effect in which 238 U is preferentially incorporated into the reactant or product phase having lower electron density around its nucleus. Mass-dependent U isotope fractionation (as is found in many lighter multi-isotopic elements) is negligible due to the small relative mass differences between 238 U and 235 U. , Uranium­(VI) reduction to U­(IV) drives the largest known natural U isotope fractionation with an isotopic enrichment factor (ε) of ∼1 ‰ favoring the preferential partitioning of 238 U in U­(IV). ,, Limited U isotope fractionation is expected during oxidative dissolution of U­(IV) to U­(VI) due to surface limitations on U availability at the mineral-fluid interface during dissolution. , Uranyl sorption also produces 238 U/ 235 U isotope fractionation, albeit, of a much smaller magnitude (ε ∼ 0.2 ‰) that preferentially partitions 235 U in the solid phase. Thus, U­(VI) sorption drives minor increases in 238 U/ 235 U of the residual fluid, while U­(VI) reduction leads to comparably large decreases in 238 U/ 235 U in solution. ,,,, …”

“…24,48,49 Limited U isotope fractionation is expected during oxidative dissolution of U(IV) to U(VI) due to surface limitations on U availability at the mineral-fluid interface during dissolution. 41,50 Uranyl sorption also produces 238 U/ 235 U isotope fractionation, albeit, of a much smaller magnitude (ε ∼ 0.2 ‰) that preferentially partitions 235 U in the solid phase. Thus, U(VI) sorption drives minor increases in 238 U/ 235 U of the residual fluid, while U(VI) reduction leads to comparably large decreases in 238 U/ 235 U in solution.…”

“…A number of recent investigations have used variations in 238 U/ 235 U ratio to study U mobility in groundwater. ,− Natural variations of 238 U/ 235 U between geological samples at any given point in time are caused by U isotope fractionation during physicochemical reactions . This fractionation is mostly driven by the nuclear field shift (NFS) effect in which 238 U is preferentially incorporated into the reactant or product phase having lower electron density around its nucleus.…”

Stability of Floodplain Subsurface Microbial Communities Through Seasonal Hydrological and Geochemical Cycles

“…Under reducing conditions, U accumulation is driven by microbially mediated reduction of highly soluble U(VI) to much less soluble U(IV) (Bargar et al, 2013;Lefebvre et al, 2019). This reduction can be performed by sulfate-or metal-reducing bacteria (e.g., Desulfobacter and Desulfovibrio, or Geobacter and Shewanella, respectively), either through direct enzymatic action (Lovely et al, 1991;Lovely and Phillips, 1992;Majumder and Wall, 2017) or indirectly through the formation of reductants such as mackinawite (Boyanov et al, 2011;Bargar et al, 2013).…”