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3348. Variations in U concentrations and isotope signatures in two Canadian lakes impacted by U mining: A combination of anthropogenic and biogeochemical processesWei Wang, Duc Huy Dang, Breda Novotnik, Thai T. Phan, R. Douglas Evans, 2019, original scientific article Abstract: Temporal and vertical variations in uranium (U) concentrations and U isotope (δ238U, ‰) signatures were
examined in sediment cores collected seven times over a one year period, from two lakes in Ontario, Canada,
which are contaminated with U by historical mining activities. Bow Lake is holomictic, experiencing seasonal
anoxia, while the sediments of meromictic Bentley Lake are permanently anoxic. Average annual peak concentrations
of U in Bow Lake subsurface sediments were approximately 300 μg L−1 and 600 μg g−1 in porewater
and bulk sediments, respectively. Similar ranges of concentrations (900 μg L−1 and 600 μg g−1, respectively)
were observed in Bentley Lake sediments. The exceedingly high levels of U observed in the porewaters of both
lakes, as well as the seasonal variability in U levels, challenge the traditional paradigm regarding U chemistry,
i.e., that reduced U(IV) should be insoluble under anoxic conditions.
The average annual δ238U ‰ values at the sediment-water interface of both lakes were similar (i.e.,
0.47 ± 0.09‰ and 0.50 ± 0.16‰, relative to IRMM-184). The deep sediments in both Bentley Lake and Bow
Lake record U isotope composition with a typical fractionation of 0.6‰ relative to the surface water, confirming
authigenic U accumulation, i.e., negligible contribution of particulate material from the tailings. Also, the δ238U
values in porewater have an average offset of ca. −0.1‰ relative to bulk sediments in anoxic zones and are
reversed in the oxic sediment layer.
Keywords: Uranium
Isotope composition
Uranium tailings
Biogeochemical cycling
Redox chemistry
Published in RUNG: 09.10.2019; Views: 3083; Downloads: 0
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3349. Uranium bearing dissolved organic matter in the porewaters of uranium contaminated lake sedimentsBreda Novotnik, Wei Chen, R. Douglas Evans, 2018, original scientific article Abstract: Uranium (U) mobility in the environment strongly depends on its oxidation state and the presence of complexing
agents such as inorganic carbon, phosphates, and dissolved organic matter (DOM). Despite the importance of
DOM in U mobility, the exact mechanism is still poorly understood. Therefore, the aim of our investigation was
to characterise sediment porewater DOM in two lakes in Ontario, Canada (Bow and Bentley Lakes) that were
historically contaminated with U and propose possible composition of UO2-bearing DOM. Depth profiles of U
concentrations in porewaters and total sediment digests reveal U levels of up to 1.3 mg L−1 in porewater and up
to 0.8 mg−1 g in sediment. Depth profiles of U did not correlate with Fe, Mn, SO4
2−, or Eh profiles. Therefore,
porewater DOM was analysed and taken into consideration as the primary source of U mobility. Porewater DOM
in each sediment section (1 cm sections, 20 cm core length) was analysed by high-resolution electrospray ionisation
mass spectrometry. PCA analyses of porewater DOM mass spectra showed grouping and clear separation
of DOM in sediment sections with elevated U concentrations in comparison to sections with background U
concentrations. Several criteria were set to characterise UO2-bearing DOM and more than 70 different molecules
were found. The vast majority of these UO2-DOM compounds fell in the category of carboxyl-containing aliphatic
molecules (H/C between 0.85 and 1.2 and O/C≤0.4) and had a mean value of m/z about 720.
Keywords: Uranium
Lake sediments
Porewater
Disolved organic matter
High resolution mass spectrometry
Published in RUNG: 09.10.2019; Views: 3226; Downloads: 0
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