Uranium delivery and uptake in a montane wetland, north-central Colorado, USA

Abstract

Comprehensive sampling of peat, underlying lakebed sediments, and coexisting waters of a naturally uraniferous montane wetland are combined with hydrologic measurements to define the important controls on uranium (U) supply and uptake. The major source of U to the wetland is groundwater flowing through locally fractured and faulted granite gneiss of Proterozoic age. Dissolved U concentrations in four springs and one seep ranged from 20 to 83 ppb (µg/l). Maximum U concentrations are ~300 ppm (mg/kg) in lakebed sediments and >3000 ppm in peat. Uranium in lakebed sediments is primarily stratabound in the more organic-rich layers, but samples of similar organic content display variable U concentrations. Post-depositional modifications include variable additions of U delivered by groundwater. Uranium distribution in peat is heterogeneous and primarily controlled by proximity to groundwater-fed springs and seeps that act as local point sources of U, and by proximity to groundwater directed along the peat/lakebeds contact. Uranium is initially sorbed on various organic components of peat as oxidized U(VI) present in groundwater. Selective extractions indicate that the majority of sorbed U remains as the oxidized species despite reducing conditions that should favor formation of U(IV). Possible explanations are kinetic hindrances related to strong complex formation between uranyl and humic substances, inhibition of anaerobic bacterial activity by low supply of dissolved iron and sulfate, and by cold temperatures.

Redox Fluctuations and Organic Complexation Govern Uranium Redistribution from U(IV)-Phosphate Minerals in a Mining-Polluted Wetland Soil, Brittany, France

“…Natural and artificial wetlands such as peats and flooded soils have been recognized for their ability to accumulate U since they harbor reducing zones. − However, long-term scavenging of U in such environments may depend on seasonal hydrological fluctuations and organic matter content since these factors play a major role in the control of redox conditions and U mobility. − Furthermore, investigations of U speciation in peat using chemical extractions have shown significant fraction of sorbed or easily extractable U species, such as 0.1 M bicarbonate extractable U, organically bound (0.1 M Na 4 P 2 O 7 ) and dilute-acid extractable U. , Only a few studies have directly addressed the speciation of U in such environments using X-ray absorption spectroscopy (XAS) that allows one to directly determine U oxidation state and molecular level speciation in complex environmental samples. Several of these studies reported U­(VI) as being dominant in wetlands and organic-rich soils, − with U­(VI) bound to carboxylic moieties, to additional organophosphates or silicates moieties, or associated with U­(VI)-phosphate minerals .…”

Geochemical Controls on the Uranium Cycle in a Lake Watershed

“…The dry bulk density could not be measured and was considered equivalent to that of the lake sediments, i.e., ∼0.2 g/cm 3 . Again, the soil U content is highly variable, spanning a range of 100–5200 μg/g depending on the distance to U sources . We chose an order of magnitude of 1500 μg/g (median of 38 soil measurements from this study and ref ), resulting in a roughly estimated U stock in the wetland soils of ∼600 kg.…”

“…The heterogeneous distribution of U concentrations in soils and sediments across the watershed is in line with the location of U source rocks (Figure 3). 31 The highest U concentrations�up to >1000 μg/ g in surface sediments and even >5000 μg/g in soil cores of ZH1 19 �are found mainly on the eastern side of the watershed, downstream of the U-bearing rocks and on the path of the eastern stream. This is consistent with higher dissolved U in the eastern stream than in the western one, as recorded in our previous study.…”

“…19 The most likely explanation to this observation is that spatial and temporal variability in the chemical erosion rate of the bedrock can induce variations in the ( 234 U/ 238 U) signature of dissolved U. 30,35,36 Hence, the ( 234 U/ 238 U) signatures in the wetland soils and in the stream waters would record the variability of the U sources ( 234 U/ 238 U) both in space (e.g., potential underground water flow feeding different parts of the wetland in addition to creeks 31 ) and in time (e.g., seasonal variations of weathering and erosion intensity, long-term changes in the creek paths). 37 In this regard, because they integrate most of U fluxes downstream of the watershed and because of their relative homogeneity, the lake water column and lake sediments would display the average ( 234 U/ 238 U) signature of the watershed.…”

Speciation of Uranium and Plutonium From Nuclear Legacy Sites to the Environment: A Mini Review

“…The interaction between uranium and NOM is especially important in wetlands. There are many examples of wetlands in which this interaction governs the partitioning (Kaplan et al, 2016 , 2017 ; Koster van Groos et al, 2016 ; Mikutta et al, 2016 ; Schumann et al, 2017 ; Dublet et al, 2019 ; Stetten et al, 2020 ). One of the main characteristics of these places is the significant seasonal change.…”