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Landscape control of uranium and thorium in boreal streams: spatiotemporal variability and the role of wetlands
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Dept. of Geology and Geochemistry, Stockholm University, Stockholm, Sweden.
Dept. of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
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. Vol. 9, no 11, 4773-4785 p.
National Category
Geochemistry
Identifiers
URN: urn:nbn:se:umu:diva-64461DOI: 10.5194/bg-9-4773-2012OAI: oai:DiVA.org:umu-64461DiVA: diva2:602918
Available from: 2013-02-04 Created: 2013-01-29 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Radionuclide transport in the boreal landscape: Uranium, thorium and other metals in forests, wetlands and streams
Open this publication in new window or tab >>Radionuclide transport in the boreal landscape: Uranium, thorium and other metals in forests, wetlands and streams
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
uranium thorium metal transport radionuclide wetland forest stream peat mire
National Category
Environmental Sciences Geochemistry
Research subject
Physical Geography
Identifiers
urn:nbn:se:umu:diva-80485 (URN)978-91-7459-714-1 (ISBN)
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

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