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Radionuclide transport in the boreal landscape: Uranium, thorium and other metals in forests, wetlands and streams
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The boreal landscape is complex mosaic of vast forests, lakes and wetlands. Through the landscape flows a fine network of streams and rivers, carrying dissolved and suspended material from atmospheric deposition and weathering of soils and bedrock to downstream recipients. This thesis investigates the transport of U, Th and other metals in the boreal landscape by comparing a set of catchments with contrasting characteristics, ranging from 0.12-68 km2 in area. Using uranium (234U/238U) and oxygen isotopes (δ18O) it was demonstrated that catchment size has a strong impact on the hydrological pathways and on the mobilisation of uranium. Both tracers also displayed a consistent shift towards more superficial sources and more superficial flow pathways when going from winter baseflow conditions to the spring flood. Large spatiotemporal variability was observed with U fluxes ranging from 1.7 -30 g km-2 a-1. Using a wide set of hydrochemical parameters and landscape characteristics it was demonstrated that wetlands play a decisive role for the biogeochemical cycling of many metals. Comparing normalised fluxes of 13 different elements (Al, Ba, Ca, Cr, Cu, La, Mg, Na, Ni, Si, Sr, U and Y) 73% of the spatial variance could be explained based on the wetland coverage and the affinity for organic matter, the latter of which was quantified using thermodynamic modelling. Hence, it was possible to link the large-scale transport patterns of a wide range of metals to fundamental biogeochemical properties. When restraining the analysis to the smaller streams (<10 km2), the explanatory power increased to 88%. For elements such as Na and Si with low affinity for organic matter the decrease in wetland-dominated catchments corresponded closely to the area of mineral soils that had been replaced by peat, indicating that reduced weathering was the main cause of the decrease. For organophilic metals the decrease in wetland-dominated catchments was even greater, suggesting that there also was an accumulation of these metals in the peat. This was confirmed by investigating the distribution of radionuclides in local mire, which revealed considerable accumulation of uranium and thorium along the edges of the mire. Based on the inventories of uranium and thorium and their distribution in the peat it was concluded that the mire historically had been a sink for these metals and that it most likely will continue to be so for a long time to come. All and all, wetlands were estimated to decrease the fluxes of metals from the boreal forests to downstream lakes and oceans by 20-40%, depending on how strongly they bind to organic matter.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2013. , 36 p.
Keyword [en]
uranium thorium metal transport radionuclide wetland forest stream peat mire
National Category
Environmental Sciences Geochemistry
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:umu:diva-80485ISBN: 978-91-7459-714-1 (print)OAI: oai:DiVA.org:umu-80485DiVA: diva2:649401
Public defence
2013-10-11, KBC-huset, Lilla hörsalen, Umeå universitet, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2013-09-20 Created: 2013-09-18 Last updated: 2013-09-18Bibliographically approved
List of papers
1. Landscape control of uranium and thorium in boreal streams: spatiotemporal variability and the role of wetlands
Open this publication in new window or tab >>Landscape control of uranium and thorium in boreal streams: spatiotemporal variability and the role of wetlands
2012 (English)In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 9, no 11, 4773-4785 p.Article in journal (Refereed) Published
Abstract [en]

