Tritium and 14C background levels in pristine aquatic systems and their potential sources of variability

Tritium and 14C currently are the two main radionuclides discharged by nuclear industry. Tritium integrates into and closely follows the water cycle and, as shown recently the carbon cycle, as is the case with 14C (Roussel-Bebet; 2014a, 2014b, Eyrolle-Boyer et al. 2014). As a result, these two elements persist in both terrestrial and aquatic environments according to the recycling rates of organic matter. Although on average the OBT activity of sediments in pristine rivers does not significantly differ today (2007-2012) from the mean HTO content on record for rainwater (2.4±0.6 Bq/L and 1.6±0.4 Bq/L, respectively), regional differences are expected based on the biomass inventories affected by atmospheric global fallout from nuclear testings and the recycling rate of organic matter within watersheds. The results obtained between 2007 and 2012 for 14C show that the levels vary between 94.5±1.5 and 234±2.7 Bq/kg of C for the sediments in French rivers and across a slightly higher range of 199±1.3 to 238±3.1 Bq/kg of C for fish. This variation is most probably due to preferential uptake of organic carbon. These ranges of values overall are below the mean baseline value for the terrestrial environment (232.0±1.8 Bq/kg of C in 2012, Roussel-Debet; 2014b) in relation with dilution by carbonates and/or fossil organic carbon present in aquatic systems. This emphasises yet again the value of establishing for these two radionuclides ranges of regional baseline values in order to account for paleoclimatic and lithological variations. We also show that sedimentary archives can be used to reconstruct past 14C and OBT levels. These results have made it possible to confirm delayed contamination of aquatic sediments by atmospheric fallout from past nuclear tests, as recently demonstrated (Eyrolle-Boyer et al., 2014). Lastly, our results show that the OBT and 14C levels within the suspended particles or sediments downstream of liquid discharges from nuclear facilities are affected by inputs from tributaries, and thus by hydrology, floods origin and chronologies.