234U/238U in a boreal stream network — Relationship to hydrological events, groundwater and scale

•The 234U/238U ratio in stream water increases with catchment area both during winter baseflow conditions and at peak flow•High 234U/238U ratios are associated with low δ18O and, therefore, deep hydrological pathways and long transit times•The 234U/238U ratio of stream water generally decreases when the spring flood arrives and activates more superficial hydrological pathways•The 234U/238U ratio has no apparent connection to the uranium concentration•The 234U/238U ratio of deep groundwater is similar to that of stream water at peak flow

Uranium isotopes (234U/238U) were measured in eight well-studied boreal streams of different sizes and characters and in seven deep groundwater wells (up to 11 m 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 234U/238U ratio a wide range of hydrochemical and geographical data were included in the analysis. Special attention was given to δ18O, the most widely used isotopic tracer for hydrological pathways and groundwater transit times, because high 234U/238U 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 234U/238U ratios in groundwater with increasing catchment area, which based on δ18O 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 234U/238U ratios in the larger streams. This relationship remained as the peak flow arrived, although it coincided with a general decrease in the 234U/238U ratios. Simultaneously, the δ18O 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 234U/238U 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 234U/238U 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 234U/238U ratios and δ18O generally provide a consistent picture of the hydrological and biogeochemical functioning of the investigated catchment.