U-234/U-238 in a boreal stream network: relationship to hydrological events, groundwater and scale
2016 (English)In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 420, 240-250 p.Article in journal (Refereed) PublishedText
Uranium isotopes (U-234/U-238) were measured in eight well-studied boreal streams of different sizes and characters and in seven deep groundwater wells (up to 11m deep) 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. To deepen the interpretation of the spatiotemporal variability of the U-234/U-238 ratio a wide range of hydrochemical and geographical data were included in the analysis. Special attention was given to delta O-18, the most widely used isotopic tracer for hydrological pathways and groundwater transit times, because high U-234/U-238 ratios have often traditionally been attributed to input of deep groundwater and long transit times. The results demonstrated that there was generally a gradual increase in U-234/U-238 ratios in groundwater with increasing catchment area, which based on delta O-18 data was caused by deeper groundwater pathways and longer residence times in larger catchments. A similar dependence on the catchment area was observed in the streams, which during baseflow conditions generally displayed higher U-234/U-238 ratios in the larger streams. This relationship remained as the peak flow arrived, although it coincided with a general decrease in the U-234/U-238 ratios. Simultaneously, the delta O-18 signal dropped, indicating a greater influence of meltwater and younger groundwater. When combined, this confirmed that the activation of shallower groundwater pathways in connection with the spring flood also activated shallower 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 U-234/U-238 ratios, suggesting that the Quaternary deposits were the main source for uranium. Despite large differences in uranium concentrations between different landscape types no effects could be seen on the isotope signature, indicating that the U-234/U-238 ratio is a reliable tracer for the primary source and that the fractionation after mobilisation probably is negligible. All in all, the results demonstrated that U-234/U-238 ratios and delta O-18 generally provide a consistent picture of the hydrological and bio-geochemical functioning of the investigated catchment.
Place, publisher, year, edition, pages
2016. Vol. 420, 240-250 p.
Uranium, Isotope, Stream, Boreal, Groundwater, Oxygen
IdentifiersURN: urn:nbn:se:umu:diva-114003DOI: 10.1016/j.chemgeo.2015.11.014ISI: 000366652200020OAI: oai:DiVA.org:umu-114003DiVA: diva2:897320