Tritium and 14C dependencies upon particulate organic matter within the nuclearized Rhone River (France)

Purpose Radiocarbon and tritium are the dominant radionuclides released by nuclear industries over the world. Close interactions of these elements with particulate organic matter (POM) in soils and rivers are widely reported in the literature but only few studies focus on their relationships with POM within nuclearized rivers. Here, we focus on the nuclearized Rhone River (France) and its tributaries in order to highlight the behavior of these two radionuclides owing to POM amounts and nature . Materials and methods Over the 2001-2021 period, suspended sediments (TSS) samples were acquired and analyzed thanks to the MOOSE network (Mediterranean Ocean Observing System for the Environment) and of the Rhone Sediment Observatory. Over this period, TSS, particulate organic carbon (POC), dissolved organic carbon (DOC), and chlorophyll and phaeopigment contents were determined at the SORA monitoring station located at the downstream part of the Rhone River. These parameters were measured from daily to weekly after continuous filtration of the waters collected from high-frequency water samplings. Larger amounts of TSS were also collected by using time integrating particle traps (PT) at the SORA station and on several Rhone tributaries. These latter samples were analyzed for global biogeochemistry (Rock Eval 6), organically bound tritium (OBT), and radiocarbon. Results and discussion At the downstream part of the Rhone River, POC represents from 1 to 10 wt.% of the TSS even though this amount rises to 20 wt.% very occasionally. POC/TSS shows a positive relationship with the chlorophyll/phaeopigment amounts reinforcing the non-negligible production of autochthonous POM in this river. The increase of C-14 contents and Delta C-14 signatures with the amount of aquatic POM underlines that a significant part of C-14 from industrial releases is transported as fresh and labile particulate organic compounds. In tributaries, Delta C-14 signatures decrease with the rising amounts of C-14 depleted refractory carbon (RC/TOC) coming from soil erosion in a similar way. OBT contents show no relationships with neither the contents nor the nature of naturally occurring POM. Conclusions Autochthonous POM would constitute a key route to transfer radiocarbon from nuclear releases towards the food chain. Normalizing radiocarbon contents with RC/POC constitutes a new powerful tool to better assess the environmental labelling of radiocarbon discharged by the nuclear industries as the dilution effect generated by dead carbon is discarded. In the Rhone River, OBT is mostly associated with synthesized organic particles inherited from watch industries.