Radioactive Cs in the estuary sediments near Fukushima Daiichi Nuclear Power Plant

Abstract

The migration and dispersion of radioactive Cs (mainly (134)Cs and (137)Cs) are of critical concern in the area surrounding the Fukushima Daiichi Nuclear Power Plant (FDNPP). Considerable uncertainty remains in understanding the properties and dynamics of radioactive Cs transport by surface water, particularly during rainfall-induced flood events to the ocean. Physical and chemical properties of unique estuary sediments, collected from the Kuma River, 4.0km south of the FDNPP, were quantified in this study. These were deposited after storm events and now occur as dried platy sediments on beach sand. The platy sediments exhibit median particle sizes ranging from 28 to 32μm. There is increasing radioactivity towards the bottom of the layers deposited; approximately 28 and 38Bqg(-1) in the upper and lower layers, respectively. The difference in the radioactivity is attributed to a larger number of particles associated with radioactive Cs in the lower part of the section, suggesting that radioactive Cs in the suspended soils transported by surface water has decreased over time. Sequential chemical extractions showed that ~90% of (137)Cs was strongly bound to the residual fraction in the estuary samples, whereas 60~80% of (137)Cs was bound to clays in the six paddy soils. This high concentration in the residual fraction facilitates ease of transport of clay and silt size particles through the river system. Estuary sediments consist of particles <100μm. Radioactive Cs desorption experiments using the estuary samples in artificial seawater revealed that 3.4±0.6% of (137)Cs was desorbed within 8h. More than 96% of (137)Cs remained strongly bound to clays. Hence, particle size is a key factor that determines the travel time and distance during the dispersion of (137)Cs in the ocean.

Unexpected source of Fukushima-derived radiocesium to the coastal ocean of Japan

“…S4). These desorption rates were higher than those observed from riverine particles (18) or estuarine sediments (19), but consistent with a well-known property of Cs: decreasing solid-solution partitioning with increasing ionic strength (20). Therefore, beach sands appear to be capable of storing a large inventory of 137 Cs at depth that over time may be remobilized by seawater intrusion into beach aquifers and released to the coastal environment via groundwater-surface water exchange processes.…”

“…Another ongoing source of FDNPP-derived 137 Cs is from rivers, with release estimates for total 137 Cs ranging from 2 to 12 TBq·y −1 (28)(29)(30)(31). Typhoons and heavy rain events have been shown to increase the river runoff flux; the 137 Cs input from this source is largely in the particulate phase (32), of which only a small amount is capable of entering the dissolved phase via desorption in the estuarine mixing zone (19). Thus, considering that ∼90% of the Cs is irreversibly bound to riverine suspended sediments (19), the riverine flux of dissolved 137 Cs is only ∼0.2-1.2 TBq·y −1 .…”

“…Typhoons and heavy rain events have been shown to increase the river runoff flux; the 137 Cs input from this source is largely in the particulate phase (32), of which only a small amount is capable of entering the dissolved phase via desorption in the estuarine mixing zone (19). Thus, considering that ∼90% of the Cs is irreversibly bound to riverine suspended sediments (19), the riverine flux of dissolved 137 Cs is only ∼0.2-1.2 TBq·y −1 . Hence, the ocean input of 137 Cs from groundwater below the sandy beaches is similar in magnitude as the other two major sources, namely ongoing FDNPP and dissolved river sources.…”

Identification of 90Sr/40K Based on Cherenkov Detector for Recovery from the Fukushima Nuclear Accident

“…The detector has an effective area of 300 cm 2 . For a sample density of 1 g/cm 3 , 30 g is the maximum mass of heated sample that can be measured. Seafood and seawater are approximately 70% and 99% or more pure water, respectively, and the compressibility is 0.3 for seafood and 0.01 for seawater.…”

“…After the accident, methods were developed to measure the radioactive concentration of cesium isotopes (radio-cesium) in samples with precision [1][2][3], which facilitates the enforcement of the maximum radioactive concentration of radio-cesium in food at 100 Bq/kg, as defined by the Ministry of Health, Labour and Welfare in Japan. This concentration leads to a dose that is less than that due to internal exposure from natural 40 K (0.18 mSv/year).…”