猪苗代湖の湖底堆積物に記録された放射性セシウムの鉛直プロファイルに基づく福島第一原子力発電所事故由来の放射性セシウムの将来予測

抄録

 Communities and residents in Fukushima Prefecture have been adversely affected and jeopardized by radioactive contamination of the environment and associated socioeconomic reputational damage due to the TEPCO Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, which occurred immediately after the extremely large earthquake and tsunami on March 11, 2011. Therefore, how long radionuclides will persist in the regional environment is a major concern. The vertical profiles of radiocesium concentrations in sediments of Lake Inawashiro-ko in Fukushima Prefecture, 85 km west of the nuclear power plant are clarified, and decadal changes in future concentrations are predicted. Sedimentary cores, 17.0 to 40.5 cm in length, were obtained from 27 sites in the lake at water depths greater than 60 m. Lacustrine sediments consist of a black upper part (unit 1) and an olive-gray lower part (unit 2). These units, providing a stratigraphic record that covers the past 130 years, are mainly composed of clayey silt as background lake floor deposits with intercalations of the 2011 and 1888 event deposits. Radiocesium (¹³⁴Cs+¹³⁷Cs) inventory, derived from the FDNPP accident, in lacustrine sediment cores at 12 sites has a range between 39,000 and 93,000 Bq/m². These values are larger than that of the initial deposition (ca. 30,000 Bq/m²) of radiocesium on the ground around Lake Inawashiro-ko. The excess radiocesium was supplied from the river catchments feeding the lake. Together with sedimentation rates at the individual sites, the ¹³⁷Cs concentration for the 50 years after 2011 is predicted based on the exponential decay patterns of global fallouts of nuclear weapons tests in the 1960s, which were also recorded in the lacustrine sediments. Using this assumption, the concentration of ¹³⁷Cs in lake floor sediments in the 2060s is estimated to be from 159 to 815 Bq/kg at a site where ¹³⁷Cs of 4,000 to 10,866 Bq/kg was initially deposited. However, the predicted radiocesium concentration may be more persistent if the flux of radiocesium from the upper catchments increases, because the deposition of radionuclides and its impacts at the 2011 accident are larger than those in the 1960s.