A brief history of origins and contents of Organically Bound Tritium (OBT) and 14C in the sediments of the Rhône watershed

Abstract Tritium ( 3 H) and Carbon-14 ( 14 C) are radionuclides of natural (cosmogenic) origin that have also been introduced into the environment by humans since the middle of the last century. They are therefore not compounds that have only recently been released into the environment and they do not pose a recognized health threat due to their low radiotoxicity. However, they hold an important place among current concerns because they are being discharged into the environment by the nuclear industry in large quantities compared to other radionuclides. Those both radionuclides partly integrate organic matter during metabolic processes (i.e., photosynthesis) leading to organically bound forms that can be found in sediments. Organically bound tritium (OBT) analyses carried out on the sediments of the Rhone and its tributaries indicate a significant and historical tritium labelling of sedimentary particles all along the Rhone river, as well as in several northern tributaries, in particular the Ognon and the Tille rivers (tributaries of the Saone), the Doubs River and the Loue River (a tributary of the Doubs) and the Arve river. The recorded levels (10 to over 20,000 Bq/L) are very likely to be related to the presence of synthetic tritiated particles (technogenic tritium), which were used in the past in watchmaking workshops. Although overall contamination levels decrease from north to south in the Rhone watershed and fade over time, particularly due to the radioactive decay of tritium, this contamination source of technogenic tritium in the Rhone watersheds is currently still not negligible. Carbon-14 analyses show that the Rhone sediments generally display 14 C levels close to the atmospheric reference values (231 Bq·kg −1 of C in 2015) or even lower in most of cases, and show sporadic and weak labelling near nuclear facilities. The low 14 C levels in the Rhone sediments are most likely related to the solid contributions from tributaries draining areas that are rich in fossil organic matter, and therefore devoid of 14 C. In the Rhone watershed, the presence in solid particles of tritium in a form organically bound to synthetic compounds and of petrogenic (fossil) organic carbon, can potentially alter the apparent assimilation rates to the food chain of these two radionuclides.