U isotopes distribution in the Lower Rhone River and its implication on radionuclides disequilibrium within the decay...

The large rivers are main pathways for the delivery of suspended sediments into coastal environments, affecting the biogeochemical fluxes and the ecosystem functioning. The radionuclides from ²³⁸U and ²³²Th-series can be used to understand the dynamic processes affecting both catchment soil erosion and sediment delivery to oceans. Based on annual water discharge the Rhone River represents the largest river of the Mediterranean Sea. The Rhone valley also represents the largest concentration in nuclear power plants in Europe. A radioactive disequilibrium between particulate ²²⁶Ra_(p) and ²³⁸U_(p) was observed in the suspended sediment discharged by the Lower Rhone River (Eyrolle et al. 2012), and a fraction of particulate ²³⁴Th was shown to derive from dissolved ²³⁸U_(d) (Zebracki et al. 2013). This extensive study has investigated the dissolved U isotopes distribution in the Lower Rhone River and its implication on particulate radionuclides disequilibrium within the decay series. The suspended sediment and filtered river waters were collected at low and high water discharges. During the 4-months of the study, two flood events generated by the Rhone southern tributaries were monitored. In river waters, the total U_(d) concentration and U isotopes distribution were obtained through Q-ICP-MS measurements. The Lower Rhone River has displayed non-conservative U-behavior, and the variations in U_(d) concentration between southern tributaries were related to the differences in bedrock lithology. The artificially occurring ²³⁶U was detected in the Rhone River at low water discharges, and was attributed to the liquid releases from nuclear industries located along the river. The (²³⁵U/²³⁸U)_(d) activity ratio (=AR) in river waters was representative of the ²³⁵U natural abundance on Earth. The (²²⁶Ra/²³⁸U)_(p) AR in suspended sediment has indicated a radioactive disequilibrium (average 1.3 ± 0.1). The excess of ²³⁴Th in suspended sediment =(²³⁴Th_xs(p)) was apparent solely at low water discharges. The activity of ²³⁴Th_xs(p) was calculated through gamma measurements and ranged from unquantifiable to 56 ± 14 Bq kg^-1. The possibility of using ²³⁴Th as a tracer for the suspended sediment dynamics in large Mediterranean river was then discussed.