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

Abstract

Tritium (H) and Carbon-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 Rhône and its tributaries indicate a significant and historical tritium labelling of sedimentary particles all along the Rhône 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 Rhône watershed and fade over time, particularly due to the radioactive decay of tritium, this contamination source of technogenic tritium in the Rhône watersheds is currently still not negligible. Carbon-14 analyses show that the Rhône sediments generally display C levels close to the atmospheric reference values (231 Bq·kg of C in 2015) or even lower in most of cases, and show sporadic and weak labelling near nuclear facilities. The low C levels in the Rhône sediments are most likely related to the solid contributions from tributaries draining areas that are rich in fossil organic matter, and therefore devoid ofC. 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 apparent assimilation rates to the food chain of these two radionuclides.

Assimilation and transport of organic bound tritium in an irrigated pine forest

“…24 Bomb pulse 14 C from nuclear testing activities contributed large amounts of bomb pulse 14 C in the late 1960s but the levels of 14 C in the youngest MWMF tree core tissues are indicative of another more recent source of 14 C which we attribute to irrigation with 14 C-contamined pond water. It has been demonstrated that in the absence of newer sources of 14 C contamination, the levels of 14 C in more recent tree ring tissue and the atmosphere should continue to decrease with time due to the radioactive decay of 14 C to stable 14 N and intensive atmospheric mixing 22,26 but we do not observe this in the tree tissues that formed during the ~20-year irrigation period.…”

“…17 In addition to T, carbon-14 ( 14 C) is another radionuclide that is sometimes found in contaminated areas and its levels have been measured in tree rings. 22,23,24,25,26 These studies suggest tree ring 14 C levels can be fairly representative of historic 14 C releases from nuclear reactors as well as from nuclear weapons testing.…”

Understanding of Tritium Behavior Toward Sediment in Aquatic System

“…This shows the persistence of tritium initially integrated into the organic matter within these matrices. [1], [3], [5], [6]. Another study in Qinshan Nuclear Power Plant, China, showed that for soils collected at ± 2 km from the nuclear power plant, there is a correlation between NE-OBT and HTO ratio in soil samples.…”

“…Because tritium is part of the organic matter cycle, OBT is accumulated in organic matter storage media, especially in the sediments of streams and rivers. [6]. Tritium can persist in both terrestrial and aquatic environments according to the recycling rates of organic matter.…”

“…On the other hand, tritium can be bound with slow metabolism species of organic matter, and in this form, tritium can be bound for several years or decades. [3], [6].…”