Le tritium organique dans les écosystèmes d’eau douce : évolution à long terme dans l’environnement des centres nucléaires de production d’électricité français

Abstract

RÉSUMÉ (2011) of aquatic ecosystems. Upstream of all NPP, OBT levels in sediments are higher than in plants and fish, themselves larger than HTO in surface water. The magnitude of these differences and the long-term trends depend on the river basin and can be explained by the varying nature of tritium sources. In river catchments, where atmospheric test fallout is the main source of tritium, the observed levels result from the exposure of aquatic organisms to two distinct tritium pools of different ages : atmospheric tritiated water (representing present fallout), and organic tritium from soils (formed over several decades) which supplies particulate matter to surface waters. In the Rhône and Rhine river basins, an additional source of organic tritium of very low bio-availability, probably originating from the luminescent paint industry, is responsible for the spiking of sediment organic matter up to 100 to 100 000 Bq.L -1 combustion water. The comparison of upstream and downstream NPP tritium levels shows that the influence of tritium discharges is detectable only in rivers, with low background OBT activities, i.e in basins other than the Rhône and Rhine. The observed increase in plant and fish OBT is lower than the added HTO activity in water due to discharge, which supports the absence of bioaccumulation for tritium originating from HTO and the absence of highly bio-available tritiated organic molecules in NPP discharges.

Evidence for tritium persistence as organically bound forms in river sediments since the past nuclear weapon tests

“…This shift directly reflects the delayed transfer of terrestrial POM from the catchment to the river. This transfer time gap of POM (around one decade), from its production during the growth of terrestrial plants (photosynthesis) to its detrital export by rivers, is of the same order of magnitude as those evaluated by other authors throughout the French territory 5,9,35 . Interestingly, the symmetric shape of the OBT peak would suggest that POM in riverine sediments is a continuum of progressively decomposing organic compounds in soils as recently proposed by 36 for soil OM.…”

“…B). Additionally, once corrected for the radioactive decay, OBT concentrations measured within the archive from 24.2 ± 3.0 Bq/L in 1986 to 6.0 ± 0.8 Bq/L in the more recent years (2006) approach those in surface sediments collected since the middle of the 80 s in various French Rivers upstream nuclear facilities, including the Loire River 5,9 (Fig. 4).…”

“…Nevertheless, any perturbation due to artificial inputs of tritium into the environment can alter this equilibrium since water masses are recycled faster than OM. These processes are assumed today to explain most OBT/HTO disequilibrium widely observed by several authors in various environmental compartments such as terrestrial plants 2–4 , aquatic plants 5,6 , soils 7,8 and river sediments collected far from the influence of releases from nuclear facilities 5,9 . Among other hypotheses such as isotopic fractionation or hypothesis dealing with tritium bioaccumulation 10 , current consensual explanation for OBT/HTO disequilibrium is that tritium integrated into the OM would persist for the long term while the free forms rapidly exchange with the surrounding environment 1 .…”

“…Even though tritium is strongly mobile in the environment through its HTO form and rapidly disperses and dilutes within water masses, the OBT form is frequently found in some reservoirs where OM is preserved such as in soils 7,8 and in riverine, lacustrine or marine surface sediments 5,9,28,30 . Consequently, if such OBT storages exist, we can expect that biogeochemical processes in these reservoirs due to OM degradation would act as a delayed source of HTO and/or dissolved OBT for the atmosphere and hydrosphere.…”

Hydrogen dynamics in soil organic matter as determined by <sup>13</sup>C and <sup>2</sup>H labeling experiments

“…Since then, tritium levels have greatly declined because of its relatively short half-life (12.3 years). However the concentration of organically bound tritium can often be higher than the concentration of water tritium due its longer residence time (Gontier and Siclet, 2011;Kim et al, 2012;Eyrolle-Boyer et al, 2014;Thompson et al, 2015). Models used to predict the fate and behavior of tritium in the environment often simplify processes linked to the formation and degradation of organically bound tritium.…”