Mass balance and latest fluxes of radiocesium derived from the fukushima accident in the western North Pacific Ocean and coastal regions of Japan

Abstract

This article summarizes and discusses mass balance calculations of the activities of Fukushima-derived ¹³⁷Cs released to the atmosphere and ocean prior to 2018 as well as the ¹³⁷Cs inventories on land and in the ocean, biota, and sediment. We propose that the consensus value of the total amount of ¹³⁷Cs released to the atmosphere was 15-21 PBq; atmospheric deposition of ¹³⁷Cs on land was 3-6 PBq; atmospheric deposition of ¹³⁷Cs on the North Pacific was 12-15 PBq; and direct discharge of ¹³⁷Cs to the ocean was 3-6 PBq. We also evaluated the movement of ¹³⁷Cs from one domain to another for several years after the accident. We calculated that the amount of ¹³⁷Cs transported by rivers might be 40 TBq. The annual deposition of ¹³⁷Cs due to resuspension at Okuma during the period 2014-2018 was 4-10 TBq year^-1. The ¹³⁷Cs discharged to the ocean was 0.73-1.0 TBq year^-1 in 2016-2018. The integrated amount of FNPP1-derived ¹³⁷Cs that entered the Sea of Japan from the Pacific Ocean from 2011 until 2017 was 270 ± 20 TBq, 6.4% of the estimated amount of FNPP1-derived ¹³⁷Cs in Subtropical Mode Water in the North Pacific. The integrated amount of FNPP1-derived ¹³⁷Cs that returned to the North Pacific Ocean through the Tsugaru Strait from the Sea of Japan was 110 ± 10 TBq. Decontamination efforts removed 134 TBq of ¹³⁷Cs from surface soil prior to February 2019, an amount that corresponded to 4% of the¹³⁷Cs deposited on land in Japan.