Organic tritium in freshwater ecosystems: long-term trends in the environment of French nuclear power plants

From 1977 to 2009, more than 600 measurements of organic tritium were performed on fish, aquatic plants and sediments upstream and downstream of the 15 French NPP located along rivers. Examination of the results shows that organic tritium activities have exponentially decreased over the last thirty years, in all components 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 Rhone and Rhine river basins, an additional source of organic tritium of very low bin-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 Rhone and Rhine. The observed increase in plant and fish OBT is lower than the added HT 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.