Spatiotemporal distributions of Fukushima-derived radionuclides in nearby marine surface sediments

Abstract

Spatiotemporal distributions of anthropogenic radionuclides in marine surface sediments off Miyagi, Fukushima, and Ibaraki Prefectures were analyzed on the basis of data collected during the monitoring program launched by the Japanese Ministry of Education, Sports, Science and Technology in 2011 right after the Fukushima Dai-ichi Nuclear Power Plant accident began. Concentrations of ¹³⁷Cs in the surface sediments varied spatially by two orders of magnitude, from 1.7 to 580 Bq kg-dry⁻¹, and there was no obvious correlation between ¹³⁷Cs concentration and the proximity of the sampling location to the accident site. The total inventory of ¹³⁷Cs accumulated in the upper 3 cm of surface sediments in the monitoring area was estimated to be 3.78 × 10¹³ Bq, that is, 0.1–2% of the total ¹³⁷Cs flux from the plant to the ocean as a result of the accident (the percentage depends on the model used to estimate the total flux). The spatial variations of ¹³⁷Cs concentration and inventory depended on two main factors: the ¹³⁷Cs concentration in the overlying water during the first several months after the accident and the physical characteristics of the sediments (water content and bulk density). The temporal variations of the concentrations of other anthropogenic radionuclides (⁹⁰Sr, ⁹⁵Nb, ^{110 m}Ag, ¹²⁵Sb, ¹²⁹Te, and ^{129 m}Te) in the sediments were also investigated. Activity ratios of these nuclides to ¹³⁷Cs suggest that the nuclides themselves were not homogenized before they were removed from seawater to the sediments

Decline in radiocesium in seafloor sediments off Fukushima and nearby prefectures

“…During the initial monitoring period, 137 Cs concentrations generally increased with time to a variable degree among the stations, reaching maximum values of up to 520 (Stn. J1) Bq/kg in dry weight (Kusakabe et al 2013), and they subsequently decreased. In February 2016, they ranged from 0.…”

“…Details of the quantification of 134 Cs and 137 Cs activities are described in Kusakabe et al (2013).…”

“…We examined temporal changes of the 137 Cs concentrations in the surface sediment to a depth of 3 cm in this study as well as the changes during the initial monitoring period (May 2011-September 2011) reported previously (Kusakabe et al 2013) (Fig. 4).…”

“…Initially, there were eight monitoring stations, but the number has increased gradually; since May 2012, 32 monitoring stations have been in operation off Fukushima and nearby prefectures. Kusakabe et al (2013) have already reported monitoring results obtained during the period from Abstract Radioactivity has been monitored in seafloor sediments off Fukushima and nearby prefectures regularly. During the initial monitoring period (May-September 2011), 137 Cs concentrations in the surface sediments (0-3 cm) generally increased to 8-580 Bq/kg.…”

Radiocesium Concentrations in the Organic Fraction of Sea Sediments

“…Radiocesium concentration in the organic fraction of sediments (Cs org ) is an important factor because the transport of radiocesium from sediment to demersal ecosystem occurs primarily through the feeding/ingestion of carbon sediments by benthos. With regard to the FNPP accident, a large amount of data is available on the spatiotemporal distribution of radiocesium concentration in sea sediments (Cs bulk ) off the Fukushima Prefecture (Otosaka and Kobayashi 2012;Kusakabe et al 2013;Otosaka and Kato 2014;Ambe et al 2014). Unfortunately, insufficient data are available on the spatiotemporal distribution of Cs org .…”