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234U/238U and 18O in a boreal stream network: comparing two tracers for deep groundwater discharge
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Dept. of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
Swedish Defence Research Agency (FOI), Umeå, Sweden.
Dept. of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
(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 [en]
uranium oxygen isotope stream forest groundwater
National Category
Geochemistry
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:umu:diva-80483OAI: oai:DiVA.org:umu-80483DiVA: diva2:649382
Available from: 2013-09-18 Created: 2013-09-18 Last updated: 2013-09-18Bibliographically 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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