Spatial and temporal variation of tritium concentrations during a dam flushing operation

International audienceTritium is a radionuclide commonly observed worldwide in riverine systems. In the Rhône River downstream the Lake Geneva (Switzerland and France), its occurrence is also related to its use for its luminescent properties in watchmaking paints. In fact, tritium is regularly observed at anomalous levels in this river and extreme events such as flushing operations might conduct to its transport downstream. In the Rhône River, characterized by 21 dams downstream the Geneva Lake, such operations are regularly organized to remove the sediments and limit problematic consequences such as siltation and increased flooding hazards. The consequences of dam flushing operations on tritium concentrations were thus investigated. Samples of Suspended Particulate Matter (SPM) and water were collected in the Rhone River downstream of Geneva in June 2012, during a planned flushing operation of three upstream reservoirs (Verbois, Chancy-Pougny and Génissiat). The concentrations of tritiated water (HTO) and organically bound Tritium (OBT) were measured and compared to reference concentrations. The flushing operations had no impact on the HTO concentration while the increases observed were related to the authorized releases of HTO from a nuclear power plant located downstream the dams. High increases of OBT concentrations were observed at two stations (Creys-Malville and Jons) without clear spatial or temporal trends. Since similarly high OBT concentrations had already been observed in suspended and deposited sediments during normal flow condition, these anomalous peaks could be explained by the heterogeneous spatial distribution of OBT in sediment sources that were resuspended during the flushing operations. The results highlight the need to investigate the amount of OBT currently stored in the upstream Rhone River as it might be significant