The concentrations of uranium and thorium in ten partly nested streams in the boreal forest region were monitored over a two-year period. The investigated catchments ranged from small headwaters (0.1 km(2)) up to a fourth-order stream (67 km(2)). Considerable spatiotemporal variations were observed, with little or no correlation between streams. The fluxes of both uranium and thorium varied substantially between the subcatchments, ranging from 1.7 to 30 g km(-2) a(-1) for uranium and from 3.2 to 24 g km(-2) a(-1) for thorium. Airborne gamma spectrometry was used to measure the concentrations of uranium and thorium in surface soils throughout the catchment, suggesting that the concentrations of uranium and thorium in mineral soils are similar throughout the catchment. The fluxes of uranium and thorium were compared to a wide range of parameters characterising the investigated catchments and the chemistry of the stream water, e. g. soil concentrations of these elements, pH, TOC (total organic carbon), Al, Si and hydrogen carbonate, but it was concluded that the spatial variabilities in the fluxes of both uranium and thorium mainly were controlled by wetlands. The results indicate that there is a predictable and systematic accumulation of both uranium and thorium in boreal wetlands that is large enough to control the transport of these elements. On the landscape scale approximately 65-80% of uranium and 55-65% of thorium entering a wetland were estimated to be retained in the peat. Overall, accumulation in mires and other types of wetlands was estimated to decrease the fluxes of uranium and thorium from the boreal forest landscape by 30-40%, indicating that wetlands play an important role for the biogeochemical cycling of uranium and thorium in the boreal forest landscape. The atmospheric deposition of uranium and thorium was also quantified, and its contribution to boreal streams was found to be low compared to weathering.

Place, publisher, year, edition, pages
Copernicus Publications, 2012
National Category
Geochemistry
Identifiers
urn:nbn:se:umu:diva-64461 (URN)10.5194/bg-9-4773-2012 (DOI)
Available from: 2013-02-04 Created: 2013-01-29 Last updated: 2017-12-06Bibliographically approved
2. Distribution and transport of radionuclides in a boreal mire: assessing past, present and future accumulation of uranium, thorium and radium
Open this publication in new window or tab >>Distribution and transport of radionuclides in a boreal mire: assessing past, present and future accumulation of uranium, thorium and radium
2013 (English)In: Journal of Environmental Radioactivity, ISSN 0265-931X, E-ISSN 1879-1700, Vol. 121, 87-97 p.Article in journal (Refereed) Published
Abstract [en]

The spatial distribution of U-238, Ra-226, K-40 and the daughters of Th-232, Ra-228 and Th-228, were measured in a small mire in northern Sweden. High activity concentrations of U-238 and Th-232 (up to 41 Bq U-238 kg(-1)) were observed in parts of the mire with a historical or current inflow of groundwater from the surrounding till soils, but the activities declined rapidly further out in the mire. Near the outlet and in the central parts of the mire the activity concentrations were low, indicating that uranium and thorium are immobilized rapidly upon their entering the peat. The Ra-226 was found to be more mobile with high activity concentrations further out into the mire (up to 24 Bq kg(-1)), although the central parts and the area near the outlet of the mire still had low activity concentrations. Based on the fluxes to and from the mire, it was estimated that approximately 60-70% of the uranium and thorium entering the mire currently is retained within it. The current accumulation rates were found to be consistent with the historical accumulation, but possibly lower. Since much of the accumulation still is concentrated to the edges of the mire and the activities are low compared to other measurements of these radionuclides in peat, there are no indications that the mire will be saturated with respect to radionuclides like uranium, thorium and radium in the foreseen future. On the contrary, normal peat growth rates for the region suggest that the average activity concentrations of the peat currently may be decreasing, since peat growth may be faster than the accumulation of radionuclides. In order to assess the total potential for accumulation of radionuclides more thoroughly it would, however, be necessary to also investigate the behaviour of other organophilic elements like aluminium, which are likely to compete for binding sites on the organic material. Measurements of the redox potential and other redox indicators demonstrate that uranium possibly could be reduced in parts of the mire. The results of the study suggest that this mire currently is, and historically has been, an important sink for radionuclides and that it most likely will continue to be so for a long time to come.

