Experimental evaluation for stability of phytoplanktonic organically bound tritium in seawater using a deuterium...

Phytoplanktonic organically bound Tritium (OBT) in seawater is a potentially overlooked risk factor for the oceanic release of tritium from nuclear facilities, which attracted attention along the Fukushima coast due to the release of tritiated water (HTO) from the Fukushima Daiichi Nuclear Power Station. This study quantitatively evaluated the stability of phytoplanktonic OBT in a seawater incubation experiment using deuterium (D), a stable isotope tracer for tritium. First, seawater, including a natural phytoplankton population, was incubated under light with deuterium for 24 h to produce D-labeled particulate organically bound hydrogen (POBH). Subsequently, the seawater was allowed to settle in the dark for 150 days. During dark incubation, the remaining D-labeled POBH in the suspended particles was measured. The D-labeled POBH produced by light incubation decreased by 20% by day 15 in the dark. Only 6.6% of D-labeled POBH remained at the end of dark incubation period (day 150). Based on the incubation experiment, a regression equation for the decrease in phytoplanktonic OBT was created. Besides, phytoplanktonic OBT accumulation in seawater off the Fukushima coast was estimated using this equation, assuming constant daily production and decomposition in seawater (including 1500 Bq L^-1 HTO) over 10 years and no spatial transport of OBT from the production site. The theoretical ultimate estimation showed that only 0.27 Bq L^-1 (0.02% of HTO) of phytoplanktonic OBT accumulates in seawater at maximum. Results clearly suggest low stability and accumulation potential of phytoplanktonic OBT in seawater off the Fukushima coast.