Comparison of distribution of ⁴⁰K and ¹³⁷Cs in aquatic macrophyte

Aquatic macrophytes give a valuable indicator of radioactive contamination following release of radioactive material into the environment. In order to assess the contamination from the Fukushima Dai-ichi Nuclear Power Plant accident in 2011, we have measured ¹³⁷Cs and ⁴⁰K concentrations in aquatic macrophytes, water, and sediments to investigate the variation within the plant itself, variation between patches of plants, and change over time of concentration ratio values (CR_wo-media). The ¹³⁷Cs concentrations in leaf parts of Egeria densa and Ranunculus nipponicus were higher than those for stem parts. On the other hand, the higher ⁴⁰K concentrations were observed in stem parts compared to leaf parts of these plants. These results suggest that Cs and K do not behave similarly in aquatic macrophytes. ¹³⁷Cs concentration ranges in the patches of E. densa and R. nipponicus, submerged and emergent life forms of Sparaganium japonicum were very different: 75-160 Bq/kg, f.w., 280-330 Bq/kg, f.w., 31-99 Bq/kg, f.w., and 33-64 Bq/kg, f.w., respectively (n=3). In S. japonicum, the large variation of ¹³⁷Cs and ⁴⁰K concentrations between the patches is probably due to its diverse life form and wide range of water content. Regarding the variation over time, only the ¹³⁷Cs concentrations in R. nipponicus consistently decreased from 2015 through 2017. Little variation of water-to-plant concentration values (CR_wo-water) of ¹³⁷Cs was observed in these aquatic macrophytes, indicating that the bioavailable fraction of ¹³⁷Cs in water phase was in steady state. (author)