Taeko Wakahara

Initial flux of sediment-associated radiocesium to the ocean from the largest river impacted by Fukushima Daiichi Nuclear Power Plant

This study aimed to quantify the flux of radiocesium in the Abukuma Basin (5,172 km2), the largest river system affected by fallout from the Fukushima Daiichi Nuclear Power Plant (FDNPP) event. In the period from 10 August 2011 to 11 May 2012 an estimated 84 to 92% of the total radiocesium transported in the basins fluvial system was carried in particulate form. During this monitoring period Typhoon Roke (September 2011) was observed to induce a significant and temporally punctuated redistribution of radiocesium. The storm-mobilised radiocesium was an estimated 6.18 Terabecquerels corresponding to 61.4% of the total load delivered to the coastal zone during the observation period. The total flux of radiocesium into the Pacific Ocean estimated at the outlet station (basin area 5,172 km2) was 5.34 TBq for 137Cs, and 4.74 TBq for 134Cs, corresponding to 1.13% of the total estimated radiocesium fallout over the basin catchment (890 TBq). This was equivalent to the estimated amount of direct leakage from FDNPP to the ocean during June 2011 to September 2012 of 17 TBq and the Level 3 Scale Leakage on 21August 2013 (24 TBq).

Time Dependence of the 137Cs Concentration in Particles Discharged from Rice Paddies to Freshwater Bodies after the Fukushima Daiichi NPP Accident

The concentration of particulate (137)Cs in paddy fields, which can be a major source of (137)Cs entering the water system, was studied following the Fukushima Daiichi Nuclear Power Plant accident. To parametrize the concentration and to estimate the time dependence, paddy fields covering various levels of (137)Cs deposition were investigated over the period 2011-2013 (n = 121). The particulate (137)Cs concentration (kBq kg-SS(-1)) showed a significant correlation with the initial surface deposition density (kBq m(-2)). This suggests that the entrainment coefficient (m(2) kg-SS(-1)), defined as the ratio between the particulate (137)Cs concentration and the initial surface deposition density, is an important parameter when modeling (137)Cs wash-off from paddy fields. The entrainment coefficient decreased with time following a double exponential function. The decrease rate constant of the entrainment coefficient was clearly higher than that reported for other land uses and for river water. The difference in the decrease rates of the entrainment coefficient suggests that paddy fields play a major role in radiocesium migration through the water system. An understanding of the decrease rate of the entrainment coefficient of paddy fields is therefore crucial to understand the migration of radiocesium in the water system.

Spatial pattern of atmospherically deposited radiocesium on the forest floor in the early phase of the Fukushima Daiichi Nuclear Power Plant accident

Spatial patterns of atmospherically deposited radiocesium on the forest floor and the temporal evolution were measured in two Japanese cedar stands and a secondary mixed broad-leaved forest in the early phase of the Fukushima Daiichi Nuclear Power Plant accident. In situ measurements of the 137Cs gamma count were made using a portable germanium gamma ray detector. These measurements revealed that the forest floors were contaminated with radionuclides derived from the accident. In the cedar stands, the inter-canopy area had higher 137Cs count rate relative to the under-canopy area, whereas no clear relationship was found between the radiocesium pattern and canopy cover in the mixed broad-leaved forest. Repeated radiocesium measurements revealed that the spatial pattern of radiocesium activity on the forest floor did not substantially change following additional deposition inputs. Furthermore, the magnitude of canopy cover partially explained spatial variability of the 137Cs on the forest floor in cedar stands. These results suggest that canopy structure affected the genesis of the horizontal variability of atmospherically deposited radiocesium on the forest floor during the early phase of the Fukushima accident.

Small Flume Experiment on the Influence of Inflow Angle and Stream Gradient on Landslide-Triggered Debris Flow Sediment Movement

Rainfall-induced landslide might transformed into more severe disaster, namely debris flow and natural dam which both holds serious threats on human life and material. The runout distance has crucial role for determining affected areas of a landslide. Our previous research found the correlation of inflow angle and stream gradient to transformation of landslide collapsed sediment either into natural dam or debris flow. This research intended to test our previous research result with a small flume experiment and aimed to analyze the influence of sediment inflow angle and stream gradient to the sediment deposition percentages as representative of runout distance and the possibility of natural dam formation. Soil samples were taken from landslide-triggered debris flow disaster initiation zone in Hiroshima (Hiroshima Pref.) and Izu Oshima (Tokyo Pref.), Japan which were induced by heavy rainfall. The small flume was 10 cm width and 15 cm height, the inflow segment angle was varied to 60° and 90°, and the stream segment gradient was varied to 10° and 15°. From the experiment results, stream gradient influence the sediment movement effectively rather than inflow angle, and it was sufficient to examine the possibility of collapsed sediment to form natural dam or debris flow. Soil samples from natural dam initiation zones and consideration of water content factor are essential for further experiment.