Carbon-14 cycling in a nuclearized river: A first study in the downstream part of the Rhône River (France)

Carbon-14 (¹⁴C) has a natural origin but is also anthropogenically released from civil nuclear facilities. Due to its long decay period (half-life: 5700 ± 30 years), it is a persistent radionuclide in the environment. In rivers, the complex speciation of carbon makes the fate of industrial ¹⁴C difficult to track. This study reports a first overview of artificial ¹⁴C cycling in a nuclearized river. A one-year sampling campaign was conducted on the French nuclearized Rhône River and two of its non-nuclearized tributaries (Durance and Ardèche rivers). Isotopic (δ¹³C, Δ¹⁴C) and carbon concentrations analyses were performed on the particulate organic carbon (POC), dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). Chlorophyll-a (Chl-a) and tritium analyses were performed to assess the dynamic of aquatic organic matter and the nuclear industry contribution, respectively. Comparisons of Δ¹⁴C data obtained from the Rhône River with those from the tributaries highlight significant industrial radiocarbon labelling in all carbon forms, with medians of 142, 130 and 42 ‰ for POC, DOC and DIC, that are 2-3 times higher than those of the tributaries. The high values of Chl-a/POC ratios with Δ¹⁴C-enriched POC suggest a biological uptake of artificial Δ¹⁴C in DIC by aquatic photosynthesis. The relationship of Δ¹⁴C-DIC with tritium activity indicates a response to recent releases and enables the contribution of nuclear power plants to be estimated at a median of 26 %. Sampling at the Rhône's mouth would reinforce our understanding of the fate of riverine ¹⁴C when entering the marine environment.