234U/238U and 18O in a boreal stream network: comparing two tracers for deep groundwater discharge
(English)Manuscript (preprint) (Other academic)
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.
uranium oxygen isotope stream forest groundwater
Research subject Physical Geography
IdentifiersURN: urn:nbn:se:umu:diva-80483OAI: oai:DiVA.org:umu-80483DiVA: diva2:649382