Establishment of the emergency

Kato et al. [8.22] study the depth distribution of radionuclides in surface soils with a 15 cm × 30 cm scraper plate (see Fig. 8.14):

“The scraper plate has two components; a metal frame that is placed on the ground, and an adjustable metal plate that can scrape or remove fixed increments of soil-depth within the frame. Advantages of the device is following; it provides a large volume of material from a large surface area, and it has few moving parts and it is robust and of simple construction....

Samples were taken with 0.5 cm increments for the depth of 0–5 cm and 1.0 cm increments for the depth of 5–10 cm, and 2.0 cm increments for the depth of 10–30 cm. In order to avoid contamination by the surface soil which can fall down from the wall of the sampling hole, spray glue was used to fix the wall. The sampled soil was then shipped to the laboratory, placed in a plastic container and sealed without oven drying and sieving to avoid ¹³¹I release into the atmosphere.”

Kato et al. [8.22] plot the depth distributions in the topsoil of ¹³¹I, ¹³⁴Cs and ¹³⁷Cs concentrations (see Fig. 8.15) and find that:

“The maximum concentration was found at the surface for all the radionuclides and the concentrations exponentially decreased with depth.

Note: Scraper: 5 mm increment to 10 cm, and 1 cm increment to 30 cm.

FIG. 8.14. Scraper plate for collection of soil samples for depth profiles of nuclides. (Courtesy of Yu. Onda, Tsukuba University, Japan.)

FIG. 8.15. Depth distribution of ¹³⁷Cs, ¹³⁴Cs and ¹³¹I activity concentrations [8.22].

“...Approximately 80% of the deposited radionuclides were found in the upper 2 cm of the soil. Sixty-three percent of total radioactivity inventory for both radiocaesium and 54% of total ¹³¹I were contained in the top 1 cm.

Furthermore, 86% and 96% of total radiocaesium inventory were absorbed by soils above 2 cm and 5 cm depth, respectively. For ¹³¹I, 79% of total radioactivity inventory in the soil profile was contained in the top 2 cm, and all ¹³¹I existed within the soil above 5 cm depth.”

8.2.2.2. A preliminary study of soil collection methods

Based on the fact that most of the ¹³¹I, ¹³⁴Cs and ¹³⁷Cs activities are contained within 5 cm of the surface, Onda et al. [8.20] assume:

“...the radionuclide deposition flux resided in the surface soil layer (within 5 cm of the soil surface) during the collection period (June–July 2011).

Therefore, we used 100-mL U-8 containers...outfitted with calibration radiation sources; the samples were placed in these containers so as to homogenize the radioactive material contained in the soil. It was important not to heat soil samples containing ¹³¹I since because drying the soils could cause ¹³¹I to evaporate and disperse into the laboratory. However, air-drying would be time-consuming with such a large number of samples, and we therefore measured wet soil samples sealed in the field (e.g., Kato et al. [8.22]). To evaluate the accuracy and precision of the measurement of radionuclide concentrations in soil samples with a germanium detector, we undertook a preliminary test of the following three potential methods (Fig. [8.16]):

1) Unmixed soil (control): Soil was collected by inserting a U-8 container into the surface soil layer and left unmixed. Radioactivity concentration was then measured.

2) Stirred soil: Soil was collected by inserting a U-8 container into the surface soil layer, after which it was stirred with a disposable plastic knife and vibrated 150 times (see Fig. [8.16]) after sealing.

Radioactivity concentration was then measured.

3) Homogenized soil: Soil was collected by inserting a U-8 container into the surface soil layer, after which it was placed in a polyethylene bag and shaken. The soil must be loosened through pressing and crushing by hand if any aggregated soil remains after shaking. Finally, the sample was transferred back to a U-8 container for storage and the radioactivity concentration was measured.”

“Fig. [8.17] shows ¹³⁷Cs concentrations obtained using the three mixing techniques for samples collected in a paddy field in the Yamakiya region, Fukushima Prefecture (sampled on May 21, 2011). Our preliminary tests showed that soil samples collected from paddies and grasslands measured using method 1, in which the distribution of radionuclides in the sample was not uniform, produced some measurement errors arising from the application of a calibration gamma source that assumed a homogeneous distribution of radionuclides, while stirring the samples as in method 2 allowed soil to spill from the containers. Measurement variability was less than in method 1, but a scatterplot indicated that samples were still not sufficiently homogenized. Fig. [8.17] clearly indicates that soil mixed outside the containers, as in method 3, was in an adequately homogeneous state and radioactivity concentration measurements had little statistical scattering. Using these results, we decided to use method 3, stirring the sample in a polyethylene bag, for further soil collection.”

Source: Figure 1 of Ref. [8.20] (left).

Note: The terms ‘Control’, ‘Knife’ and ‘Bag’ indicate no treatment, mixed with a knife, and mixed in a plastic bag by hand, respectively. Five soil core samples were collected from the forest floor, grassland and paddy field on 21 May 2011.

FIG. 8.16. Schematic diagram of the three sampling procedures.

Figure 8.18 provides the relationship between the number of times that the sample was shaken and the measured concentration of ¹³⁷Cs and ¹³¹I. Soil sampling was conducted in April 2011 on an artificial hillslope at the Terrestrial Environmental Research Centre of the University of Tsukuba. The shaking treatment was performed reciprocally by several operators to correct individual differences. Results indicate that shaking samples at least 150 times is necessary to homogenize soil materials in the containers.

8.2.2.3. Selection of soil collection locations

Onda et al. [8.20] emphasize that collecting multiple samples from the same locations is vital to monitor long term changes in the fallout inventory of radioactive material. To achieve this, they selected sampling locations with no anticipated disturbances. They tried to choose sampling points without vegetation, and they collected five soil samples from each site within a range of 3 m of each other to account for any anticipated variation in soil radioactivity concentrations.

In some cases, soil core samples were collected together with above ground vegetation, and where “sampling locations fell inside high-radiation dose-rate Source: Figure 2 of Ref. [8.20].

Note: Refer to Fig. 8.16 for each treatment. Error bars denote the range of measured radioactivity in the five soil samples. Soil core samples were collected on 21 May 2011.

FIG. 8.17. Effect of mixing method on the measured radioactivity in soil samples (paddy field).

Source: Figure 8 of Ref. [8.20], modified.

Note: Soil sampling was conducted on an artificial hillslope at the Terrestrial Environmental Research Center of the University of Tsukuba, Japan, on 16 April 2011.

FIG. 8.18. Effect of shaking on the measured concentrations of ¹³⁷Cs and ¹³¹I.

areas (e.g., evacuation zones), only one to three samples were taken to avoid prolonged exposure of sampling workers to radiation” [8.20].

8.2.3. Protocol for assessing radionuclide contamination in soil samples