Natural and Fukushima-derived radioactivity in macroalgae and mussels along the Japanese shoreline

Abstract. Following the failure of the nuclear power plant in Fukushima Prefecture in March 2011, peer-reviewed publications describing radioactivity levels in organisms inhabiting coastal environments are scarce. This paper reports on elevated levels of ¹³⁴Cs and ¹³⁷Cs in macroalgae and mussels (up to ~ 800 Bq kg⁻¹ dry wt.) in June 2011. Cs concentrations in biota sampled in early June 2011 were higher in areas south of Fukushima than sampled in the last third of the month north of Fukushima. Activity concentrations from ^134+137Cs in organisms south of Fukushima were comparable to or lower than those from the naturally occurring ⁴⁰K in the same samples. While ²¹⁰Pb and ²¹⁰Po concentrations were generally lower than these other radionuclides, ²¹⁰Po as an α-emitter is more significant from a radiological viewpoint than γ-emitters as it can inflict greater biological damage. By applying known bioconcentration factors of Cs in biota, measured biota concentrations of Cs were also used to estimate Cs concentrations in coastal seawater to be in the range of 10²–10³ Bq m⁻³. These estimates show that, 3 months after the accident and maximal release of radioactive Cs, levels of Cs persisted in coastal waters, although at levels that were two orders of magnitude lower than at the time of release. These June coastal seawater Cs levels were four orders of magnitude above Cs concentrations off Japan prior to the Fukushima disaster.