A 30-year record reveals re-equilibration rates of 137Cs in marine biota after the Fukushima Dai-ichi nuclear power plant accident: Concentration ratios in pre- and post-event conditions.

Abstract

Concentration ratios (CRs), expressed by dividing 137Cs activity in seawater by that in marine biota (mainly fish), were obtained from the monitoring of 137Cs in coastal areas around Japan between 1984 and 2016. Before the TEPCO Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident (1984-2010), mean CRs of 137Cs, mainly from global fallout (i.e. CRGF), were almost constant for each species throughout the monitoring period, but were different among species, while the values for several species were dependent on their length (i.e. CRGF-SIZE). Thus, CRGF and CRGF-SIZE values for 29 of marketable species are given here as references for conditions where marine biota are in approximate equilibrium (or steady state) with their host water with respect to 137Cs activities in the marine environment. After the FDNPP accident (2011-2016), the impact of the accident has been sustained in eastern Japan waters as indicated by apparent CRs (CRas) which are being used here as indicators of disequilibrium between organisms and their host water. The recession rates of this disequilibrium (the effective CRa half-lives) ranged from 100 to 1100\u202fdays. The identified distinct variation was due to the sample locations, even for the same species, because of the change in 137Cs activity concentrations in their host water and diet preference differences. Variation among species, even those captured from the same area, was mainly due to diet differences as well as metabolic-physiological differences in 137Cs retention. Thus, our results from >30\u202fyears of systematically monitoring have helped quantify the recession rates of post-FDNPP disequilibrium of 137Cs in biota for assessment of how long term is required from contaminated condition by underlying spatial, inter- and intra-species factors.