Place, publisher, year, edition, pages
Elsevier, 2013
National Category
Ecology Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:umu:diva-73555 (URN)10.1016/j.jenvrad.2012.06.010 (DOI)000318962200009 ()
Conference
International Conference on Radioecology and Environmental Radioactivity (ICRER), JUN 19-24, 2011, Hamilton, CANADA
Note

Special Issue

Available from: 2013-06-25 Created: 2013-06-25 Last updated: 2017-12-06Bibliographically approved
3. 234U/238U and 18O in a boreal stream network: comparing two tracers for deep groundwater discharge
Open this publication in new window or tab >>234U/238U and 18O in a boreal stream network: comparing two tracers for deep groundwater discharge
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Uranium (234U/238U) and oxygen (δ18O) isotopes were measured in eight boreal streams and in seven deep groundwater wells adjacent to these streams. The measurements covered an entire spring flood cycle from winter baseflow conditions, through the peak flow and into the early summer, thereby capturing much of the hydrological dynamics in these systems. In the groundwater higher 234U/238U ratios were found in the larger catchments, indicating that long transit times and deep hydrological pathways are reflected in the uranium isotope signature. This was also supported by the δ18O signals and the Ca/Mg ratio in the groundwater. A similar dependence on the catchment area was observed in the streams, which during baseflow conditions generally displayed higher 234U/238U ratios in the larger streams. This relationship remained as the peak flood arrived, although it coincided with a general decrease in the 234U/238U ratios. Simultaneously, the δ18O signal dropped, indicating a greater influence of meltwater. Combined the two isotope pairs suggest that the spring flood leads to more superficial hydrological pathways and more superficial sources of uranium. In connection with the spring flood there was a strong, nearly 1:1 relationship between the streams and the adjacent groundwater wells in terms of 234U/238U ratios. However, the streams generally tended to have higher 234U/238U ratios than the deep groundwater, which in turn had higher uranium concentrations. No correlation was found between 234U/238U ratios and uranium concentrations in any of the analysed waters, and no difference in 234U/238U ratios could be observed between different landscape types. All and all, the results demonstrate that 234U/238U and δ18O generally provide a consistent view of the functioning of the investigated catchment in terms of hydrology and element transport.

Keyword
uranium oxygen isotope stream forest groundwater
National Category
Geochemistry
Research subject
Physical Geography
Identifiers
urn:nbn:se:umu:diva-80483 (URN)
Available from: 2013-09-18 Created: 2013-09-18 Last updated: 2013-09-18Bibliographically approved
4. Metal transport in the boreal landscape: the role of wetlands and the affinity for organic matter
Open this publication in new window or tab >>Metal transport in the boreal landscape: the role of wetlands and the affinity for organic matter
2014 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 48, no 7, 3783-3790 p.Article in journal (Other academic) Published
Abstract [en]

Stream water concentrations of 13 major and trace elements (Al, Ba, Ca, Cr, Cu, La, Mg, Na,Ni, Si, Sr, U, Y) were used to estimate fluxes from 15 boreal catchments. All elementsdisplayed a significant negative correlation to the wetland coverage in each catchment, butthe influence of wetlands was stronger for organophilic metals. 73% of the spatialdifferences in the normalized element fluxes could be explained based only on the wetlandcoverage and the affinity for organic matter, which was quantified using thermodynamicmodeling. When restraining the analysis to the smaller streams (<10 km2) the explanatorypower increased to 88%. The results suggest that wetlands may decrease the fluxes ofmetals from boreal forests to downstream recipients by up to 40%. We suggest that thedecrease in element fluxes is caused by a combination of low weathering in peat soils andaccumulation of organophilic metals in peat. The model could not explain the spatial pattersfor some metals with low affinity for organic matter, some redox-sensitive metals and somemetals with exceptionally high atmospheric deposition, but the results still demonstrate thatwetlands play a crucial role for the biogeochemical cycling of metals in the boreal landscape.

Keyword
metal transport, boreal landscape, forest stream, wetland, peat, mire
National Category
Environmental Sciences
Research subject
Physical Geography
Identifiers
urn:nbn:se:umu:diva-80484 (URN)10.1021/es4045506 (DOI)000333776100023 ()
Note

Originally published in dissertation in manuscript form.

Available from: 2013-09-18 Created: 2013-09-18 Last updated: 2017-12-06Bibliographically approved

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