Tritium activity concentration and behaviour in coastal regions of Fukushima in 2014

Abstract. We observed 3H activity concentrations and the 137Cs activity concentrations during the SoSo 5 rivers cruise in 2014 and at the Tomioka port in 2014–2018. The 3H activity concentrations at coastal stations located close to the Fukushima coast ranged from 90 Bq m−3 to 175 Bq m−3, and decreased between 67 Bq m−3 to 83 Bq m−3 at the stations located 12–16 km from the coast. The 3H activity concentration at the estuaries and ports, except at 56 north canal of the FNPP1 site, are around 200–500 Bq m−3 and slightly lower than the 3H activity concentration of 500–600 Bq m−3 observed in the rivers. These gradients of the 3H activity concentrations in the coastal region might indicate the large effect of 3H flux through the rivers. On the other hand, the 3H activity concentration at 56N of the FNPP1 site was significantly high compared to the 3H activity concentration in surrounding waters both north and south of the FNPP1 site and in river waters. It should also be noted that the 3H activity concentrations were similar at the stations located both north and south of the FNPP1 site, while the 137Cs activity concentrations were lower at the stations north of the FNPP1 site and higher at the stations south of the FNPP1 site. This indicated that the major sources of 137Cs could be the FNPP1 site as the point source while the source of 3H should be more diffuse and linked to riverine inputs located north and south of the FNPP1 site. The 3H / 137Cs activity ratios in coastal waters were 1.2–2.2 as obtained via the slopes by standardised major axis regressions between the 3H activity concentration and the 137Cs activity concentration of SoSo samples and Tomioka Port observed in 2014–2018, which is significantly high compared to that of the released radionuclides derived from the FNPP1 site, which was 0.01 in 2011 just after the accident. The open-water 3H activity concentration contribution to coastal waters was estimated to be 67 ± 20 Bq m−3 and 66 ± 17 Bq m−3 as the intercepts also by standardised major axis regressions. These estimates are consistent with 50 Bq m−3 obtained at the Kuroshio region as the background levels of 3H activity concentration in open water. The 3H and 137Cs fluxes to the coastal region of Fukushima based on the open-water movement, freshwater flux from the rivers based on their respective catchment, and mean monthly precipitation were estimated. The largest 3H flux is the open-water inflow from the north of the FNPP1 site and it reaches 52 GBq day−1, while the rivers north of the FNPP1 site showed 3–5 GBq day−1 fluxes. We obtained a 3H flux as 1.9–4.5 GBq day−1 of 3H using the 3H activity concentration in the port, which is comparable with the fluxes obtained from the rivers located north of the FNPP1 site. While using 3H activity concentration at the 56 north canal of FNPP1, we obtained 28–86 GBq day−1 fluxes, which is one order of magnitude larger than those estimated using 3H activity concentration in the FNPP1 port. One of the reasons could be the very high variability of the 3H levels at 56 north canal and in the port of FNPP1, explaining variable 3H/137Cs activity ratio observed at 56 north canal and in the port of FNPP1. The 3H activity concentration of TFWT in the fish filets collected close to the FNPP1 site ranged from 97 ± 11 Bq m−3 to 144 ± 11 Bq m−3, which were similar to the 3H activity concentrations in the surrounding seawater, in agreement with the knowledge that the bioconcentration factor of 3H is approximately 1. In contrast, higher values were found in TOBT, which can be linked to life-history traits.