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

(IF 5.59; Q1)International audienceTritium (3H) and Carbon-14 (14C) are radionuclides of natural (cosmogenic) origin that have also been introducedinto the environment by humans since the middle of the last century. They are therefore not compoundsthat have only recently been released into the environment and they do not pose a recognized health threat dueto their lowradiotoxicity. However, they hold an important placeamong current concerns because they are beingdischarged 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) leadingto organically bound forms that can be found in sediments. Organically bound tritium(OBT) analyses carriedout on the sediments of the Rhône and its tributaries indicate a significant and historical tritiumlabelling of sedimentaryparticles all along the Rhône river, aswell as in several northern tributaries, in particular the Ognon andthe Tille rivers (tributaries of the Saone), the Doubs River and the Loue River (a tributary of the Doubs) and theArve river. The recorded levels (10 to over 20,000 Bq/L) are very likely to be related to the presence of synthetictritiated particles (technogenic tritium),whichwere used in the past inwatchmakingworkshops. Although overallcontamination levels decrease from north to south in the Rhône watershed and fade over time, particularlydue to the radioactive decay of tritium, this contamination source of technogenic tritiumin the Rhônewatershedsis currently still not negligible. Carbon-14 analyses show that the Rhône sediments generally display 14C levelsclose to the atmospheric reference values (231 Bq·kg−1 of C in 2015) or even lower in most of cases, andshow sporadic and weak labelling near nuclear facilities. The low 14C levels in the Rhône sediments are mostlikely related to the solid contributions from tributaries draining areas that are rich in fossil organic matter,and therefore devoid of 14C. In the Rhône 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 apparentassimilation rates to the food chain of these two radionuclides