Sedimentation and remobilization of radiocesium in the coastal area of Ibaraki, 70 km south of the Fukushima Dai-ichi Nuclear Power Plant

Decline in radiocesium in seafloor sediments off Fukushima and nearby prefectures

“…Possible pathways for the release of sedimentary Cs include desorption and ion exchange. Otosaka and Kobayashi (2013) performed a sequential extraction experiment with sediment collected off Ibaraki Prefecture, south of Fukushima Prefecture, and evaluated the contributions of the surface-adsorbed and organic matter-bound 137 Cs to be less than 5% and about 20%, respectively, of total 137 Cs. Their result indicates that organic material can be a major reservoir of labile 137 Cs.…”

“…Their result indicates that organic material can be a major reservoir of labile 137 Cs. In addition, the results of a dissolution experiment in which contaminated sediment was placed in uncontaminated seawater showed that more than 85% of the 137 Cs remained in the sediments after 30 days (Otosaka and Kobayashi 2013). Ono et al (2015) estimated that the ratio of the organicbound 137 Cs inventory to the total 137 Cs inventory in the sediment off Fukushima ranges from 2.4 to 13.9%, with an average ratio (± standard deviation) of 5.7 (±2.9) %.…”

“…For example, Misumi et al (2014) indicated that for the first several months after the accident, the horizontal distribution of 137 Cs in the surface sediment was related to its deposition and adsorption from the polluted seawater. Ambe et al (2014) identified a high-concentration band with a width of about 20 km in the southern part of the monitoring area, where waters were shallower than 100 m, and showed that the higher concentrations were due to adsorption by finegrained sediment, and Otosaka and Kobayashi (2013) proposed that 137 Cs-enriched fine particles were transported offshore. Long-term monitoring of bottom waters just above the seafloor and field experiments carried out off the Fukushima coast revealed that the combination of wave and current action caused by meteorological disturbance was a key process in the transport of suspended particulate material (Yagi et al 2015;Buesseler et al 2015).…”

Three-Dimensional Distribution of Radiocesium in Sea Sediment Derived from the Fukushima Dai-ichi Nuclear Power Plant

“…This result seems superfi cially to indicate that organic content also can determine radiocesium concentration. However, a report for radiocesium concentration in the coastal area of Ibaraki Prefecture after the FNPP accident (Otosaka and Kobayashi 2013 ) indicated that the lithogenic fraction contained most of the 137 Cs in the sediment. The contribution of organic matter to the radiocesium concentration in sea sediment is also small by chemical leaching for the samples of Ambe et al ( 2014 ) (as seen in the next chapter).…”

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 .…”